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THE
MICROSCOPE
AND ITS
REVELATIONS
BY THE SAME AUTHOR.
PRINCIPLES OF HUMAN PHYSIOLOGY. With
numerous Illustrations on Steel and Wood. Eighth Edition,
Edited by Mr. Henry Power. 8vo. In the Press.
A MANUAL OF PHYSIOLOGY. With numerous
Illustrations on Steel and Wood. Fifth Edition, Crown 8vo.
In the Press.
Plate J.
Plate. H
THE
MICROS CO P E
AND ITS
EEYELATIONS
BY e^
WILLIAM B. CARPENTER, M.D. LL.D.
F.R.S. F.G.S. F.L.S.
CORRESPONDING MEMBER OE THE INSTITUTE OP FEANCE
BEGISTEAE TO THE UNIVERSITY OF LONDON
. FIFTH EDITION
PREPARED WITH THE ASSISTANCE OF H. J. SLACK, F.G.S., HON. SEC. TO THE
ROYAL MICROSCOPICAL SOCIETY
ILLUSTRATED BY TWENTY-FIVE PLATES
AND FOUR HUNDRED AND FORTY-NINE WOOD ENGRAVINGS
LONDON
J. & A. CHURCHILL, NEW BURLINGTON STREET
1875
[111 rights reserved]
PREFACE.
The Tapid increase which has recently taken place in the nse of the
Microscope, — both as an instrument of scientific research, and as a
means of gratifying a laudable curiosity and of obtaining a health-
ful recreation, — nas naturally led to a demand for information,
both as to the mode of employing the Instrument and its appur-
tenances, and as to the Objects for whose minute examination it is
most appropriate. This information the Author has endeavoured
to supply in the following Treatise ; in which he has aimed to
combine, within a moderate compass, that information in regard
to the use of his Instrument and its Appliances which is most
essential to the working Microscopist, with such an account of the
Objects best fitted for his study as may qualify him to comprehend
what he observes, and thus prepare him to benefit Science whilst
expanding and refreshing his own mind. The sale of four large
Editions of this Manual, together amounting to ten thousand
copies,— notwithstanding the competition of several cheaper and
more popular treatises, — with the numerous unsought testimonies to
its usefulness which, the Author has received from persons pre-
viously unknown to him, justify the belief that it has not inade-
quately supplied an existing want ; and in the preparation of the
new Edition now called-for, therefore, he has found no reason
to deviate from his original plan, whilst he has endeavoured to
improve its execution as to every point which seemed capable of
amended treatment.
vi PEEFACE.
In his account of the various forms of Microscopes and Accessory
Apparatus, the Author has not attempted to describe every thing
which is used in this country ; still less, to go into minute details
respecting the construction of foreign instruments. He is satisfied
that in nearly all which relates both to the mechanical and the
optical arrangements of their instruments, the chief English
Microscope-makers are quite on a level with, if not in advance of,
their Continental rivals ; but on the other hand, the latter have
supplied instruments which are adequate to all the ordinary pur-
poses of scientific research, at a lower price than such could until
recently be obtained in this country. Several British makers,
however, are now devoting themselves to the production of Micro-
scopes which shall be really good though ckea/p ; and the Author
cannot but view with great satisfaction the extension of the manu-
facture in this direction. In the selection of Instruments for
description which it was necessary for him to make, he trusts that
he will be found to have done adequate justice to those who have
most claim to honourable mention. His principle has been to make
mention of such Makers as have distinguished themselves by the
introduction of any new pattern which he regards as deserving of
special recommendation ; those who have simply copied the patterns
of others without essential modification, receiving no such recogni-
tion,— not because their instruments are inferior, but because they
are not original.
In treating of the Applications of the Microscope, the Author
has constantly endeavoured to meet the wants of such as come to
the study of the minute forms of Animal and Yegetable life with
little or no previous scientific preparation, but desire to gain some-
thing more than a mere sight of the objects to which their obser-
vation may be directed. Some of these may perhaps object to the
general tone of his work as too highly-pitched, and may think that
he might have rendered his descriptions simpler by employing
fewer Scientific terms. But he would reply that he has had much
opportunity of observing among the votaries of the Microscope a
desire for such information as he has attempted to convey ; and
that the use of scientific terms cannot be easily dispensed with,
PREFACE. vil
since there are no others in which the facts can be readily ex-
pressed. As he has made a point of explaining these in the
places where they are first introduced, he cannot think that
any of his readers need find much difficulty in apprehending
their meaning.
The proportion of space allotted to the several departments has
been determined not so much by their Physiological importance, as
by their special interest to the amateur Microscopist ; and the re-
membrance of this consideration will serve to account for much
that might otherwise appear either defective or redundant. The
Author has thought it particularly needful to limit himself in
treating of certain very important subjects which are fully dis-
cussed in Treatises expressly devoted to them (such, for example, as
the structure of Insects, and the Primary Tissues of Vertebrata),
in order that he might give more space to those on which no such
sources of information are readily accessible. For the same
reason, he has omitted all reference to the applications of the
Microscope to Pathological inquiry ; a subject which would interest
only one division of his readers, and on which it would have been
impossible for him to compress, within a sufficiently narrow com-
pass, a really -useru.1 summary of what such readers can readily
learn elsewhere. Cn the other hand, he has gone somewhat into
detail in regard to various humble forms of Vegetable and Animal
life, which the diligent Collector is not unlikely to meet with, and
which will fully reward his most attentive scrutiny.
It has been the Author's object throughout, to guide the possessor
of a Microscope to tie intelligent study of any department of
Natural History, which his individual tastes may lead him to
follow-out, and his indiTidual circumstances may give him facilities
for pursuing. And he has particularly aimed to show, under each
head, how small is the amount of trustworthy knowledge already
acquired, compared with xhat which remains to be attained by the
zealous and persevering student. Being satisfied that there is a
large quantity of valuable Microscope-power at present running to
waste in this country, — being applied in such desultory observa-
tions as are of no service whatever to Science, and of very little to
Vin PREFACE.
the mind of the observer, — he will consider himself well rewarded
for the pains he has bestowed on the production of this Manual, if
it should tend to direct this power to more systematic labours, in
those fertile fields which only await the diligent cultivator to bear
abundant fruit.
In all that concerns the working of the Microscope and its
appurtenances, the Author has mainly drawn upon his own ex-
perience, which dates-back almost to the time when Achromatic
Object-glasses were first constructed in this country. But having
of late found himself compelled to limit his attention more and more
to particular lines of scientific inquiry, and having been hence led to
fear that he might have fallen behind in his knowledge of the more
recent developments both of the theory and practice of Microscopy,
he has sought the aid of his friend Mr. H. J. Slack, whose position
as Secretary to the Royal Microscopical Society, in combination
with his general scientific attainments, pointed him cut as a trust-
worthy coadjutor. In particulars, he has left it to Mr. Slack to
estimate the practical value of the new principles and methods
recently introduced by Dr. Eoyston-Pigott, which have been the
subject of much discussion, and as to which there is still great
discrepancy of opinion.
It may be thought that fuller notice should have been taken of a
number of new processes and appliances which have been intro-
duced since the appearance of the last Edition of this Manual, and
which are daily proving of great value ia Scientific inquiry.
But to do this would be to depart from the original purpose of the
work, which was to impart general guidance, rather than special
instruction : and in the belief that a wicli and not too minute
survey of the principal forms of Organic structure and modes of
Living action, presented by the Vegetable and Animal Kingdoms,
constitutes the best possible preparation for the detailed study of
any one department, the Author has purposely abstained from
making such considerable additions ay would be useful only to
those who are devoting themselves to the latter object, and who
need the full information which they can only obtain from special
Treatises.
PEEFACE. ix
For tlie same reason he has abstained from noticing a large
number of new pieces of Apparatus, many of which have doubtless
a special value to those who have devised them, but which have
not yet established their claim to rank as part of the ordinary
armamentum of the Micro scopist. To have described a long series
of these would have added greatly to the bulk of his volume,
without adding to its utility in the same proportion ; and the
Author has deemed it preferable to limit himself in most instances
to those which he has himself tried and found to be serviceable, —
his object being, not the impossible one of teaching his reader all
that has to be learned, but the putting him in the way of learning
it from that best of all teachers, Experience.
The whole Treatise has been subjected to a careful revision ; and
much new matter, with many additional illustrations, have been
introduced, especially under the following heads : —
Microscopes. — Stephenson's Binocular, p. 64. — Field's Dissecting
and Mounting, p. 81. — Browning's Botating, p. 95. — Boss's New
Boss- Jackson Model, p. 102. — Beck's New First-class Model,
p. 104— Swift's New Portable, p. 818.
Microscopic Appliances. — - Dr. Boyston-Pigott's Aplanatic
Searcher, p. 40. — Browning's Bright-line Spectruni-Micrometer,
p. 117. — Wenham's Keflex Bluminator, p. 142. — Swift's New
Achromatic Condenser, p. 820. — Blankley's Bevolving Mica- Selenite
Stage, p. 820. — Swift's Portable Microscope Lamp, p. 822. — Beck's
Beversible Compressoriums, p. 163.
Results of Microscopic Study. — Dr. Woodward's Photographs of
Test-Objects, pp. 213, 701. — Nature of Markings on Diatoms,
pp. 308-312. — Belation of low forms of Fungi, Bacteria, and
Vibriones to Fermentation, &c, pp. 379-382. — Coccoliths, Cocco-
spheres, and Bathybius, pp. 464-466, 816. — Life-History of Cerco-
monad, pp. 494-496. — New Types of Arenaceous Foraniinifera, pp.
529-539. — Nummuline Tubulation of Eozoon Canadense, p. 556. —
Siliceous Sponges, pp. 569, 570. — Embryonic Development, pp. 572,
727. — Structure of Scales of Insects, pp. 692-702. — Nervous System
of Comatula, p. 771. — Formation of Chalk on Atlantic Sea-bed,
pp. 795-798. — Concretionary Calcareous Deposits, pp. 815, 816.
x PEEFACE.
The Author (who holds himself more particularly responsible
for the division which treats of the Applications of the Mi-
croscope), is perfectly aware that he may be found charge-
able with many faults of omission, through his not having taken
note of later researches upon various topics referred to in his
pages, whereby he might have made his account of them more
accurate or more complete. He must plead in mitigation of such
criticism, first, the impossibility of his keeping pace with the rapid
extension of knowledge over every part of the constantly-widening
field of Microscopic study ; and, secondly, the necessity of restricting
his treatise within the limited compass that adapts it to the class
for which it is intended. He has greatly increased, however, the
number of references to recent and trustworthy sources of informa-
tion ; and he hopes that these will prove serviceable alike to such
as desire to extend their own inquiries, and to such as merely
wish to acquaint themselves with what has been done by others.
To the former class he would give this word of encouragement, —
that, notwithstanding the number of recruits continually being
added to the vast army of Microscopists, and the rapid extension of
its conquests, the inexhaustibility of Nature is constantly becoming
more and more apparent ; so that no apprehension need arise
that the Microscopists research can ever be brought to a stand for
want of an object !
University op London,
December) 1874.
TABLE OF CONTENTS.
INTRODUCTION.
Sketch cf the History of the Microscope and Microscopic Discovery
Educational value of the Microscope .....
1
22
CHAPTER I.
OPTICAL PRINCIPLES OF THE MICROSCOPE.
Laws of Refraction : —Spherical and Chromatic Aberration . . .30
Simple Microscope . 48
Compound Microscope . . . . . . . . .52
Principles of Binocular Vision ........ 57
Stereoscopic Binocular Microscopes 59
Nachet's. 60
Wenham's .......... 62
Stephenson's .......... 64
Cachet's Stereo-pseudoscopic .67
Special value of Stereoscopic Binoculars . . . . .69
CHAPTER II.
CONSTRUCTION OF THE MICROSCOPE.
General principles .
Simple Microscopes .
Ross's
Quekett's Dissecting .
Field's Dissecting and
Mounting
Beck's and Nachet's Binocular
Compound Microscopes ,
Third-Class Microscopes
Field's Educational .
Crouch's Educational,
Pillischer's Student's .
1-Class Microscopes .
Beck's Student's
Ladd's Student's
Nachet's Student's
Browning's Rotating
74
77
78
80
81
83
85
87
BL
Crouch's Student's
cular . . . .96
Beck's Popular . .96
Collins's Harley Binocular , 97
First- Class Microscopes . .99
106
107
108
108
110
Powell and Lealand's . . 102
Beck's . . . .104
Microscopes for Special Purposes 106
Beale's Pocket and Demon-
strating ....
Baker's Travelling ,
King's Aquarium
Dr. L, Smith's Inverted
Nachet's Double-bodied
Powell and Lealand's Non-
stereoscopic Binocular
111
Xli
TABLE OF CONTENTS.
CHAPTER III.
ACCESSORY APPARATUS.
PAGE
PAGE
Draw-Tube ....
112
Wenham's Reflex Illuminator .
142
Lister's Erector
113
White-Cloud Illuminator .
144
Nachet's Erecting Prism .
114
Polarizing Apparatus
145
Micro-Spectroscope .
115
Side. Illuminators for Opaque
Micrometric Apparatus
121
Objects .
147
Goniometer ....
125
Parabolic Speculum .
150
Diaphragm Eye-piece and Indi-
Liebcrkiihn .
151
cator .....
125
Beck's Vertical Illuminator
153
Camera Lucida and other Draw-
Stephenson's Safety Stage .
154
ing Apparatus
126
Stage- Forceps and Vice
155
Nose-piece ....
130
Disk-holder and Object-holder .
156
Object-Marker
130
Glass Stage-Plate and Growing
Object-Finders
131
Slide .
157
Diaphragm ....
133
Live Boxes and Cells
158
Achromatic Condensers
134
Zoophyte- Trough
160
Webster Condenser .
136
Compressoriums
161
Oblique Illuminators
137
Dipping Tubes
164
Amici's Prism ....
138
Glass Syringe ....
165
Reade's Hemispherical Condenser 139
Forceps .....
166
Black-Ground Illuminators
140
CHAPT
ER TV.
MANAGEMENT OF
THE MICROSCOPE.
Support ....
168
Arrangement for Transparent Ot
-
Light ....
169
jects .
182
Position of Light
171
Arrangement for Opaque Objects
190
Care of the Eyes
172
Errors of Interpretation .
193
Care of the Microscope
173
Comparative Values of Object
General Arrangements
174
Glasses .
200
Focal Adjustment
176
Test-Objects .
205
Adjustment of Object- Glass
179
Determination of Magnifying
Power . . . . .
214
CHAPT
ER V.
PREPARATION, MOUNTING, A
NB COLLECTING OP OBJECTS.
Microscopic Dissection
217
Preparation of Specimens in
Cutting Sections of Soft Sub
stances
Cutting Sections of Harder Sub
220
Viscid Media
231
Glass Slides .
233
stances . . "
221
Thin Glass .
234
Grinding and Polishing of Sec
Varnishes and Cements
236
tions ....
222
Mounting Objects Dry
239
Chemical Actions
227
Mounting Objects in Canada
Staining Processes .
230
Balsam and Gum Damar
242
TABLE OF CONTEXTS.
xm
PAGE
PAGB
Preservative Media .
. 252
Built-up Ce'ls .
. 261
Mounting Objects in Fluid
. 255
Mounting Objects in Cells
. 262
Cement- Cells .
. 257
Importance of Cleanliness
. 264
Thin-Glass Cells .
. 258
Labelling- and Preserving
. 265
Sunk and Plate-Glass Cells
. 259
Collection of Objects
. 266
Tube-Cells
. 260
CHAPTER VI.
MICROSCOPIC FORMS
OP VEGETABLE LIFE. PROTOPHTTES
Boundary between Animal and
Ulvaeeae .
. 348
Vegetable Kingdoms
. 272
Oscillatoriacese
. 350
Characters of Vegetable Cell
. 272
Nostochaeese .
. 352
Life-History of Simplest Proto-
Siphonaceee
. 353
phytes
. 274
Confervaceae .
. 358
Volvocinea?
. 282
Conjugatese
. 362
Desmidiacese .
. 290
Chaetophoracea?
. 363
Pediastreae
. 300
Batrachospermese
. 364
Diatomacese .
. 304
Characese
. 365
Palmellacese .
. 346
Alg83
Lichens .
Fungi
Hepa ticae
CHAPTER VII.
MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA.
370
Mosses
377
Ferns
378
Equisetaceae
395
399
406
412
CHAPTER VIII.
MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
Elementary Tissues .
Structure of Stem and Root
. 415
. 433
| Structure of Cuticle and Leaves . 445
l Structure of Flowers and Seeds . 452
CHAPTER IX.
MICROSCOPIC FORMS OP ANIMAL LIFE : — PROTOZOA ; ANIMALCULES.
Protozoa .
. 462
Gregarinida
Rhizopoda
. 466
Thalassicollida
Reticularia
. 468
Animalcules
Radiolaria
. 470
Infusoria
Lobosa
. 473
Rotifera
Reproduction of Pvhizopoda
. 477
. 479
. 481
. 482
. 483
. 501
TABLE OF CONTEXTS.
CHAPTER X.
FORAHINIFERA, POLYCYSTINA, AND SPONGES.
Foraminifera .
PAGE
. 514
Foraminifera — continued.
PAGE
Miiiolida .
. 520
Nummulinida
. 545
Lituolida .
. 529
Polyeystina
. 5b2
Lagenida .
. 539
Aeanthometrina
. 566
Gflobigerinida .
. 540 Porifera (Sponges)
CHAPTER XI.
ZOOPHYTES.
. 567
Hydra
. 574 1 Acalephse . .
. 584
Compound Hydrozoa
.578 Actinozoa
. 588
Production of Medusoids
. 579 J Ctenophora
. 592
CHAPTER XII.
ECHINODERMATA.
Structure of Skeleton
. 596
Echinoderm-Larvse .
. 608
Polyzoa
CHAPTER XIII. ■
POLYZOA AND TUNICATA.
. 616 | Tunicata.
CHAPTER XIV.
MOLLUSCOUS ANIMALS GENERALLY.
. 623
Structure of Shells. .
Palate of Gasteropods
Development of Mollusks
. 632
. 644
. 648
Ciliary motion on Grills
Organs of Sense of Mollusks
Chromatophores of Cephalopods
656
656
657
CHAPTER XV.
ANNULOSA OR WORMS.
Entozoa .
Turbellaria
. 659 ] Annelids ....
. 662 j Development of Annelids .
CHAPTER XVI.
CRUSTACEA.
664
666
Pycnogonidse' .
Entomostraca .
Suctoria . .
. 674
. 676
. 683
Cirrhipeda .
Shell of Decapoda .
Metamorphosis of Decapoda
684
686
687
TABLE OF CONTEXTS.
CHAPTER XVII.
INSECTS AND
ARACHNIDA.
PAGE
PAGE
Number and variety of Objects
Wings ....
. 719
afforded by Insects
. 689
Feet ....
721
Structure of Integument
. 691
Stings and Ovipositors
. 724
Tegumentary Appendages
. 692
Eggs ....
. 725
Eyes
. 704
Agamic Reproduction
. 726
Antenna? ....
. 707
Embryonic Development .
. 727
Mouth .
. 709
,
Circulation of the Blood
. 713
Acarida ....
. 728
Respiratory Apparatus
. 715
Parts of Spiders
. 729
CHAPTER XVIII.
VERTEBRATED ANIMALS.
Elementary Tissues .
.
732
Epidermis
. 759
Bone
.
736
Pigment-Cells .
. 760
Teeth
740
Epithelium
. 761
Scales of Fish .
743
Fat .
. 763
Hairs
746
Cartilage .
. 764
Feathers .
750
Glands
. 765
Hoofs, Horns, &c.
750
Muscle
. 766
Blood
.
751
Nerve
. 770
White and Yellow Fibres .
756
Circulation of the Blood
. 771
Skin, Mucous and
Serous
InJ
3Cted Preparations
. 780
Membranes .
758
Vessels of Respiratory Org
ans . 786
CHAPTER XIX.
APPLICATION OF THE MICROSCOPE TO G-EOLOGT.
Fossilized Wood, Coal . . 790
Fossil Foraminifera ; Chalk . 793
Organic materials of Rocks . 798
Structure of Fossil Bones, Teeth,
&c 801
Inorganic materials of Rocks . 804
CHAPTER XX.
INORGANIC OR MINERAL KINGDOM.— POLARIZATION.
Mineral Objects . . . 807
Crystallization of Salts . .808
Molecular Coalescence . .813
Organic Structures
Polariscope .
Micro-Chemistry
suitable for
813
816
APPENDIX.
EXPLANATIONS OF THE PLATES.
PLATE I. (Frontispiece.)
VARIOUS FORMS OF DIATOMACE.E.
Fig. 1. Actinocyclus Ralfsii.
2. Asterolampra concinna.
3. Eeliopelta (as seen with black-ground illumination).
4. Aster omphalus BrooTceii.
5. Aulacodlscus Oreganus.
PLATE II. (Frontispiece).
echinus -spine (Original), and podura-scale (after R. Beck).
Fig. 1. Transverse section of Spine of Echinometra heteropora.
2. Markings on Scale of Podura, as seen by transmitted light under a
well-corrected l-8th inch Objective.
3. Partial obliteration of the markings by the insinuation of moisture
between the Scale and the Covering-glass.
4. Appearance of the markings, when the Scale is illuminated from above
by oblique light falling at right angles to them.
5. The same, when the light falls on the Scale in the direction of the
markings.
PLATE III. (p. 96).
crouch's student's binocular.
PLATE IV. (p. 97).
beck's popular microscope.
PLATE V. (p. 102).
ROSS'S JACKSON-MODEL MICROSCOPE.
PLATE VI. (p. 104).
POWELL AND LEALAND's LARGE MICROSCOPE.
b
xvm EXPLANATIONS OF THE PLATES.
PLATE VII. (p. 105).
MESSRS. BECK'S LARGE MICROSCOPE.
PLATE VIII. (p. 276).
development of PALMOGLiEA and protococcus (after Braun and Cohn).
Fig. 1, a — i. Successive stages of binary subdivision of Palmoglcea ; k — M,
successive stages of conjugation.
2, a — c. Binary subdivision of ' still ' form of Protococcus ; D — G, multi-
plication of ' motile ' form ; h — L, different phases of ' motile ' condition.
PLATE IX. (p. 284).
development op volvox globator (after Williamson),
Fig. 1. Young Volvox; a, prirnoi'dial cell of secondary sphere ; b, poly-
gonal masses of endochrome, separated by hyaline substance.
2. Tie same more advanced ; a, a, polygonal masses cf endochrome ;
b, b, their connecting processes ; c, primordial cell of secondary sphere.
3. The same more advanced, showing an increase in the size of the con-
necting processes, a, a, and a duplicative subdivision of the primordial cell.
4. The same more advanced, showing the masses of endochrome more
widely separated by the interposition of hyaline substance, and each furnished
with a pair of cilia ; whilst the primordial' cell, /, has undergone a second
segmentation.
5. Portion of the spherical wall of a mature Volvox, showing the wide
separation of the endochrome-masses still connected by the processes b, b, the
lines of areolation, c, dividing the hyaline substance, and the long cilia, e.
6. 7, 8. Secondary sphere, or macro-gonidium, developed by the progressive
segmentation of the primordial cell.
9. Single cell from the wall of a mature Volvox, showing the endochrome
mass, b, to contain two vacuoles a, a, and to be surrounded by a hyaline
envelope, d, having polygonal borders.
10. Portion of the wall of a young Volvox, seen edgeways, showing that its
sphere is still invested by the hyaline envelope of the original cell, which the
cilia penetrate but do not pierce.
11. Two cells from a mature Volvox, seen edgeways, showing the enclosure
of the endochrome-masses in their own hyaline investment, and the persistence
of the general investment (here pierced by the cilia) around the entire sphere.
PLATE X. (p. 330).
arachnoidiscus japonicus (after K. Beck).
Tho specimens, attached to the surface of a Sea-weed, are represented as
seen under a l-4th Objective, with Lieberkiihn illumination : — a, internal
surface ; B, external surface ; c, front view, showing incipient subdivision.
PLATE XI. (p. 360).
development and reproduction op sph^roplea annulina (after Cohn).
Fig. 1. Oo-spore, of a red colour, having its outer membrane furnished with
stellate prolongations.
EXPLANATIONS OF THE PLATES. xix
2, 3, 4. Successive stages of segmentation of the oo-spore.
5, Fusiform ciliated zoospores set free by the rupture of the coats of the
oo-spore.
6, 7, 8. Successive stages of its development into a filament.
9. Immature filament, showing at a the annulation of the endochrome pro-
duced by the regular arrangement of vacuoles, and at b the frothy appearance
produced by the multiplication of vacuoles.
10. More advanced stage, showing at a the aggregation of the endochrome
into definite masses, which become star-shaped as seen at b.
11. The star-shaped masses of endochrome, a, draw themselves together
again and become ovoidal, as at b ; definite openings, c, show themselves in the
cell- wall.
12. Entrance of the antherozoids, d, through the openings c, c.
13. Formation of mature oo-spores within the filament.
14. Contents of another filament, a, becoming converted into antherozoids,
which move about at b within their containing cell, and escape ^as seen at d )
through the opening c.
15. Antherozoids swimming freely by means of two motile filaments.
PLATE XII. (p. 440).
TRANSVERSE AND VERTICAL SECTIONS OF EXOGENOUS STEMS (Original).
Fig. 1. Portion of transverse section of a Fossil Wood, showing the medullary
rays a a, a a, a a, running nearly parallel to each other, and the openings of
large ducts in the midst of the woody fibres.
2. Vertical (tangential) section of the same wood ; showing the woody
fibres separated by the medullary rays, and by the large ducts, b b, b b.
3 and 4. Transverse and vertical (tangential) sections of a Fossil Wood,
showing the separation of the woody plates, a, a, by the very large medullary
rays, b, b.
PLATE XIII. (p. 465.)
Coscinodiscus (after Stephenson) ; Podura-scale (after Woodward) ;
Bathybius and Coccoliths (after Huxley and Haeckel).
Fig. 1. Hexagonal areola of inner or ' eye-spot ' layer of Coscinodiscus
oculus iridis, viewed in bisulphide of carbon, showing fracture through ' eye-
spot' (p. 328).
2. Areola of outer layer of the same.
3. Portion of a Podura- scale, as represented in a Photograph taken by Col.
Dr. Woodward (U.S.), with somewhat oblique illumination, and the objective
slightly withdrawn from the focal position which renders the ' exclamation -
marks' most distinctly (p. 701).
4. Portion of Bathybius Huxleyi, with imbedded coccoliths.
5. Discolith, seen in front view.
.6. Cyatholith, seen in front view: — (i) Central corpuscle; (2) Granular
zone ; (3) Transparent outer zone.
8, 9. Discolith s seen edgeways.
7, 10, 11. Cyatholiths seen obliquely.
12: Coccosphere, with imbedded cyatholiths.
b 2
xx EXPLANATIONS OF THE PLATES.
PLATE XIV. (p. 497).
sexual reproduction oe INFUSORIA (after Balbiani).
Fig. 1. Cod j ligation of Paramecium aurelia : a, ovarium (nucleus) ;
b, seminal capsule (nucleolus) ; c, oviducal canal ; d, seminal canal ; e, buccal
fissure.
2. The same, more advanced ; a, ovary, showing lobulated surface ; b, 6,
secondary seminal capsules.
3. One of the individuals in a still more advanced state of conjugation,
showing the ovary a, a, broken up into fragments connected by the tube m ;
b, b, seminal capsules ; v, contractile vesicle.
4. Paramecium, ten hours after the conclusion of the conjugation; a, a,
unchanged granular masses of the ovary; of which other portions have been
developed into the ova, o, o, still contained within the connecting tube m ;
b, b, seminal capsules.
5. The same, three days after the completion of the conjugation.
6 — 12. Successive stages in the development of the seminal capsules.
1'6 — 18. Successive stages in the development of the ovules.
19. Acinetce in different stages, a, b, o.
20. Paramecium containing three A cwieta-parasites, q, q, q', lying in
introverted pouches, of which the external openings are seen at x, x.
21. Stentor in conjugation.
PLATE XV. (p. 517).
VARIOUS FORMS OF FORAMINIFERA (Original).
Fig.
1. Comuspira.
Fig. 11. Cristellaria.
2. Spiroloculina.
12. Globigerina.
3. Trilocidina.
13. Polymorphina
4. Bilocidina.
14. Textularia.
5. Peneroplis.
15. Discorbina
6. Orbiculina (cyclical form).
16. Polystomella.
7. Orbiculina (young).
17. Planorbulina.
8. Orbicidina (spiral form).
18. Rotalia.
9. Lagena.
19. Nonionina.
0. Nodosaria.
PLATE XVI. (p. 548).
VARIOUS FORMS OF FORAMINIFERA (Original).
Fig. 1. CycloclypeuS) showing external surface, and vertical and horizontal
sections.
2. Operculina, laid open to show its internal structure : — a, margina cord,
seen in cross section at a' ; b, b, external walls of the chambers ; c, c, cavities
of the chambers ; d c', their alar prolongations ; d, d, septa, divided at d' d'
and at d", so as to lay open the interseptal canals, the general distribution of
which is seen in the septa e, e; the lines radiating from e, e, point to the
secondary pores ; g, g, non-tubular columns.
3. Calcarina, laid open to show its internal structure : — a, chambered
portion ; 6, intermediate skeleton ; c, one of the radiating prolongations
proceeding from it, with extensions of the canal-system.
EXPLANATIONS OF THE PLATES. xxi
PLATE XYIL (p. 558).
STRUCTURE OF EOZOON CANADENSE (Original).
Fig. 1. Portion of its calcareous Shell, as it would appear if the Serpentine
that fills its chambers could be dissolved away : — a1, a1, chambers of lower
story, opening into each other at a, a, but occasionally separated by a septum
b, b ; A2, A2, chambers of upper story ; B, B, proper walls of the chambers,
formed of a finely-tubular or nummuline substance ; c, c, intermediate skele-
ton, occasionally traversed by large stolon-passages, d, connecting the chambers
of different st- Ties, and penetrated by the arborescent systems of canals E, e, e.
2. Decalcified portion, showing the Serpentinous internal cast of the
chambers, canals, and tubuli of the original ; presenting an exact model of
the Animal substance which originally filled them.
PLATE XVIII. (p. 562).
various forms of polycystina (after Ehrenberg).
Fig. 1. Podocyrtis Schomburgkii .
2. Rhopalocanium ornatam.
3. Haliomma hystrix.
4. Pterocanium, with animal.
PLATE XIX. (p. 566).
various forms of radiolaria (after Haeckel) .
Fig 1. Eth?nospha?ra siplionophora.
2. Actinomma inerme.
3. Acanthometra xiphicantha.
4. Arachnos/.hcera obligacaniha.
5. Cladococcus viminalis.
PLATE XX. (p. 5S1).
campanularia gelatinosa (after Van Beneden).
A, Upper part of the stem and branches, of the natural size.
b, Small portion enlarged, showing the structure of the animal ; a, terminal
branch bearing polypes ; b, polype-bud partially developed ; c, horny cell con-
taining the expanded polype d ; e, ovarian capsule, containing medusiform
gemmae in various stages of development ; f, fleshy substance extending through
the stem and branches, and connecting the different polype-cells and ovarian
capsules ; g, annular constrictions at the base of the branches.
PLATE XXI. (p, 615).
PENTACRINOID LARVA OF ANTEDON (Original).
Fig. 1. Skeleton of early Pentacrinoid, under Black-ground illumination,
showing its component plates : — b, b, basals, articulated below to the highest
point of the stem ; r1, r1, first radials, between two of which is seen the
xxn EXPLANATIONS OF THE PLATES.
single anal plate, a ; r2, second radials ; r3, third radials, giving off the
bifurcating arms at their summit ; o, o, orals.
2, 3. Back and front views of a more advanced Pentacrinoid, as seen by in-
cident light, one of the pair of arms being cut away in Fig. 3, in order to bring
the mouth audits surrounding parts into view : — b, b, basals ; r\ r2, r3, first,
second, and third radials ; a, anal, now carried upwards by the projection of
the vent v ; o, o, orals ; cir, dorsal cirrhi, developed from the highest joint
of the stem.
PLATE XXII. (p. 618).
structure op laguncula repens (after Van Beneden).
A, Polypide expanded ; b, Polypide retracted ; c, another view of the same,
with the visceral apparatus in outline, that the manner in which it is doubled
on itself, with the tentacular crown and muscular system, may be more
distinctly seen : — a, a, tentacula ; b, pharynx ; c, pharyngeal valve ; d,
oesophagus ; e, stomach ; /, its pyloric orifice ; g, cilia on its inner surface ; k,
biliary -follicles lodged in its wall ; i, intestine ; k, particles of excrementitious
matter ; Z, anal orifice ; m, testis ; n, ovary ; o, ova lying loose in the peri-
visceral cavity; p, outlet for their discharge ; q, spermatozoa in the perivisceral
cavity ; r, s, t, u, v, w, x, muscles.
D, Portion of the Lophophore more enlarged: — a, a, tentacula ; b, b, their
internal canals ; c, their muscles ; d, lophophore ; e, its retractor muscles.
PLATE XXIII. (p. 670).
STRUCTURE AND DEVELOPMENT OF TOMOPTERIS ONISCTFORHIS (Original).
A. Portion of caudal prolongation, containing the spermatic sacs, a, a.
b. Adult Male specimen.
c. Hinder part of adult Female specimen, more enlarged, showing ova lying
freely in the perivisceral cavity and its caudal prolongation.
d. Ciliated canal, commencing externally in the larger and smaller rosette
like disks, a, b.
E. One of the pinnulated segments, showing the position of the ciliated
canal, c, and its rosette-like disks, a, b ; showing also the incipient develop-
ment of the ova, d, at the extremity of ihe segment.
P. Cephalic Ganglion, with its pair of auditory (?) vesicles, a, a, and its two
ocelli, b, b.
G. Very young Tomopteris, showing at a, a the larval antennae; b, b,
the incipient long antennas of the adult ; c, d, e, f, four pairs of succeeding
pinnulated segments, followed by bifid tail.
PLATE XXIV. (p. 778).
circulation in the tadpole (after Whitney).
Fig. 1. Anterior portion of young Tadpole, showing the external gills, with
the incipient tufts of the internal gills, and the pair of minute tubes between
the heart and the spirally-coiled intestine, which are the rudiments of the
future lungs.
EXPLANATIONS OF THE PLATES. xxm
2. More advanced Tadpole, in which the external gills have almost disap-
peared : — a, remnant of external gills on the left side ; b, operculum ; c, rem-
nant of external gill on the right side, turned in.
3. Advanced Tadpole, showing the course of the general Circulation : —
a, heart; b, branchial arteries ; c, pericardium ; d, internal gill ; e, first or
cephalic trunk ; /, branch to lip ; g, branches to head ; k, second or branchial
trunk ; i, third trunk, uniting with its fellow to form the abdominal aorta,
which is continued as the caudal artery Jc, to the extremity of the tail ; I,
caudal vein ; m, kidney ; n, vena cava ; o, liver ; p, vena portse ; q, sinus
venosus, receiving the jugular vein, r, and the abdominal veins, t, u, as also
the branchial vein, v.
4. The branchial Circulation on a larger scale : — A, B, c, three primary
branches of the branchial artery ; a, cartilaginous arches ; b, additional frame-
work ; c, e, twigs of branchial artery ; d, f, rootlets of branchial vein.
5. Origin of the vessels of the internal gills, g, from the roots of those of
the external.
6. The heart, systemic arteries, pulmonary arteries and veins, and lungs,
in the adult Frog : the heart beinw turned up in the right hand figure, to
show the junction of the pulmonary veins and their entrance into the left
auricle.
PLATE XXV. (p. 784).
DISTRIBUTION OP CAPILLARY BLOODVESSELS, AS SHOWN IN TRANSPARENT
injections (Original),
Fig. 1. Transverse section of small intestine of Rat, showing the villi
in situ.
2. Section of the toe of a Mouse : — a, a, a, tarsal bones ; 5, digital
artery ; c, vascular loops in the papillae forming the thick epidermic
cushion on the under surface ; d, distribution of vessels in the matrix of the
claw.
3. Distribution of Bloodvessels in the cortical layer of the brain, showing
the manner in which the arteries, carried-in by the pia mater, dip-djwninto
the furrows of the convolutions.
ERRATUM.
Page 328, line 5, for " Plate XL" read a Plate XIII."
LIST OF WOOD-CUT ILLUSTRATIONS.
1. Diagram illustrating Eefraction .....
2. Refraction of Parallel rays by plano-convex lens .
3. Ditto by double convex lens .
4. Eefraction of rays diverging from distance of diameter .
5. Eefraction of Diverging rays
6. Eefraction of Converging rays
7. Formation of images by Convex lenses
8. Spherical Aberration
9. Chromatic Aberration ......
10. Section of Achromatic Object-glass ....
11. Effect of Covering-glass ......
12. Optical action of Simple Microscope ....
13. Optical action of simplest form of Compound Microscope
14. Optical action of complete Compound Microscope
15. Huyghenian Eye-piece . . . ...
16. Stereoscopic Pyramids ......
17. Arrangement of Prisms in Nachet's Stereoscopic Binocular Micro
scope ........
18. Nachet's Stereoscopic Binocular ....
19. Wenham's Prism for Stereoscopic Binocular
20. Sectional view of Wenham's Stereoscopic Binocular
21. Exterior view of Wenham's Stereoscopic Binocular
22. Arrangement of Prisms in Stephenson's Binocular
23. Erecting Prism for Stephenson's Erecting Binocular
24. Exterior view of Stephenson's Erecting Binocular
25. Condenser for Stephenson's Binocular
26. Diaphragm with double aperture for ditto .
27. Arrangement of Prisms in Nachet's Stereo-Pseudoscopic Binocular
28. Exterior of Nachet's Stereo-Pseudoscopic Binocular
29. Diagram illustrating Angle of Aperture suitable for Binocular
Objectives
30. Ditto Ditto ....
31. Eoss's Simple Microscope .......
32. Quekett's Dissecting Microscope .....
33. Field's Dissecting and Mounting Microscope
34. Beck's Dissecting Microscope, with Nachet's Binocular Magnifier
35. Crouch's Educational Microscope^
36. Pillischer's Student's Microscope
37. Messrs. Beck's Student's Microscope .
38. Ladd's Student's Microscope
39. Nachet's Student's Microscope .
40. Browning's Eota ting Microscope
LIST OF WOOD-CUT ILLUSTRATIONS.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
61.
62.
63.
64.
65.
66.
67.
68.
69.
70.
71.
72.
73.
74.
75.
76.
77.
78.
79.
80.
81.
82.
83.
84.
85.
86.
87.
90,
92-
95.
Lealand's B
of Prism
Lealand
Collins's Harley Binocular
Ross's First Class Microscope
Powell and Lealand's Smaller Microscope
Beale's Demonstrating Microscope
Baker's Travelling Microscope .
Dr. Lawrence Smith's Inverted Microscope
Diagram of Reversing Prism of ditto .
Nachet's Double bodied Microscope
Arrangement of Prism, &c, in Powell and
high powers .....
Draw-tube with Erector .
Nachet's Erecting Eye-piece, with Diagram
Sorby-Browning Micro-Spectroscope .
Arrangement of Prisms in ditto
Bright line Spectro-Micrometer .
Solar Spectrum and Absorption- spectrum
Spectroscopic appearances of Blood, &c, after Sorby
Jackson's Eye-piece Micrometer .
Hartnack's Eye-piece Micrometer
Microscope arranged for Drawing
Diagram of Chevalier's Camera Lucida
Diagram of Nachet's Camera Lucida .
Brooke's Nose-piece, modified by Powell and
Collins's Graduating Diaphragm
Messrs. Beck's Achromatic Condenser
Ross's ditto .....
Webster Condenser, fitted with Collins's Graduating
Amici's Prism
Parabolic Illuminator
Diagram of action of ditto .
Wenham's Reflex Illuminator
White-cloud Illuminator .
Fitting of Polarizing Prism
Fitting of Analyzing Prism
Selenite Object- Carrier
Condensing Lens
Bull's-eye Condenser
Beck's Parabolic Speculum
Crouch's Adapter for ditto
Diagram of Lieberkiihn
Beck's Vertical Illuminator
Stephenson's Safety-stage .
Stage-forceps .
Beck's Disk-holder .
Morris's Object-holder
Maddox's Growing- Slide .
Aquatic Box .
Zoophyte-Trough
Compressorium
Ross's Compressorium
91. Messrs. Beck's Parallel-plate Compressor
-94. Messrs. Beck's Reversible Cell Compressor
Dipping Tubes
in ocular for
Diaph
LIST OF WOOD-CUT ILLUSTRATIONS.
96. Glass Syringe .......
97. Forceps ........
98. Bockett-Lamp
99. Section of Adjusting Objective ....
100. Arrangement of Microscope for Transparent Objects
101. Effect of different modes of Illumination on Pleurosigma formosum.
after Beck . . . . .
102. Arrangement of Microscope for Opaque Objects .
103. False hexagonal Areolation of Pleurosigma angulatum
104. Valve of Surirella gemma, after Hartnack and Woodward
105. Spring-Scissors
106. Curved Scissors
107. Valentin's Knife
108. Section-Instrument .
109. Lever of Contact
110. Spring-Clip
111. Wooden Slide for Opaque Objects
112. Smith's Mounting Instrument
113. Slider-Forceps
114. Spring-Press .
115. Dropping-Bottle
116. Shadbolt's Turn-Table .
117. Sunk Cells
118. Plate-Glass Cells
119. Tube-Cells
120. Built-up Cells .
121. Volvox globator, after Ehrenberg
122. Formation of Amoeboid bodies in Volvox, after Hicks
123. Various species of Staurastrum, after Ralfs
124. Circulation in Closterium, after S. G. Osborne
125. Binary Subdivision of Micrasterias , after Lobb
126. Conjugation of Cosmarium, after Ralfs
127. Ditto of Closterium, after Ralfs
128. Binary Subdivision and Conjugation of Didymoprium, after Ralfs
129. Development of Pcdiastrum granulatum, after Bi
130. Various forms of Pediastrum, after Ralfs
131. Portion of Isthmia nervosa, after Smith
132. Triceratium favus, after Smith .
133. Pleurosigma quadratum, after R. Beck
134. Bididphia pulchella, after Smith
135. Conjugation of Epithemia,. after Thwaites
136. Conjugation of Melosira, after Thwaites
137. Meridian circulare, after Smith
138. Bacillaria paradoxa, after Smith
139. Licmophora Jlabellata, after Smith
140. Diatoma vulgare, after Smith .
141. Grammatophora serpentina, after Smith
142. Surirella constricta, after Smith , .
143. Campylodiscus costatus, after Smith .
144. Melosira subflexilis, after Smith
145. Melosira varians, after Smith .
146. Actinoptychm undulatus, after Smith
147. Isthmia nervosa, after Smith .
PAGE
166
166
170
179
184
LIST OF WOOD-CUT ILLUSTRATIONS.
XXVll
148. Chcetoceros Wighamii, after T. West
149. Bacteriastrum furcatum, after T. West
150. Rhizosolenia imbricata, after Brightwell
151. Achnanthes longipes, after Smith
152. Gomphonema geminatum, after Smith
153. Separate frustules of ditto, after Smith
154. Schizoaema Grevillii, after Smith
155. Mastogloia Smithii, after Smith
156. Mastogloia lanceolata, after Smith
157. Fossil Diatomaceai, from Oran, after Ehrenberg
158. Fossil Diatomacece, from Mourne mountains, after Ehrenberg
1 59. Htematococcus sanguineus, after Hassall .
160. Successive stages of development of Ulva, after Kutzing
161. Zoospores of Ulva, after Thuret .....
162. Oscellatoria contexta, after D'Alquen .
163. Nostoc, after Hassall
164. Generation of Vauckeria, after Pringsheim
165. Zoospores of Achlya, after Unger ....
166. Cell-multiplication of Conferva, after Mohl
167. Sexual Reproduction of (Edogonium ciliatum, after Pringsh
168. Zygnema quininum, after Kutzing ....
169. Ch&tophora elegans, after Thuret ....
170. Batrachospermum moniliforme .....
171. Nitella flexilis, after Slack
172. Antheridia of Chara, after Thuret ....
173. Mesogloia vermicidaris. after Payer ....
174. Sphacelaria cirrhosa (original), with antheridium of S. tribuloides,
after Pringsheim .......
175. Receptacle of Fucus, after Thuret ....
176. Antheridia and Antherozoids of Fucus, after Thuret
177. Tetraspores of Carpocaulon, after Kutzing .
178. Torula cerevisio?, after Mandl .....
179. Sarcina ventricidi, after Robin .....
180. Botrytis bassiana, after Robin .....
181. Enterobryus spiralis, after Leidy ....
182. Structure of Enterobryus, after Leidy ....
183. Fungoid Vegetation from Passulus, after Leidy .
184. Shell of A nomia penetrated by parasitic Fungus .
185. Stysanus caput -medusce, after Payer . . . .
186. Puccinia graminis .......
187. JEcidium tussilaginis, after Payer ....
188. Clavaria crispula, after Payer .....
189. Fructification of Marchantia, after Payer
190. Stomata of Marchantia, after Mirbel ....
191. Conceptacles of Marcha.ntia, after Mirbel
192. Arch egonia of Marchantia, after Payer
193. Elater and Spores of Marchantia, after Payer
194. Structure of M osses, after Jussieu ....
195. Antheridia and Antherozoids of Folytrichum, after Thuret
196. Mouth of Capsule of Funariei .....
197. Peristome of Fontinalis, after Payer ....
198. Ditto of Bryum, ditto ....
199. Ditto of Cinclidium, ditto ....
LIST OF WOOD-CUT ILLUSTRATIONS.
200.
201.
202.
203.
204.
205.
206.
207.
208.
209.
210.
211.
212.
213]
214.
215.
216.
217.
218.
219.
220.
221.
222.
223.
224.
225.
226.
227.
228.
229.
230.
231.
232.
233.
234.
235.
236.
237.
238.
239.
240.
241.
242.
243.
244.
245.
246.
247.
248.
249.
250.
251.
252.
Portion of Leaf of Sphagnum
Section of Petiole of Fern .
Sori of Polypodium, after Payer
Ditto of Hcemionitis, ditto
Sorus and Indusium of Aspidium
Ditto of Deparia, after Payer
Development of Prothallium of Pteris, after Suminski
Antheridia and antherozoids of Pteris, after Suminski
Archegoniuni of Pteris, after Suminski
Spores of Equisetum, after Payer
Section of leaf of Agave, after Hartig .
Section of Aralia (rice-paper) . . .
Stellate Parenchyma of Rush ....
Cubical Parenchyma of Nuphar ....
Development of leaf-cells of Anacharis, after Wenham
Circulation in hairs of Tradescantia, after Slack
Testa of Star- Anise .....
Section of Cherry-stone ....
Section of Coquilla-nut ....
Spiral cells of Oncidium ....
Spiral fibres of Collomia ....
Cells of Paiony filled with Starch
Starch-grains under polarized light .
Glandular fibres of Coniferous Wood .
Vascular tissue of Italian Reed, after Schleiden
Transverse section of Stem of. Palm . . ,
Ditto ditto Wanghie Cane
Diagram of formation of Exogenous Stem .
Transverse section of Stem of Clematis
Ditto ditto Rhamnus
Portion of the same, more highly magnified
Transverse section of Hazel
Portion of Transverse section of Stem of Cedar
Transverse section of Fossil Conifer .
Vertical section of Fossil Conifer, radial
Ditto . Ditto tangential
Ditto of Mahogany .
Transverse section of Aristolochia (?) .
Ditto of Burdock
Cuticle of Yucca .....
Ditto of Indian Corn
Ditto of App'e, after Brongniart
Ditto of Rochea Ditto .
Vertical Section of Leaf of Rochea, after Brongniart
Cuticle of Pris,
Vertical Section of Leaf of Iris,
Longitudinal Section of ditto
Cuticle of Petal of Geranium
Pollen-grains of Althaea, &c.
Seeds of Poppy, &c .
Gromia oviformis, after Schulze
Actinophrys sol, after Claparede
Amoeba princeps, after Ehrenberg
Ditto
Ditto
Ditto
LIST OF WOOD-CUT ILLUSTRATIONS.
XXIX
253. Various forms of Amcebina, after Ebrenberg
254. Gregarina from Earthworm, after Lieberkiihn
255. Sphcerozoum ovodimare, after Haeckel
2L6. Kerona silur •us, and Paramecium caudatum, after Milne- Edwards
257. Group of Vorticello?, after Ehrenberg ....
258. Fissiparous Multiplication of Chilodon, after Ehrenberg
259. Encysting process in Vorticella, after Stein .
260. Metamorphosis of Trichoda, after Haime .
261. Brachionus pala, after Milne-Edwards
262. JRo'ifer vulgaris, after Ehrenberg ....
263. Manducatory apparatus of Euchlanis deflexa, after Gosse
264. Stephanoceros Eichornii, after Cubitt
265. Noteus qitadricomis, after Ebrenberg
266. Rotalia ornata, after Schulze .....
367. Alveolina Quoii .......
268. Disk of Simple type of Orbitolites ....
269. Animal of Ditto
270. Portion of animal of Complex type of Orbitolites .
271. Rhabdammina ; Nodosarine and Moniliform Lituolce .
272. Saccamina spherica and Pilulina Jeffreysii
273. Globigerine, Orbuline> and Nodosarine Lituolce ; Proteonina
274. Nautiloid Lituola, with internal structure .
275. General view of Parheria ......
276. Portion of Parlceria, more highly magnified
277. Internal casts of Textularia and Rotalia, after Ehrenberg
278. Tinoporus baculatus .
279. Section of Faujasina, after Williamson
280. Internal cast of Polystomella
281. Vertical Section of Nummulina .
282. Portion of ditto more highly magnified
283. Horizontal Section of Nummulina
284. Internal cast of Nummulina
285. Heterostegina .....
286. Section of Orbitoides Fortisii parallel to its
287. Portions of ditto more highly magnified
288. Vertical Section of Orbitoides Fortisii
289. Internal cast of Orbitoides Fortisii
290. Vertical Section of calcareous Shell of Eozooi
291. Varietal modifications of Astromrna
292. Haliomma Humboldtii, after Ehrenberg
293. Perichlamydium prcetextum, Ditto
294. Stylodyctya gracilis, Ditto
295. Astromrna A ristolelis, Ditto
296. Polycystina, from Barbadoes Ditto
297. Structure of Grantia, after Dobie
298. Portion of Halichondria .
299. Siliceous spicules of Pachymatisma .
300. Hydra fusca, after Milne-Edwards .
301. Ditto, in gemmation, after Trembley
302. Medusa-buds of Syncoryna, after Sars
303. Sertularia cupressina, after Johnston
304. Thaumantias pilosella, after E. Forbes
305. Development of Medusa-buds, after Daly ell
urface
n Canadense
srx
LIST OF Y\rOOD-CUT ILLUSTRATIONS.
306. Development of Medusa, after Dalyell
307. Filiferous capsules of Actinia, &c, after Gosse
308. Spicules of Alcyonium and Gorgonia .
309. Spicules of Gorgonia guttata and Muricea elongata
310. Cydippe and Beroe, after Milne-Edwards .
311. Noctiluca miliaria, after Quatrefages
312. Section of Shell of Ech inus .
313. Calcareous reticulation uf Spine of Echinus.
314. Ambulacral Disk of Echinus
315. Transverse Section of Spine of Acrocladia .
316. Spines of Spatangus .....
317. Structure of Tooth of Echinus, after Salter .
318. Calcareous skeleton of Astrophyton
349. Calcareous skeleton of Holothuria
320. Ditto of Synapta .
321. Ditto of Chirodota .
322. Bipinnarian larva of Star-fish, after Muller
323. PI uteus-larva of Echinus, after Mliller
324. Avdedon rosaceus (Comatula rosacea) .
325. Pentacrinoid larva of Antedon, after Thomson
326. Ceils of Lepralice, after Johnston
327. Bird's-head processes of Cellularia and Bugula, after Johnston
and Busk
328. Amaroucium proliferum, after Milne-Edwards
329. Botryllus violaceus, . Ditto
330. Perophora, after Lister . . ...
331. Transverse Section of Shell of Pinna .
332. Membranous basis of ditto ....
333. Vertical Section of ditto ....
334. Oblique Section of Shell of Pinna
335. Nacre of Avicula
336. Section of hinge-tooth of Mya .
337. Vertical Section of Shell of Unio
338. Internal and external surfaces of Shell of Terebratula
339. Vertical Sections of ditto ditto
340. Horizontal Section of Shell of Terebratula bidlata
341. Ditto ditto of Megerlia lima
342. Ditto ditto of Spiriferina rostrata
343. Palate of Helix hortensis
344. Ditto of Zonites cellarius
345. Ditto of Trochus zizyphinus ....
346. Ditto of Doris tuberculata ....
347. Ditto of Buccinum, under Polarized light
348. Parasitic Larva? (Glochidium) of Anodon, after Hought
349. Embryonic development of Doris, after Eeid
350. Embryonic development of Purpura .
351. Later stages of the same .....
352. Structure of Polycelis, after Quatrefages
353. Circulation of Terebella, after Milne- Edwards
354. Actinotrocha branchiata, after Wagener
355. Development of Nemertes from Pilidium, after Krohn
356. Ammothea pycnogonoides, after Quatrefages
357. Cyclops quadricornis, after Baird
LIST OF WOOD-CUT ILLUSTRATIONS.
XXXI
358.
359.
360.
361.
362.
363.
364.
365.
366.
367.
368.
369.
370.
371.
372.
373.
374.
375-
376.
377.
378.
379.
380.
381.
382.
383.
384.
385.
386.
387.
390.
391.
392.
393.
394.
395.
396.
397.
398.
399.
400.
401.
402.
403.
404.
405.
406.
407.
408.
409.
Development of Balanus, after Bate
Metamorphosis of Carcinus, after Couch
Scale of Morpho Menelaus .....
Scales of Polyommatus argus, after Boyston-Pigott
Battledoor Scale of Polyommatus argus, after Quekett
Scale of Lepisma saccharina, after Beck
Scale of Machilis polypoda, after Beck
Scales of Lepidocyrtus curvicollis (test)
Scale of Lepidocyrtus curvicollis (ordinary), after Beck
Hairs of Myriapod and Dermestes
Head and Eyes of Bee
Section of Eye of Melolontha, after Strauss-Durckheim
Eye of Bee ......
Antenna of Cockchafer ....
Portions of Ditto more highly magnified
Tongue of Fly
Tongue, &c, of Honey Bee
Proboscis of Vanessa ....
Tracheal system of N&pa, after Milne-Edwards
Trachea of Dytiscus ....
Spiracle of Fly .
Spiracle of Larva of Cockchafer
Foot of Fly, after Hepworth
Foot of Dytiscus .....
Eggs of Insects, after Burmeister
Foot, with combs, of Spider
Ordinary and glutinous threads of Spider .
Minute structure of Bone, after "Wilson
Lacuna? of ditto, highly magnified, after Mandl
Section of bony Scale of Lepidosteus .
Vertical section of Tcoth of Lamna, after Owen
Transverse Ditto of Pristis ditto
Ditto Ditto oiMyliobates
Vertical section of Human Tooth, after Mandl
Portion of Skin of Sole
Scale of Sole .....
Hair of Sable .....
Hair of M usk-deer ....
Hair of Squirrel and Indian Bat
Transverse section of Hair of Pecari .
Structure of Human Hair, after Wilson
Transverse section of Horn of Rhinoceros
Blood-corpuscles of Frog, after Donne .
Ditto of Man ditto
Comparative sizes of Bed Blood-corpuscles, after Gulliver
Altered White corpuscle of Human Blood, after Beale
Fibrous Membrane of Egg-shell .
White Fibrous Tissue
Portion of young Tendon, showing Connective-tissue-corpuscles
after Beale
Yellow Fibrous Tissue ....
Vertical Section of Skin of Finger, after Ecker
Pigment-cells of Choroid, after Henle
xxxu LIST OF WOOD-CUT ILLUSTRATIONS.
410. Pigment-cells of Tadpole, after Schwann ....
411. Epithelium-cells, from Mucous Membrane of Mouth, after Lebert
412. Ciliated Epithelium, after Mandl . . '.
413. Areolar and Adipose Tissue, after Mandl
414. Cartilage of Ear of Mouse .
415. Cartilage of Tadpole, after Schwann .....
416. Follicles of Mammary Gland, with Secreting Cells, after Lebert
417. Fasciculus of Striated Muscular Fibre, after Mandl'
418. Fibrilk* of Striated Muscular Fibre of Terebratula
419. Fusiform Cells of Non-striated Muscular Fibre, after Kolliker
420. Nerve-cells and Nerve-fibres, after Ecker ....
421. Gelatinous Nerve- fibres, from Olfactory nerve . . .
422. Distribution of Tactile Nerves in Skin, after Ecker
423. Capillary Circulation in Webb of Frog's foot, after Wagner .
424. Villi of Small Intestine of Monkey
425. Capillary network around Fat-cells '
426. Capillary network of Muscle
427. Distribution of Capillaries in Mucous Membrane .
428. Distribution of Capillaries in Skin of Finger
429. Portion of Gill of Eel
430. Interior of Lung of Frog ... ....
431. Section of Lung of Fowl
432. Section of Human Lung .......
433. Microscopic organisms in Levant Mud, after Williamson
434. Ditto ditto in Chalk, after Ehrenberg .
435. Ditto ditto ditto ditto ....
436. Eye of Trilobite, after Buckland
437. Section of Tooth of Labyrinthodon, after Owen .
438. Crystallized Silver
439. Radiating Crystallization of Santonine, after Davies
440. Eadiating Crystallization of Sulphate of Copper and Magnesia
after Davies .........
441. Spiral Crystallization of Sulphate of Copper, after R. Thomas
442. Artificial Concretions of Carbonate of Lime, after Rainey
443. Swift's Portable Microscope, as set up for use
444. Ditto ditto as folded for packing
445. Blankley's Revolving Mica-Selenite Stage .
446. Swift's New Achromatic Condenser
447. Swift's Portable Microscope-Lamp, as set up for use
448. Ditto ditto, as packed in tube
449. Nachet's Optical Illusion
4*6* MEDTg
^° JUN1 01922
)PE.
INTRODUCTION.
Of all the instruments which have been yet applied to Scientific
research, there is perhaps not more than one (the Spectroscope)
which has undergone such important improvements within so brief
a space of time, as the Microscope has received during the second
third of the present century ; or whose use under its improved
form has been more largely or more rapidly productive of most
valuable results. As an optical instrument, the Microscope is now
at least as perfect as the Telescope ; for the 6-feet parabolic specu-
lum of Lord Rosse's gigantic instrument is not more completely
adapted to the Astronomical survey of the heavenly bodies, than
the achromatic combination of lenses, so minute that they can
scarcely be themselves discerned by the unaided eye, is to the
scrutiny of the Physiologist into the mysteries of life and organi-
zation. Nor are the revelations of the one less surprising to those
who find their greatest charm in novelty, or less interesting to
those who apply themselves to the study of their scientific bearings,
than are those of the other. The universe which the Microscope
brings under our ken, seems as unbounded in its limit as that
whose remotest depths the Telescope still vainly attempts to
fathom. Wonders as great are disclosed in a speck of whose
minuteness the mind can scarcely form any distinct conception, as
in the most mysterious of those vast but remote nebulae, which the
Telescope fails to resolve, and concerning which the information
furnished by the Spectroscope, highly valuable as it is, is still very
imperfect. And the general doctrines to which the labours of
Microscopists are manifestly tending in regard to the laws of
Organization and the nature of Yital Action, seem fully deserving
to take rank in comprehensiveness and importance with the highest
principles yet attained in Physical or Chemical Science.
As the primary object of this treatise is to promote the use of
the Microscope, by explaining its construction, by instructing the
learner in the best methods of employing it, and by pointing-out
the principal directions in which these may be turned to good
account, any detailed review of its history would be misplaced. It
B
2 HISTOEY OF THE MICEOSCOPE.
will suffice to state that, whilst the simple microscope or mag-
nifying-glass was known at a very remote period, the compound
microscope, — the powers of which, like those of the telescope, depend
upon the combination of two or more lenses, — was not invented
until about the end of the sixteenth century ; the earlier micro-
scopes having been little else than modified telescopes, and the
essential distinction between the two not having been at first ap-
preciated. Still, even in the very imperfect form which the instru-
ment originally possessed, the attention of scientific men was early
attracted to the Microscope, for it opened to them a field of research
altogether new, and promised to add largely to their information
concerning the structure of every kind of organized body. The
^Transactions of the Eoyal Society contain the most striking evi-
dence of the interest taken in microscopic investigations two cen-
turies ago. Their early volumes, as Mr. Quekett truly remarked,
' literally teem' with accounts of improvements in the construction
of the Microscope, and of discoveries made by its means. The
Micrognathia of Robert Hooke, published in 16b7, was, for its time,
a most wonderful production ; but this was soon thrown into the
shade by the researches of Leeuwenhoek, whose name first appears
in the Philosophical Transactions in the year lo73. That with
such imperfect instruments at his command, this accurate and
painstaking observer should have seen so much and so well, as to
make it dangerous for any one, even now, to announce a discovery
without having first consulted his works, in order to see whether
some anticipation of it may not be found there, must ever remain a
marvel to the Microscopist. This is partly to be explained by the
fact that he trusted less to the compound microscope, than to single
lenses of high power, the use of which is attended with difficulty,
but which are comparatively free from the errors inseparable from
the first-named instrument in its original form. The names of
Grew and Malpighi also appear as frecpient contributors to the
early volumes of the Philosophical Transactions, the researches of
the former having been chiefly directed to the minute structure of
Plants, and those of the latter to that of Animals. Both were
attended with great success. The former laid the foundation of
our anatomical knowledge of the Vegetable tissues, and described
their disposition in the roots and stems of a great variety of plants
and trees, besides making-out many important facts in regard to
their physiological actions : the latter did the same for the Animal
body, and he seems to have been the first to witness the marvellous
spectacle of the movement of Blood in the capillary vessels of the
Frog's foot, — thus verifying, by ocular demonstration, that doctrine
of the passage of blood from the smallest arteries to the smallest
veins, which had been propounded as a rational probability by the
fugacious Harvey.
Glimpses of the invisible world of Animalcular life were occa-
sionally revealed to the earlier Microscopists, by which their curio-
sity must have been strongly excited ; yet they do not appear to
EAELY DISCOVEEIES ^ITH THE MICEOSCOPE. 3
have entered on this class of investigations with any large portion
of that persevering zeal which they devoted to the analysis of the
higher forms of organic structure. Its wonders, however, were
gradually unfolded ; so that in the various treatises on the Micro-
scope published during the eighteenth century, an account of the
Plants and Animals (but especially of the latter) too minute to be
seen by the unaided eye occupies a conspicuous place. It was
towards the middle of that period, that M. Trembley of Geneva
first gave to the world his researches on the ' fresh-water Polype,'
or Hydra; the publication of which may be considered to have
marked a most important epoch in the history of microscopic
enquiry. For it presented to the Naturalist the first known ex-
ample of a class of animals (of which the more delicate and flexible
Zoophytes of dry collections are the skeletons) whose claim to that
designation had been previously doubted or even denied, — the term
' sea-mosses,' ' sea-ferns,' &c, having been applied to them, not
merely as appropriately indicating their form and aspect, but as
expressive of what even the most eminent Zoologists, as well as
Botanists, considered to be their vegetable nature. And it pre-
sented to the Physiologist an entirely new type of animal life;
the wonderful nature of which was fitted not only to excite the
•liveliest interest, but also to effect a vast extension in the range
of the ideas entertained up to that time regarding its nature and
capacities. For what Animal previously known could propagate
itself by buds like a plant, — could produce afresh any part that
might be cut away, — could form any number of new heads by the
completion of the halves into which the previous heads had been
slit (thus realizing the ancient fable of the Hydra), — could even
regenerate the whole from a minute portion, so that when the body
of one individual was positively minced into fragments, each of
these should grow into a new and complete polype, — could endure
being turned inside-out, so that what was previously the external
surface should become the lining of the stomach and vice versa,
— and could sustain various other kinds of treatment not less
strange (such as the grafting of two individuals together, head to
head, or tail to tail, or the head of one to the tail of another), net
only without any apparent injury, but with every indication, in the
vigour of its life, of being entirely free from suffering or damage ?
(See §§ 471, 472.) It was by our own countryman, Ellis, that the
discoveries of Trembley were first applied to the elucidation of the
really animal nature of the so-called Corallines ;* the structure of
which was so carefully investigated by him, that subsequent ob-
servers added little to our knowledge of it until a comparatively
recent period.
The true Animalcules were first systematically studied, in the
latter part of the last century, by Gleichen, a German microscopki,
* The structures to -which this term is now scientificaVy restricted, are really
Vegetable (§ 285.)
b2
i HISTOEY OF MICKOSCOPIC DISCOVERY.
who devised the ingenious plan of feeding them with particles of
colouring matter, so as to make apparent the form and position of
their digestive cavities ; and this study was afterwards zealously
pursued by the eminent Danish naturalist, Otho Fred Miiller, to
the results of whose labours in this field but little was added by
others, until Professor Ehrenberg entered upon the investigation
with the advantage of greatly improved instruments. It was at
about the same period with Miiller, that Yaucher, a Geneyese
botanist, systematically applied the Microscope to the investiga-
tion of the lower forms of Vegetable life ; and made many curious
discoveries in regard both to their structure and to the history of
their lives. He was the first to notice the extraordinary pheno-
menon of the spontaneous movement of the zoospores of the
humbler Aquatic Plants, which is now known to be the means pro-
vided by Nature for the dispersion of the race (see §§ 265, 269) ;
but being possessed with the idea (common to all Naturalists of
that period, and still very generally prevalent) that spontaneous
motion evinces Animal life, he interpreted the facts which he ob-
served, as indicating the existence of a class of beings which are
Plants at one phase of their lives and Animals at another, — a
doctrine which, if true in any case (§§ 364, 365), is certainly not ap-
plicable to the forms he studied. Notwithstanding this and other
errors of interpretation, however, the'work of Yaucher on the ' Fresh-
water Confervse' contains such a vast bo^y of accurate observation
on the growth and reproduction of the Microscopic Plants to the
study of which he devoted himself, that it is quite worthy to take
rank with that of Trembley, as having laid the foundation for all
our scientific knowledge of these very interesting forms. Although
the curious phenomenon of ' conjugation' (§ 276) had been previ-
ously observed by Miiller, yet its connection with the function of
Reproduction had not been even suspected by him ; and it was by
Yaucher that its real import was first discerned, and that its
occurrence (which had been regarded by Miiller as an isolated
phenomenon, peculiar to a single species) was found to be common
to a large number of humble aquatic forms of vegetation. But
little advance was made upon the discoveries of Yaucher in regard
to these, save by addition to the number of their specific forms,
until a fresh stimulus had been given to such investigations by
the improvement of the instrument itself. At present, they are
among the most favourite objects of study among a large number
of observers, both in this country and on the Continent ; and are
well deserving of the attention they receive.
Less real progress seems to have been made in Microscopic
enquiry during the first quarter of the present century, than during
any similar period since the invention of the instrument. The
defects inseparable from its original construction formed a bar to
all discovery beyond certain limits ; and although we are now con-
tinually meeting with new wonders, which patient and sagacious
FALLACIES OF OBSERVATION. 5
observation would have detected at any time and with any of the
instruments then in use, yet it is not surprising that the impres-
sion should have become general, that almost everything which it
could accomplish had already been done. The instrument fell
under a temporary cloud from another cause ; for having been ap-
plied by Anatomists and Physiologists to the determination of the
elementary structure of the animal body, their results were found
to be so discordant, as to give rise to a general suspicion of a want
of trustworthiness in the Microscope and in everything announced
upon its authority. Thus both the instrument and its advocates
were brought into more or less discredit ; and as they continue to
lie under this, in the estimation of many, to the present day, it
will be desirable to pause here for a while, to enquire into the
sources of that discrepancy, to consider whether it is avoidable, and
to enquire how far it should lead to a distrust of Microscopic obser-
vations, carefully and sagaciously made, and accurately recorded.
It is a tendency common to all observers, and not by any means
peculiar to Microscopists, to describe what they believe and infer,
rather than what they actually witness. The older Microscopic
observers were especially liable to fall into this error ; since the
want of definiteness in the images presented to their eyes, left a
great deal to be completed by the imagination. And when, as
frequently happened, Physiologists began with theorizing on the
elementary structure of the body, and allowed themselves to twist
their imperfect observations into accordance with their theories, it
was not surprising that their accounts of what they professed to
have seen should be extremely discordant. But from the moment
that the visual image presented by a well-constructed Microscope,
gave almost as perfect an idea of the object as we could have
obtained from the sight of the object itself if enlarged to the same
size and viewed with the unassisted eye, Microscopic observations
admitted of nearly the same certainty as observations of any other
class ; it being only in certain cases, when high powers are used,
that a doubt can fairly remain about any question of fact as to
which the Microscope can be expected to inform us.
Another fallacy, common like the last to all observations, but
with which the Microscopic observations of former times were per-
haps especially chargeable, arises from a want of due attention to
the conditions under which the observations are made. Thus one
observer described the human Blood-corpuscles as flattened disks
resembling pieces of money, another as slightly concave on each
surface, a third as slightly convex, a fourth as highly convex, and
a fifth as globular ; and the former prevalence of the last opinion is
marked -by the habit which still lingers in popular phraseology, of
designating these bodies as ' blood-globules.' Yet all microscopists
are now agreed, that their real form, when examined in freshly-
drawn blood, is that of circular disks with slightly concave surfaces ;
and the diversity in previous statements was simply due to the
alteration effected in the shape of these disks, by the action of water
6 HISTORY OF MICROSCOPIC DISCOVERY.
or other liquids added for the sake of dilution ; the effect of this
being to render their surfaces first fiat, then slightly convex, then
more highly convex, at last changing their form to that of perfect
spheres. But Microscopical enquiries are not in themselves more
liable to fallacies of this description, than are any other kinds of
scientific investigation ; and it will always be found here, as well
as elsewhere, that — good instruments and competent observers
being pre- supposed — the accordance in results will be precisely pro-
portional to the accordance of conditions, that is, to the similarity
of the objects, the similarity of the treatment to which they may
be subjected, and the similarity of the mode in which they may be
viewed. Objects of difficulty should be viewed under various modes
of illumination, and sometimes in fluids of different refractive
powers : and errors may often be eliminated by carefully com-
paring the various appearances that are thus obtained.
The more completely, therefore, the statements of Microscopic
observers are kept free from those fallacies to which observations
of any kind are liable, when due care has not been taken to guard
against them, the more completely will it be found that an essential
agreement exists among them all, in regard to the facts which they
record. And although the influence of preconceived theories still
too greatly modifies, in the minds of some, the descriptions they
profess to give of the facts actually presented to their visual sense,
yet on the whole it is remarkable to what a unity of doctrine the
best Microscopists of all countries are converging, in regard to all
such subjects of this kind of enquiry as have been studied by
them with adequate care and under similar conditions. Hence it is
neither fair to charge upon the Microscopists of the present day the
errors of their predecessors ; nor is it just to lay to the account of
the instrument, what entirely proceeds from the fault of the
observer, in recording, not what he sees in it, but what he supposes
himself to see.
It was at the commencement of the second quarter of the present
century, that the principle of Achromatic correction, which had long
before been applied to the Telescope, was first brought into efficient
operation in the construction of the Microscope ; for although its
theoretical possibility was well known, insuperable difficulties were
believed to exist in its practical application. The nature of this most
important improvement will be explained in its proper place (§ 13) ;
and at present it will be sufficient to say that, within eight or ten
years from the date of its first introduction, the character of the
Microscope was in effect so completely transformed, that it soon
acquired the deserved reputation of being one of the most perfect
instruments ever devised by Art for the investigation of Nature.
To this reputation it has a still higher claim at the present time ;
and though it would be hazardous to deny the possibility of any
further improvement, yet the statements of theorists as to what
may be accomplished, are so nearly equalled by what has been
MICROSCOPIC STUDY OF PLANTS. 7
effected, that little room for improvement can "be considered to
remain, until chemists furnish opticians with new varieties of glass
whose refractive and dispersive powers shall be better suited to
their requirements.
Neither Botanists nor Zoologists, Anatomists nor Physiologists,
were slow to avail themselves of the means of perfecting and
extending their knowledge, thus unexpectedly put into their hands ;
and the records of Scientific Societies, and the pages of Scientific
Journals, have ever since teemed, like the early Transactions of
the E/oyal Society, with accounts of discoveries made by its
instrumentality. — A slight sketch of what has thus been accom-
plished by the assistance of the Microscope in the investigation of
the phenomena of Life, seems an appropriate Introduction to the
more detailed account of the instrument and its uses, which the
present Treatise is designed to embrace.
The comparative simplicity of the structure of Plants, and the
relatively large scale of their elementary parts, had allowed the
Vegetable Anatomist, as we have seen, to elucidate some of its
most important features, without any better assistance than the
earlier Microscopes were capable of supplying. And many of those
humbler forms of Oryptogamic vegetation, which only manifest
themselves to the unaided eye when by their multiplication they
aggregate into large masses, had been made the objects of careful
study, which had yielded some most important results. Hence
there seemed comparatively little to be done by the Microscopist
in Botanical research ; and it was not immediately perceived what
was the direction in which his labours were likely to be most pro-
ductive. Many valuable memoirs had been published, from time
to time, on various points of vegetable structure ; the increased
precision and greater completeness of which bore testimony to the
importance of the aid which had been afforded by the greater
efficiency of the instruments employed in such researches. But it
was when the attention of Vegetable Physiologists first began to
be prominently directed to the history of development, as the most
important of all the subjects which presented themselves for inves-
tigation, that the greatest impulse was given to Scientific Botany ;
and its subsequent progress has been largely influenced by that
impulse, both in the accelerated rate at which it has advanced,
and in the direction which it has taken. Although Robert Brown
had previously observed and recorded certain phenomena of great
importance, yet it is in the Memoir of Prof. Schleiden, first pub-
lished in 1837, that this new movement may be considered to have
had its real origin ; so that, whatever may be the errors with
which his statements (whether on that occasion or subsequently)
are chargeable, there cannot be any reasonable question as to the
essential service he has rendered to science, in pointing out the
way to others on whose results greater reliance may be placed. It
was by Schleiden that the fundamental truth was first broadly
8 LATER RESULTS OF MICROSCOPIC RESEARCH.
enunciated, that as there are many among the lowest orders of
Plants in which a single cell constitutes the entire individual, every
one living for and by itself alone, so each of the cells by the aggre-
gation of which any individual among the higher Plants is built
up, has an independent life of its own, besides the ' incidental ' life
which it possesses as a part of the organism at large ; and it was
by him that the doctrine was first proclaimed, that the life-history
of the individual cell is therefore the very first and absolutely in-
dispensable basis, not only for Yegetable Physiology, but (as was
even then foreseen by his far-reaching mental vision) for the
Science of Life in general. The first problem, therefore, which he
set himself to investigate, was — how does the cell itself originate ?
It is unfortunate that he should have had recourse, for its solution,
to some of those cases in which the investigation is attended with
peculiar difficulty, instead of making more use of the means and
opportunities which the ' single-celled ' plants afford ; and it is
doubtless in great part to this cause, that we are to attribute certain
fallacies in his statements, of which subsequent researches have
furnished the correction.
In no department of Botany has recent Microscopy been more
fertile in curious and important results, than in that which relates
to the humblest forms of Cryptogamia that abound not only in
our seas, rivers, and lakes, but even .more in our marshes, pools,
and ditches. For, in the first place, those present us with a num-
ber of most beautiful and most varied forms, such as on that
account alone are objects of great interest to the Microscopist ;
as is especially the case with the curious group (ranked among
Animalcules by Prof. Ehrenberg,) which, from the bipartite form
of their cells, has received the designation of Besmidiacece (§ 219).
In another group, that of Diatomacece (regarded as Animalcules,
by Ehrenberg, and by many other Naturalists), not only are the
forms of the plants often very remarkable (§ 232), but their sur-
faces exhibit markings of extraordinary beauty and symmetry,
which are among the best ' test-objects ' that can be employed for
the higher powers of the instrument (§ 146) : moreover, the mem-
brane of each cell being infiltrated with silica, which not only
takes its form, but receives the impress of its minutest markings,
the siliceous skeletons remain unchanged after the death of the
plants which formed them, sometimes accumulating to such an
amount as to give rise to deposits of considerable thickness at the
bottoms of the lakes or pools which they inhabit ; and similar
deposits, commonly designated as beds of ' fossil animalcules,' are
not unfrequently found at a considerable distance from the surface
of the ground, on the site of what must have probably once been a
lake or estuary, occasionally extending over such an area, and
reaching to such a depth, as to constitute no insignificant part of
the crust of the globe.
It is not only in the foregoing particulars, however, that these
and other humble tribes of Plants have special attractions for the
LOWER FORMS OF VEGETABLE LIFE. 9
Microscopist ; since the study of their living actions "brings to view
many phenomena, which are not only well calculated to excite the
interest of those who find their chief pleasure in the act of observ-
ing, but are also of the highest value to the Physiologist, who
seeks to determine from the study of them what are the acts
wherein Vitality may be said essentially to consist, and what are
the fundamental distinctions between Animal and Yegetable life.
Thus it is among these plants, that we can best study the history
of the multiplication of cells by ' binary subdivision,' which seems
to be the most general mode of growth and increase throughout
the Vegetable kingdom (§ 204) ; and it is in these, again, that
the process of sexual generation is presented to us under its
simplest aspect, in that curious act of ' conjugation' to which
reference has already been made (p. 4). But further, nearly all
these Plants have at some period or other of their lives a power
of spontaneous movement, which in many instances so much
resembles that of Animalcules, as to seem unmistakeably to indi-
cate their animal nature, more especially as this movement is
usually accomplished by the agency of visible cilia (§§ 208, 265) :
and the determination of the conditions under which it occurs, and
of the purpose it is intended to fulfil, is only likely to be accom-
plished after a far more extensive as well as more minute study of
their entire history, than has yet been prosecuted, save in a small
number of instances. It is not a little remarkable, moreover, that
in several of thi cases -in which the life-history of these plants has
been most completely elucidated, they have been found to present
a great variety of forms and aspects at different periods of their
existence, and also to possess several different methods of repro-
duction ; and hence it can be very little doubted, that numerous
forms which are commonly reputed to be distinct and unrelated
species, will prove in the end to be nothing else than successive stages
of one and the same type (§ 210). One of the most curious results
attained by Microscopic enquiry of late years, has been the succes-
sive transfer of one group of reputed Animalcules after another,
from the Animal to the Vegetable side of the line of demarcation
between the two kingdoms ; and although, as to the precise points
across which this line should be drawn, there is not yet an unani-
mous agreement, yet there is now an increasing accordance as to
its general situation, which, even a few years since, was energeti-
cally canvassed. Those who see for the first time the well-known
Volvox (commonly termed the ' globe-animalcule') will be surprised
to learn that this, with its allies, constituting the family Volvocinece,
is now to be considered as on the Vegetable side of the boundary
(§§212-218).
'Not only this lowest type of Vegetable existence, but the
Cryptogamic series as a whole, has undergone of late years a very
close scrutiny, which has yielded results of the highest importance ;
many new and curious forms having been brought to light (some
of them in situations in which their existence might have been
10 LATER RESULTS OF MICROSCOPIC RESEARCH.
least anticipated), and some of the most obscure portions of their
history having received an unexpectedly clear elucidation. Thus
the discovery was announced by M. Audouin in 1837, that the
disease termed muscardine, which annually carried off large num-
bers of the silkworms bred in the south of France, really consists
in the growth of a fungous vegetation in the interior of their bodies,
the further propagation of which may be almost entirely prevented
by appropriate means (§ 294) ; in the succeeding year, the fact
was brought forward by several Microscopists, that yeast also is
composed of vegetable cells, which grow and multiply during the
process of fermentation (§ 288) ; and subsequent researches have
shown that the bodies of almost all animals, not even excepting
Man himself, are occasionally infested by Vegetable as well as by
Animal Parasites, many of them remarkable for their beauty of
configuration, and others for the variety of the forms they assume
(§ 296). The various parasites which attack our cultivated plants,
again, — such as the ' blights ' of corn, the potato-fungus, and the
vine-fungus (§§ 301,302), — have received a large measure of attention
from Microscopists, and much valuable information has been col-
lected in regard to them. It is still a question, however, which
has to be decided upon other than microscopic evidence, how far
the attacks of these Fungi are to be considered as the causes of the
diseases to which they stand related, or whether their development
(as is undoubtedly the case in many parallel, instances) is the conse-
quence of the previously-unhealthy condition of the plants which
they infest : the general evidence appears to the Author to incline
to the latter view, which does not exclude their injurious action.
Of all the additions which our knowledge of the structure and
life-history of the higher types of Cryptogamic vegetation has re-
ceived, since the achromatic microscope has been brought to bear
upon them, there is none so remarkable as that which relates to
their Reproductive function. For the existence in that group, of
anything at all corresponding to the sexual generation of Flowering-
Plants, was scarcely admitted by any Botanists ; and those few
who did affirm it were unable to substantiate their views by any
satisfactory proof, and were (as the event has shown) quite wrong
as to the grounds on which they based them. Various isolated
facts, the true meaning of which was quite unrecognized, had been
discovered from time to time, — such as the existence of the moving
filaments now termed ' antherozoids,' in the ' globules' of the Chara
(first demonstrated by Mr. Varley in 1834), and in the ' antheridia'
of Mosses and Liverworts (as shown by Unger and Meyen in 1837),
and the presence of ' antheridia' upon what had been always pre-
viously considered the embryo-frond of the Ferns (first detected by
ISTageli in 1844) : but of the connection of these with the generative
function, no valid evidence could be produced ; and the sexual re-
production of the Cryptogamia was treated by many Botanists of
the greatest eminence, as a doctrine not less chimerical, than the
doctrine of the sexuality of Flowering-Plants had been regarded by
HIGHEE FOEMS OF VEGETABLE LIFE. 11
tlie opponents of Linnaeus. It was by the admirable researches of
Count Suminski upon the development of the Ferns (1848), that
the way was first opened to the right comprehension of the repro-
ductive process in that group (§ 316) ; and the doctrine of the
fertilizing powers of the ' antherozoids,' once established in a single
case, was soon proved to apply equally well to many others. Com-
plete evidence of the like sexuality in the several groups of the
Cryptogamic series has since been obtained by Microscopic research ;
this having been especially furnished by Hofmeister in regard to
the higher types, by Thuret and Decaisne as to the marine Algae,
and by Tulasne with respect to Lichens and Fungi ; and the doc-
trine may now be considered as established beyond the reach of
cavil. — With the study of the Eeproduction of these plants, that of
the history of their development has naturally been connected ; and
some of the facts already brought to light, especially by the study
of certain forms of Fungous vegetation, demonstrate the extreme
importance of this enquiry in settling the foundations of Classifi-
cation. For whereas the arrangement of Fungi, as of other Plants,
has been based upon the characters furnished by their fructifica-
tion, these characters have been found by Tulasne to be frequently
subject to variations so wide, that one and the same individual
shall present two or more kinds of fructification, such as had been
previously considered to be peculiar to distinct orders (§ 299). In
this department of study, which has been comparatively little culti-
vated by Microscopists of our own country, there is a peculiarly
wide field for careful and painstaking research, and a sure prospect
of an ample harvest of discovery. (See Chap. VII.)
Although it has been in Cryptogamic Botany that the zealous
pursuit of Microscopic enquiry has been most conducive to scien-
tific progress, yet the attention of Vegetable Anatomists and Phy-
siologists has been also largely and productively directed to the
minute structure and life-history of Flowering-Plants. For although
some of the general features of that structure had been discovered
by the earlier observers, and successive additions had been made
to the knowledge of them, previously to the new era to which refe-
rence has so often been made, yet all this knowledge required to be
completed and made exact by a more refined examination of the
Elementary Tissues than was before possible ; and little was cer-
tainly known in regard to those processes of growth, development,
and reproduction, in which their activity as living organisms con-
sists. All the researches which have been made upon this point
tend most completely to bear-out the general doctrine so clearly set
forth by Schleiden, as to the independent vitality of each integral
part of the fabric ; and among the most curious results of the
enquiries which have been prosecuted in this direction, may be
mentioned the discovery, that the movement of ' rotation' of the
protoplasm (or viscid granular fluid at the expense of which the
nutritive act seems to take place) within the cells, which was
first observed by the Abbe Corti in the Chara in 1776 (§ 279), is by
12 LATER RESULTS OF MICROSCOPIC RESEARCH.
no means an unique or exceptional case ; for that it may be detected
in so large a number of instances, among Phanerogamia no less
than among Cryptogamia (§§ 322-324) as apparently to justify
the conclusion that it takes place in Vegetable cells generally, at
some period or other of their evolution. In studying the phenomena
of Vegetable Nutrition, the Microscope has been most effectually
applied, not merely to the determination of changes in the form and
arrangement of the elementary parts, but also to the detection of
such changes in their composition as ordinary Chemistry would be
quite at fault to discover : each individual cell being (so to speak)
a laboratory in itself, within which a transformation of organic
compounds is continually taking-place, not only for its own re-
quirements, but for those of the economy at large ; and these
changes being at once made apparent by the application of che-
mical reagents to microscopic specimens whilst actually under
observation. Hence the Vegetable Physiologist finds, in this
Microscopic Chemistry, one of his most valuable means of tracing
the succession of the changes in which Nutrition consists, as well
as of establishing the chemical nature of particles far too minute to
be analyzed in the ordinary way : and he derives further assistance
in the same kind of investigation, from the application of Polarized
Light (§ 98), which immediately enables him to detect the presence
of mineral deposits, of starch-granules, and of certain other sub-
stances peculiarly affected by it ; as also, from Spectroscopic
examination of the colour-properties of the fluid contents of the cells
(§§ 71-75), which throws great light upon their chemical relations.
One of the most interesting among the general results of such re-
searches, has been the discovery that the true cell-wall of the Plant
(the ' primordial utricle' of Mohl) has the same albuminous compo-
sition as that of the Animal ; the external cellulose envelope, which
had been previously considered as the distinctive attribute of the
Vegetable cell, being in reality but a secretion from its surface
(§ 201). Of all the applications of the Microscope, however, to the
study of the life-history of the Flowering-Plant, there is none which
has excited so much interest, or given-rise to so much discussion, as
the nature of the process by which the Ovule is fecundated by the
penetration of the pollen- tube (§ 359). This question, however, may
be considered as now determined ; and the conclusion arrived-at is
one so strictly in harmony with the general results obtained by the
study of the (apparently) very different phenomena presented by
the Generative process of the Cryptogamia, that it justifies the
Physiologist in advancing a general doctrine as to the nature of
the function, which proves to be no less applicable to the Animal
kingdom than it is to the Vegetable.
Among the objects of interest so abundantly offered by the
Animal Kingdom to the observation of Microscopists furnished
with vastly-improved instruments of research, it was natural that
those minuter forms of Animal life which teem in almost every
LOWER FORMS OF ANIMAL LIFE. 13
stationary collection of water, should engage their early attention ;
and among those Naturalists who applied themselves to this study,
the foremost rank must undoubtedly be assigned to the celebrated
German Microscopist, Prof. Ehrenberg. For although it is now
unquestionable that he has committed numerous errors, — many
doctrines which at first gained considerable currency on the
strength of his high reputation, having now been abandoned by
almost every one save their originator, — yet when we look at the
vast advances which he unquestionably made in our knowledge of
Animalcular life, the untiring industry which he has displayed in
the study of it, the impulse which he has given to the investigations
of others, and the broad foundation which he has laid for their
enquiries in the magnificent works in which his own observations
are recorded, we cannot but feel that his services have been almost
invaluable, since, but for him, this department of microscopic
enquiry would certainly have been in a position far behind that to
which it has now advanced. Yet, great as has been the labour
bestowed by him and by his followers in the same line of pursuit,
it has become increasingly evident of late years that our knowledge
of Infusory Animalcules is still in its infancy ; that the great fabric
erected by Prof. Ehrenberg rests upon a most insecure foundation ;
and that the Anatomy, Physiology, and Systematic arrangement
of these beings need to be re-studied completely ah initio. For, in
the first place, there can be no doubt whatever, that a considerable
number of the so-called Animalcules belong to the Vegetable king-
dom ; consisting, as already pointed-out (p. 9), of the motile forms
of the humbler Plants, of which a very large proportion pass, at
some period of their existence, through a stage of activity that
serves for their diffusion. Moreover, in another group whose cha-
racter has been entirely misconceived by the great German
Microscopist, and was first clearly discriminated by M. Dujardin,
there is neither mouth nor stomach of any kind (§§ 369-377) ; the
minute organic particles which serve as the food of these crea-
tures, being incorporated, as it were, with the soft animal jelly
which constitutes their almost homogeneous bodies, and this jelly
further extending itself into ' pseudopodial' prolongations, whereby
these alimentary particles are laid-hold-of and drawn-in. It was
by the same distinguished French Microscopist that the important
fact was first discovered, that animals of this Rhizopod type are
really the fabricators of those minute shells, which, from their
Nautilus-like aspect, had been previously regarded as belonging to
the highest class of the Molluscous Sub-Kingdom ; and the whole
of this most interesting group (Chap. X.), which had received from
M. D'Orbigny (who first perceived the speciality of its nature, and
made a particular study of it) the designation of Foraminifera,
has thus had its place in the Animal scale most strangely reversed ;
being at once degraded from a position but little removed from
Yertebrated animals, to a level in some respects even lower than
that of the ordinary Animalcules.
14 LATER RESULTS OF MICROSCOPIC RESEARCH.
Bnt even when Prof. Ehrenberg's class of Polygastrica has been
thus reduced, by the removal of those forms which are true Plants,
and by the detachment of snch as belong to the Bhizopod group,
we find that our knowledge of its real nature is almost wholly to
be gained ; since little else has yet been accomplished, than a de-
scription of a multitude of forms, of whose history as living beings
scarcely anything else is known, than that they take food into the
interior of their bodies by means of an oral orifice, that they digest
this food and appropriate it to their own growth, and that they
multiply themselves by binary subdivision (§§ 386-392). This
binary subdivision is not to be regarded, however, as the true
generative process, being simply one of multiplication ; and various
notions have been put forth from time to time as to the sexual
organs of Animalcules, and the mode of their operation. The re-
cent observations of Stein, Balbiani, and others, have thrown much
light upon this point ; and under their guidance it is probable that
large additions to our knowledge regarding the Eeproduction of this
group will ere long be made. It is still an open question, however,
how far changes of form and condition may take place during the
development of these organisms ; and this enquiry can only be effi-
ciently prosecuted, by limiting the range of observation for a time
to a small number of forms, and pursuing these through all the
phases of their existence.
Among the most important of Prof. Ehrenberg's unquestioned
discoveries, we are undoubtedly to place that of the comparatively
high organization of the Botifera, or Wheel- Animalcules and their
allies (§§ 404-413) ; for which, though previously confounded with
the simpler Infusoria, he asserted and vindicated a claim to a far
more elevated rank. Eor although in this instance, too, some of
his descriptions have been shown to be incorrect, and many of his
inferences to be erroneous, and although subsequent observers are
not agreed among themselves as to many important particulars,
yet all assent to the general accuracy of Prof. Ehrenberg's state-
ments, and recognize the title of the Eotifera to a place not far
removed from that of the Yermiform tribes.
A parallel discovery was made about the same time by MM.
Audouin and Milne-Edwards, in regard to the Flustrw and their
allies, which had previously ranked among those flexible Zoophytes
popularly known as ' corallines,' and are often scarcely to be dis-
tinguished from them in mode of growth or general aspect ;* but
which were separated as a distinct order by these observers, on
account of their possession of a second orifice to the alimentary
* " You go down," says Mr. Kingsley, " to any shore after a gale of wind,
and pick up a few delicate little sea-ferns.-- You have two in your hand {Sertu-
laria operculata and Gemellaria loricvlatcC), which probably look to yon, even
under a good pocket-magnifier, identical or nearly so. But you are told, to your
surprise, that however alike the dead horny polypidoms which you hold may
be, the two species of animals which have formed them, are at least as far
apart in the scale of creation as a Quadruped is from a Fish."
LOWER FORMS OF ANIMAL LIFE. 15
canal, and the general conformity of their plan of organization to
that which characterizes the inferior Mollusca (§§ 507-513). The
importance of this distinction was at once recognized; and the
group received the designation of Polyzoa from Mr. J. Y. Thomp-
son, and of Bryozoa from Prof. Ehrenberg. The organization of
this very interesting group was further elucidated, some years sub-
sequently, by the admirable observations of Dr. Arthur Farre upon
a newly-discovered form (named by him Bowerbankia) , the trans-
parence of whose envelopes allowed its internal structure to be
distinctly macle-out ; and the additional features which he detected,
were all such as to strengthen the idea already entertained of
its essentially Molluscan character. This idea received its final
and complete confirmation from the admirable researches of
M. Milne-Edwards on the Compound Ascidians, which are the
lowest animals whose Molluscous nature had been previously
acknowledged (§§ 514-518) ; these having been discovered by him
to agree with Zoophytes in their plant-like attribute of extension
by ' gemmation' or budding, and to present, in all the most im-
portant features of their organization, an extremely close approxi-
mation to the Polyzoa. — Thus whilst Microscopic research has
degraded the Foraminif era from their supposed rank with the ISTau-
tilus and Cuttle-fish to the level of the Sponge, it has raised the
"Wheel- Animalcules into proximity with the aquatic Worms, and
the humble ' Sea-mat,' formerly supposed to be a Plant, to a posi-
tion not much below that of the Oyster and Mussel.
Another most curious and most important field of Microscopic
enquiry has been opened-up in the study of the transformations
which a large proportion of the lower animals undergo during the
early stages of their existence ; and notwithstanding that it has
even yet been very imperfectly cultivated, the unexpected result
has been already attained, that the fact of ' metamorphosis,' — pre-
viously known only in the cases of Insects and Tadpoles, and com-
monly considered as an altogether exceptional phenomenon, — is
nearly universal among the inferior tribes ; it being a rare occur-
rence for the offspring to come forth from the egg in a condition
bearing any resemblance to that which characterizes the adult, and
the latter being in general attained only after a long series of
changes, in the course of which many curious phases are presented.
One of the earliest and most remarkable discoveries which was
made in this direction, — that of the metamorphosis of the Cirrhipeds
(Barnacles and their allies) by Mr. J. Y. Thompson, — proved of
most important assistance in the determination of the true place
of that group, which had previously been a matter of controversy ;
for although in their outward characters they bear such a resem-
blance to Mollusks, that the Barnacles which attach themselves to
floating timber, and the Acorn-shells which incrust the surfaces of
rocks, are unhesitatingly ranked by Shell-collectors among their
' multivalves,' yet the close resemblance which exists between their
early forms and the little Water-fleas which swarm in our pools
16 LATER RESULTS OF MICROSCOPIC RESEARCH.
(§ 572), makes it quite certain that the Barnacles not only belong
to the Articulated instead of to the Molluscous series, but that they
must be ranked in close proximity to the Entomostracous division
of the Crustacea, if not actually as members of it. To the same
discoverer, moreover, we owe the knowledge that even the common
Grab undergoes metamorphoses scarcely less strange, its earliest
form being a little creature of most grotesque shape, which had
been previously described as an adult and perfect Entomostracan
(§ 574) ; so that, although scarcely any two creatures can appa-
rently be more unlike than a Barnacle and a Crab, they have (so to
speak) the same starting-point ; the difference in their ultimate
aspect chiefly arising from the difference in the proportionate de-
velopment of parts which are common to both.
A still more remarkable series of metamorphoses was subse-
quently shown by Prof. Miiller to exist among the Echinoderms
(Star-fish, Sea-urchins, &c.) ; whose development he studied with
great perseverance and sagacity. Thus the larva of the Star-fish
is an active free-swimming animal (§ 502), having a long body
with six slender arms on each side, from one end of which the
young star-fish is (so to speak) budded-off ; and when this has
attained a certain stage of development, the long twelve-armed
body separates from it and dies away, its chief function having
apparently been to carry the young Star-fish to a distance from
its fellows, and thus to prevent overcrowding by the accumulation
of individuals in particular spots, which would be liable to occur
if they never had any more active powers of locomotion than they
possess in their adult state. — Scarcely less remarkable are the
changes which are to be witnessed in the greater number of aquatic
Mollusks, almost all of which, however inert in their adult con-
dition, possess active powers of locomotion in their larval state ;
some being propelled by the vibratile movement of cilia disposed
upon the head somewhat after the fashion of those of Wheel-
animalcules (§ 541), and others by the lateral strokes of a sort of
tail which afterwards disappears like that of a Tadpole (§ 518). —
Among the Annelids or marine Worms, again, there is found to be
an extraordinary dissimilarity, though of a somewhat different
nature, between the larval and the adult forms : for they commonly
come-forth from the egg in a condition but little advanced beyond
that of Animalcules ; and, although they do not usually undergo any
metamorphosis comparable to that of Insects, they pass through
a long series of phases of development (chiefly consisting in the
successive production of new joints or segments, and of the organs
appertaining to these) before they acquire their complete type
(§ 554). — In nearly all the foregoing cases it may be remarked that
the larval forms of different species bear to one another a far
stronger resemblance than exists among their adults, the distin-
guishing characters of the latter being only evolved as life ad-
vances ; and every new discovery in this direction only gives fresh
confirmation to the great law of development early detected by the
LOWER FOEMS OF ANIMAL LIFE. 17
sagacity of Yon Baer, that the more special forms of structure arise
out of the more general, and this by a gradual change. The meaning
of this law will become obvious hereafter, when some of the prin-
cipal cases to which it applies shall have been brought in illustra-
tion of it (Chap. XII.).
A still more curious series of discoveries has been made by
means of the Microscope, in regard to the early development of the
Medusan Acal&phs (jelly-fish, &c), and the relationship that exists
between them and the Hydroid Zoophytes ; — two groups of animals,
which had been previously ranked in different classes, and had not
been supposed to possess anything in common. For it has been
clearly ascertained by the careful observations of Sars, Siebold,
Dalyell, and others, that those delicate arborescent Zoophytes, each
polype of which is essentially a Hydra (§ 473), not only grow by
extending themselves into new branches, like Plants, — sometimes
also budding-off detached gemmce, which multiply their kind by
developing themselves into Zoophytic forms like those whence
they sprang ; but also produce peculiar buds having all the cha-
racters of Medusce, which contain the proper generative organs of
the Zoophyte, but which, usually detaching themselves from the
stock that bore them, swim freely through the ocean as minute
jelly-fish, without exhibiting the slightest trace of their originally
attached condition (§§ 474-477). The Medusa? in due time pro-
duce fertile eggs ; and each egg developes itself, not into the form
of its immediate progenitor, but into that of the Zoophyte from
which the Medusa was budded-off. And thus a most extraordinary
alternation of forms is presented, between the Zoophyte, which may
be compared to the growing or vegetating stage of a Plant (its
polypes representing the leaf -buds), and the Medusa, the develop-
ment of which marks its flowering stage. So again, from the
investigation of the early history of those larger forms of ' jelly-
fish' with which every visitor to the sea-coast is familiar, it has been
rendered certain that they too are developed from Polype-larvae,
usually of very minute size, which give-off Medusa-buds (§ 481) ;
so that whilst they are best known to us in their Medusan state,
and the Hydroid Zoophytes in their polypoid state, each of these
groups is the representative of a certain stage in the life-history of
one and the same tribe of these curious beings, which, when com-
plete, includes both states. — Changes very similar in kind, and in
many respects even more remarkable, have been found by micro-
scopic enquiry to take place among the Entozoa (intestinal worms) ;
but being interesting only to professed Naturalists and scientific
Physiologists, they scarcely call for particular notice in a treatise
like the present.
It has not been among the least important results of the new
turn which Zoological enquiry has thus taken, that a far higher
spirit has been introduced into the cultivation of this science than
previously pervaded it. Formerly it was thought, alike in Zoology
c
18 LATER EESULTS OF MICROSCOPIC RESEARCH.
and in Botany, that classification might be adequately based on
external characters alone ; and the scientific acquirements of a
Naturalist were estimated rather by the extent of his familiarity
with these, than by any knowledge he might possess of internal
organization. The great system of Cuvier, it is true, professed
to rest upon organization as its basis ; but the acquaintance with
this which was considered requisite for the purpose, was very
limited in its amount and superficial in its character ; and no
Naturalist formerly thought of studying the history of Develop-
ment as a necessary adjunct to the Science of Classification. How
essential a knowledge of it has now become, however, if only as a
basis for any truly natural arrangement of Animals, must have
been made apparent by the preceding sketch ; and it has thus
come to be felt and admitted amongst all truly-philosophic Natu-
ralists, that the complete study of any particular group, even for
the purposes of classification, involves the acquirement of a know-
ledge, not only of its intimate structure, but of its entire life-
history. And thus Natural History and Physiology, — two depart-
ments of the great Science of Life, which the Creator inextricably
blended, but which Man has foolishly striven to separate, — are
now again being brought into their original and essential harmony ;
and it is coming to be thought more creditable to give a com-
plete elucidation of the history of even a single species, than to
describe any number of new forms about which nothing else is
made-out than what shows itself on the surface.
Thus every Microscopist, however limited may be his oppor-
tunities, has a wide range of observation presented to him in the
study of the lower forms of Animal life ; with the strongest incite-
ment to persevering and well-directed enquiry, that the anticipation
of novelty and the expectation of valuable results can afford. For,
notwithstanding the large number of admirable records which
have been already published (chiefly, we must admit with regret,
by Continental Naturalists) upon the developmental history of the
lower tribes of Animals, there is no one of the subjects that have
been just passed in review, of which the knowledge hitherto gained
can be regarded as more than a sample of that which remains to be
acquired. Eecords like those already referred-to might easily be
multiplied a hundred-fold, with infinite advantage to Science ; if
those Microscopists who spend their time in desultory observation,
and in looking at some favourite objects over and over again, would
but concentrate their attention upon some particular species or
group, and work-out its entire history with patience and determi-
nation. And the observer himself would find this great advantage
in so doing, — that an enquiry thus pursued gradually becomes to
him an object of such attractive interest, that he experiences a zest
in its pursuit to which the mere dilettante is an entire stranger,
besides enjoying all that mental profit which is the almost neces-
sary result of the thorough performance of any task not in itself
unworthy. And what can be a more worthy occupation, than the
ELEMENTAEY STEUCTUEE OF HIGHEE ANIMALS. 19
attempt to gain an insight, however limited, into the operations of
Creative Wisdom ? — these being not less wonderfully displayed
among the forms of Animal life which are accounted the simplest
and least attractive, than in those which more conspicuously solicit
the attention of the Student of ISTature, by the beauty of their
aspect or the elaborateness of their organization.
It has not been, however, in the study of the minuter forms of
Animal life alone, that the Microscope has been turned to valuable
account ; for the Anatomist and the Physiologist who had made the
Human fabric the especial object of their study, and who had been
led to believe that the knowledge accumulated by their repeated
and persevering scrutiny into every portion accessible to their
vision, was all which it lay within their power to attain, have
found in this new instrument of research, the means of advancing
far nearer towards the penetralia of Organization, and of gaining
a much deeper insight into the mysteries of Life, than had ever
before been conceived possible. For every part of the entire organ-
ism has been, so to speak, decomposed into its elementary tissues,
the structure and actions of each of which have been separately
and minutely investigated ; and thus a new department of study,
which is known as Histology (or Science of the Tissues) has not
only been marked out, but has already made great advances towards
completeness. In the pip-suit of this enquiry, the Microscopists of
our day have not limited themselves to the fabric of Man, but have
extended their researches through the entire range of the Animal
kingdom ; and in so doing, have found, as in every other depart-
ment of Nature, a combination of endless variety in detail, with a
marvellous simplicity and uniformity of general plan.
Thus the bones which constitute the skeleton of the Yertebrated
animal, however different from each other in their external con-
figuration, in the arrangement of their compact and their can-
cellated portions, and such other particulars as specially adapt
them for the purposes they have to perform in each organism, — all
consist of a certain kind of tissue, distinguished under the micro-
scope by features of a most peculiar and interesting kind ; and
these features, whilst presenting (like those of the Human counte-
nance) a certain general conformity to a common plan, exhibit
(as was shown by Prof. Quekett) such distinctive modifications of
that plan in the different Classes and Orders of the Yertebrated
series, that it is generally possible by the microscopic examination
of the merest fragment of a bone, to pronounce with great pro-
bability as to the natural family to which it has belonged (§§ 612,
665). — Still more is this the case in regard to the teeth, whose
organic structure (originally detected by Leeuwenhoek) has been
newly and far more completely elucidated by Purkinje, Eetzius:
Owen, and Tomes ; for the enquiry into the comparative struc.
ture of these organs, which has been prosecuted by Prof. Owen
in particular through the entire range of the Yertebrated series
c2
20 LATER RESULTS OF MICROSCOPIC RESEARCH.
has shown that, with an equally close conformity to a certain general
plan of structure, there are at the same time still wider diversities
in detail, which are so characteristic of their respective groups,
that it is often possible to discriminate, not only families, but even
genera and species, by careful attention to the minute features of
their structure (§§ 615, 616, 664). — Similar enquiries, with results
in many respects analogous, have been carried-out by the Author,
in regard to the shells of Mollusks (§§ 521-534), Crustaceans
(§ 573), and Echinoderms (§§ 491-500) ; his researches having not
only demonstrated the existence of an organic structure in these
protective envelopes (which had been previously affirmed to be mere
inorganic exudations, presenting in many instances a crystalline
texture), but having shown that many natural groups are so dis-
tinctly characterized by the microscopic peculiarities they present,
that the inspection of a minute fragment of Shell will often serve to
determine, no less surely than in the case of bones and teeth, the
position of the animal of which it formed part.
The soft parts of the Animal body, moreover, such as the carti-
lages which cover the extremities of the bones and the ligaments
which hold them together at the joints, the muscles whose contrac-
tion developes motion and the tendons which communicate that
motion, the nervous ganglia which generate nervous force and the
nerve-fibres which convey it, the shin which clothes the body and
the mucous and serous membranes which line its cavities, the
assimilating glands which make the blood and the secreting glands
which keep it in a state of purity, — these, and many other tissues
that might be enumerated, are severally found to present charac-
teristic peculiarities of structure, which are more or less distinctly
recognizable throughout the Animal series, and which bear the
strongest testimony to the Unity of the Design in which they all
originated. As we descend to the lower forms of Animal life,
however, we find these distinctions less and less obvious ; and we
at last come to fabrics of such extreme simplicity and homogeneous-
ness, that every part seems to resemble every other in structure
and actions ; no provision being made for that ' division of labour'
which marks the higher types of organization, and which, being
the consequence of the development of separate organs each having
its special work to do, can only be effected where there is a ' diffe-
rentiation' of parts that gives to the entire fabric a character of
lieterogeneousness (Chap. XYIII).
The Microscopic investigations whose nature has thus been
sketched, have not only been most fruitful in the discovery of indi-
vidual facts, but have led to certain general results of great value
in Physiological Science. Among the most important of these, is
the complete metamorphosis which has been effected in the ideas
previously entertained regarding living action : such having been
essentially based on the Circulation of the blood, as the only vital
phenomenon of which any direct cognizance could be gained
DEVELOPMENTAL HISTORY OF ANIMALS. 21
through the medium of the senses. For it gradually came to be
clearly perceived, that in the Animal as in the Plant, each integral
portion of the Organism possesses an independent Life of its oivn,
in virtue of which it performs a series of actions peculiar to itself,
provided that the conditions requisite for those actions "be supplied
to it ; and that the Life of the body as a whole (like a symphony
performed by a full orchestra) consists in the harmonious combi-
nation of its separate instrumental acts,— the Circulation of the
blood, instead of making the tissues, simply affording the supply of
prepared nutriment at the expense of which they evolve themselves
from germs previously existing. This general doctrine was first
put prominently forward by Schwann, whose " Microscopical
Researches into the Accordance in the Structure and Growth of
Animals and Plants," published in 1839, mark the commencement
of a new era in all that department of Animal Physiology which
comprises the simply- vegetative life of the organized fabric. These
researches, avowedly based upon the ideas advanced by Schleiden,
were prosecuted in the same direction as his had been ; the object
which this admirable observer and philosophic reasoner specially
proposed to himself, being the study of the development of the
Animal tissues. He found that although their evolution cannot
be watched while in actual progress, its history may be traced-out
by the comparison of the successive stages brought to light by
Microscopic research ; and in so far as this has been accomplished
for each separate part of the organism, the structure and actions of
its several components, however diverse in their fully-developed
condition, are found to resemble each other more and more closely,
the more nearly these parts are traced-back to their earliest ap-
pearance. Thus we arrive in our retrospective survey, at a period
in the early history of Man, at which the whole embryonic mass is
but a congeries of cells, all apparently similar and equal to each
other ; and going still further back, it is found that all these have
had their origin in the subdivision of a single primordial cell,
which is the first defined product of the generative act. On this
single cell the Physiologist bases his idea of the most elementary
type of Organization ; whilst its actions present him with all that
is essential to the notion of Life. And in pursuing the history of
the germ, from this, its simplest and most homogeneous form, to the
assumption of that completed and perfected type which is marked
by the extreme heterogeneousness of its different parts, he has
another illustration of that law of progress from the general to the
special (p. 17), which is one of the highest principles yet attained
in the science of Vitality.
But further, the Physiologist, not confining his enquiries to Man,
but pursuing the like researches into the developmental history of
other living beings, is soon led to the conclusion that the same is
true of them also : each Animal, as well as each Plant, having the
same starting-point in the single cell ; and the distinctive features
by which its perfected form is characterized, how striking and im-
22 GEOLOGICAL EESULTS OF MICROSCOPIC RESEARCH.
portant soever these may be, arising in the course of its develop-
ment towards the condition it is ultimately to present. In the
progress of that Evolution, those fundamental differences which
mark-out the great natural divisions of the Animal and the Vege-
table Kingdoms respectively, are the first to manifest themselves ;
and the subordinate peculiarities which distinguish classes, orders,
families, genera, and species, successively make their appearance,
usually (but not by any means constantly) in the order of im-
portance which Systematists have assigned to them. And it is in
thus pursuing, by the aid which the Microscope alone can afford to
his visual power, the history of the Organic Germ, from that simple
and homogeneous condition which seems common to every kind of
living being, either to that complex and most heterogeneous
type of which Man is the highest representative, or only to that
humble Protophyte or Protozoon which lives and grows and mul-
tiplies without showing any essential advance upon its embryonic
form, — that the Physiologist is led to recognize the essential con-
formity in the method of this Evolution, to that which he learnt
from Palasontological research to have been the mode of Evolution
in Geological Time of the Organic Creation now existing.
Most important services have also been rendered by the Micro-
scopist to the Geologist ; who has not only been enabled to arrive
at the precise nature of fragments of 'fossilized teeth, bones, shells,
wood, &c, by a minute examination of their internal structure, in
many cases in which their external features did not afford the
means of identifying them ; but has also been brought by its
means to the knowledge that numerous deposits which form no
insignificant part of the solid crust of the globe, are made-up by
the accumulation of the skeletons of organic forms too minute to be
discerned by the unaided eye. Various examples of both of these
applications of the instrument will be given in their proper place
(Chap. XIX.) ; and it will be here necessary only to refer to the
determination of the large share which the calcareous-shelled
Foraminifera have had in the formation of Chalk (§ 659), and to
the discovery of the Diatomaceous nature of many extensive
siliceous deposits (§ 260), in proof of the assertion, that the
Geologist has no right to assume an acquaintance with the nature
of any formation whatever, until he has subjected it to Microscopic
examination. In this line of enquiry Prof. Ehrenberg has taken
the lead from the first ; and his discovery that the green sands which
present themselves in various formations from the Silurian upwards,
and which form a considerable layer beneath the Chalk, are chiefly
composed of siliceous casts of the interior of Foraminifera and
minute Mollusca, the calcareous shells of which have disappeared,
is one of the most remarkable of his many contributions to Micro-
geology (§ 661).
It has been the purpose of the foregoing sketch, to convey an
idea, not merely of the services which the Microscope has already
EDUCATIONAL VALUE OF THE MICKOSUOPE. 23
rendered to the collector of facts in every department of the Science
of Life, but also of the value of these facts as a foundation for
philosophic reasoning. For it is when thus utilized, that observa-
tions, whether made with the Microscope or with the Telescope, or
by any other instrumentality, acquire their highest value, and
excite the strongest interest in the mind. But as it is not every one
who is prepared by his previous acquirements to appreciate such
researches according to the scientific estimate of their importance,
it may be well now to address ourselves to that large and increasing
number, who are disposed to apply themselves to Microscopic
research as amateurs, following the pursuit rather as a means of
wholesome recreation to their own minds, than with a view to the
extension of the boundaries of existing knowledge ; and to those in
particular who are charged, whether as parents or as instructors,
with the direction and training of the youthful mind.
All the advantages which have been urged at various times, with
so much sense and vigour,* in favour of the study of Natural
History, apply with full force to Microscopical enquiry. What
better encouragement and direction can possibly be given to the
exercise of the observing powers of a child, than to habituate him
to the employment of this instrument upon the objects which im-
mediately surround him, and then to teach him to search-out
novelties among those less immediately accessible ? The more we
limit the natural exercise of these powers, by the use of those
methods of education which are generally considered to be specially
advantageous for the development of the Intellect, — the more we
take him from fields and woods, from hills and moors, from river-
side and sea- shore, and shut him up in close school-rooms and
narrow play-grounds, limiting his attention to abstractions, and
cutting him off even in his hours of sport from those sights and
sounds of Nature which seem to be the appointed food of the
youthful spirit, — the more does it seem important that he should
in some way be brought into contact with her, that he should have
his thoughts sometimes turned from the pages of books to those of
Creation, from the teachings of Man to those of God. Now if we
attempt to give this direction to the thoughts and feelings in a
merely didactic mode, it loses that spontaneousness which is one
of its most valuable features. But if we place before the young a
set of objects which can scarcely fail to excite their healthful
curiosity, satisfying this only so far as to leave thern still enquirers,
and stimulating their interest from time to time by the disclosure
of such new wonders as arouse new feelings of delight, they come
to look upon the pursuit as an ever-fresh fountain of happiness
and enjoyment, and to seek every opportunity of following it for
themselves.
There are no circumstances or conditions of life, which need be
* By none more forcibly than by Mr. Kingsley, in bis charming little volume
entitled " Glaucus, or the Wonders of the Shore."
24 EDUCATIONAL VALUE OF THE MICROSCOPE.
altogether cut-off from these sources of interest and improvement.
Those who are brought-up amidst the wholesome influences of the
country, have, it is true, the greatest direct opportunities of thus
drawing from the Natural Creation the appropriate nurture for their
own spiritual life. But their very familiarity with the objects
around them prevents them from receiving the full benefit of their
influence, unless they be led to see how much there is beneath the
surface even of what they seem to know best ; and in rightly train-
ing them to look for this, how many educational objects, — -physical,
intellectual, and moral, — may be answered at the same time !
" A walk without an object," says Mr. Kingsiey, " unless in the
most lovely and novel scenery, is a poor exercise ; and as a re-
creation utterly nil. If we wish rural walks to do our children any
good, we must give them a love for rural sights, an object in every
walk ; we must teach them — -and we can teach them — to find
wonder in every insect, sublimity in every hedge-row, the records
of past worlds in every pebble, and boundless fertility upon the
barren shore ; and so, by teaching them to make full use of that
limited sphere in which they now are, make them faithful in a few
things, that they may be fit hereafter to be rulers over much."
What can be a more effectual means of turning such opportunities
to the best account, than the employment of an aid which not only
multiplies almost infinitely the sources of interest presented by the
objects with which our eyes are most familiar, but finds inexhaustible
life where all seems dead, constant activity where all seems motion-
less, perpetual change where all seems inert ? — Turn, on the other
hand, to the young who are growing up in our great towns, in the
heart of the vast Metropolis, whose range of vision is limited on
every side by bricks and mortar, who rarely see a green leaf or a
fresh blade of grass, and whose knowledge of animal life is practi-
cally limited to the dozen or two of creatures that everywhere
attach themselves to the companionship of Man, and shape their
habits by his. To attempt to inspire a real love of Nature by books
and pictures, in those wbo have never felt her influences, is almost
hopeless. A child may be interested by accounts of her wonders,
as by any other instructive narrative ; but they have little of life or
reality in his mind — far less than has the story of adventure which
appeals to his own sympathies, or even than the fairy tale which
charms and fixes his imagination. — Here the Microscope may be
introduced with all the more advantage, as being almost the only
means accessible under such circumstances for supplying what is
needed. A single rural or even suburban walk may afford stores
of pleasurable occupation for weeks, in the examination of its col-
lected treasures. A large glass jar may be easily made to teem
with life, in almost as many and as varied forms as could be found
by the unaided eye in long and toilsome voyages over the wide
ocean ; and a never-ending source of amusement is afforded by the
observation of thejr growth, their changes, their movements, their
habits. The school-boy thus trained looks forward to the holiday
EDUCATIONAL VALUE OF THE MICEOSCOPE. 25
which shall enable him to search afresh in some favourite pool, or
to explore the wonders of some stagnant basin, with as much zest
as the keenest sportsman longs for a day's shooting on the moors,
or a day's fishing in the best trout-stream ; and with this great ad-
vantage— that his excursion is only the beginning of a fresh stock
of enjoyment, instead of being in itself the whole.
This is no imaginary picture, bnt one which we have constantly
under our eyes ; and no argument can be needed to show the
value of such a taste, to such, at least, as have set clearly before
their minds the objects at which they should aim in the great
work of Education. For we have not merely to train the intel-
lectual powers and to develope the moral sense ; but to form those
tastes — those ' likes and dislikes ' — which exercise a more abiding
and a more cogent influence on the conduct, than either the reason
or the mere knowledge of duty. It is our object to foster all the
higher aspirations, to keep in check all that is low and degrading.
But the mind must have recreation and amusement ; and the more
closely it is kept, by the system of education adopted, to the exer-
cise of any one set of powers, the more potent will be that reaction
which will urge it, when restraint is removed, to activity of some
other kind ; and the more important is it that this reaction should
receive a direction to what is healthful and elevating, instead of to
what is weakening and degrading. It is quite a mistake to imagine
that those evil habits - which result from a wrong exercise of the
natural powers, a wrong direction of the natural tendencies, can
be effectually antagonized by the simple effort at repression. The
constant exercise either of external coercion or of internal restraint,
tends to keep the attention directed towards the forbidden object
of gratification ; the malady is only held in check, not cured ; and
it will break-out, perhaps with augmented force, whenever the
perpetually-present impulses shall derive more than ordinary
strength from some casual occurrence, or the restraining power
shall have been temporarily weakened. The only effectual mode of
keeping in check the wrung, is by making use of these same powers
and tendencies in a right mode ; by finding-out objects whereon
they may be beneficially exercised ; and by giving them such a
direction and encouragement, as may lead them to expend themselves
upon these, instead of fretting and chafing under restraint, ready
to break loose at the first opportunity. There is no object on
which the youthful energy can be employed more worthily, than in
the pursuit of Knowledge ; no kind of knowledge can be made more
attractive, than that which is presented by the Works of Creation ;
no source is more accessible, no fountain more inexhaustible ; and
there is none which affords, both in the mode of pursuing it, and.
in its own nature, so complete or so beneficial a diversion from
ordinary scholastic pursuits.
If there be one class more than another, which especially needs
to have its attention thus awakened to such objects of interest, as,
by drawing its better nature into exercise, shall keep it free from
26 EDUCATIONAL VALUE OF THE MICKOSCOPE.
the grovelling sensuality in which it too frequently loses itself, it
is our Labouring pojDulation ; the elevation of which is one of the
great social problems of the day. On those who are actively con-
cerned in promoting and conducting its education, the claims and
advantages of the Study of Nature can scarcely be too strongly
urged ; since experience has fully proved, — what might have been
a priori anticipated, — that where the taste for this pursuit has
been early fostered by judicious training, it becomes so completely
a part of the mind, that it rarely leaves the individual, however
unfavourable his circumstances may be to its exercise, but con-
tinues to exert a refining and elevating influence through his whole
subsequent course of life. Now for the reasons already stated, the
Microscope is not merely a most valuable adjunct in such instruc-
tion, but its assistance is essential in giving to almost every Natural
object its highest educational value ; and whilst the country
Schoolmaster has the best opportunities of turning it to useful
account, it is to the city Schoolmaster that, in default of other
opportunities, its importance as an educational instrument should
be the greatest. — It was from feeling very strongly how much
advantage would accrue from the introduction of a form of Micro-
scope, which should be at once good enough for Educational pur-
poses, and cheap enough to find its way into every well- supported
School in town and country, that the author suggested to the
Society of Arts in the summer of 1854 that'it should endeavour to
carry-out an object so strictly in accordance with the enlightened
purposes which it is aiming to effect ; and this suggestion having
been considered worthy of adoption, a Committee, chiefly con-
sisting of experienced Microscopists, was appointed to carry it
into effect. It was determined to aim at obtaining two instru-
ments ;— a simple microscope for the use of Scholars, to whom
it might be appropriately given as a reward for zeal and pro-
ficiency in the pursuit of Natural History, not in books, but in
the field ; — and a compound microscope for the use of Teachers,
of capacity sufficient to afford a good view of every kind of object
most likely to interest the pupil or to be within the reach of the
instructor. Notwithstanding the apprehensions generally expressed,
that no instruments at all likely to answer the intended purpose
could possibly be produced- at the prices specified, the result proved
their fallacy ; and the Compound Microscope of Messrs. Field of
Birmingham, to which the Society's Prize was awarded, has been
the progenitor of a whole brood of cheap ' Students' Microscopes '
by different makers, many of which are equal, for working pur-
poses, to the best instruments which could be obtained no more
than twenty years ago at three or four times their cost.
It is not alone, however, as furnishing an attractive object of
pursuit for the young — fitted at once to excite a wholesome taste
for novelty, ever growing with what it feeds-on, and to call forth
the healthful exercise of all those powers, both physical and mental,
which can minister to its gratification, — that Natural History
DISCIPLINE OF THE OBSERVING FACULTIES. 27
studies in general, and Microscopic enquiry in particular, are to be
specially commended as a means of intellectual and moral disci-
pline ; for there is no capacity, however elevated, to which they do
not furnish ample material for the exercise of all its best powers,
no period of life which may not draw from them its purest plea-
sures. Even to observe well is not so easy a thing as many persons
imagine. Some are too hasty, imagining that they can take-in
everything at a glance, and hence often forming very erroneous or
imperfect notions, which may give an entirely -wrong direction not
only to their own views but to those of others, and may thus
render necessary an amount of labour for the ultimate determi-
nation of the truth, many times as great as that which would have
sufficed in the first instance, had the original observations been
accurately made and faithfully recorded. Others, again, are too
slow and hesitating ; and fix their attention too much upon details,
to be able to enter into the real significance of what may be pre-
sented to the vision. Although ignorance has doubtless much to
do in producing both these faults, yet they both have their source
in mental tendencies which are not corrected by the mere acquisi-
tion of knowledge, and which are very inimical, not only to its fair
reception, but also to the formation of a sound judgment upon any
subject whatever. The habit of guarding against them, therefore,
once acquired in regard to Microscopic observation, will be of in-
valuable service in every walk of life. Not less important is it
(as has been already shown), to keep our observations free alike
from the bias of preconceived ideas, and from the suggestive in-
fluence of superficial resemblances ; and here, too, we find the
training which Microscopic study affords, especially when it is
prosecuted under the direction of an experienced guide, of the
highest value in forming judicious habits of thought and action.
To set the young observer to examine and investigate for himself,
to tell him merely ivhere to look and (in general terms) ivhat to
look for, to require from him a careful account of what he sees,
and then to lead him to compare this with the descriptions of
similar objects by Microscopists of large experience and unques-
tionable accuracy, is not only the best training he can receive as a
Microscopist, but one of the best means of preparing his mind, for
the exercise of its powers in any sphere whatever.
It cannot be too strongly or too constantly kept in view, that
the value of the results of Microscopic enquiry will depend far
more upon the sagacity, perseverance, and accuracy of the Ob-
server, than upon the elaborateness of his instrument. The most
perfect Microscope ever made, in the hands of one who knows not
how to turn it to account, is valueless ; in the hands of a careless,
a hasty, or a prejudiced observer, it is worse than valueless, as
furnishing new contributions to the already large stock of errors
that pass under the guise of scientific truths. On the other hand,
the least costly Microscope that has ever been constructed, how
limited soever its powers, provided that it gives no false appear-
28 DISCIPLINE OF THE OBSEEVIXG FACULTIES.
ances, shall furnish, to him who knows what may be done with it,
a means of turning to an acconnt, profitable alike to science and
to his own immortal spirit, those hours which might otherwise be
passed in languid ennui, or in frivolous or degrading amusements,*
and even of immortalizing his name by the discovery of secrets in
Nature as yet undreamed of. A very large proportion of the great
achievements of Microscopic research that have been noticed in
the preceding outline, have been made by the instrumentality of
microscopes which would be generally condemned in the present
day as unfit for any scientific purpose ; and it cannot for a moment
be supposed that the field which Nature presents for the prosecu-
tion of enquiries with instruments of comparatively limited capa-
city, has been in any appreciable degree exhausted. On the con-
trary, what has been done by these and scarcely superior instru-
ments, only shows how much there is to be done. — The Author
may be excused for citing, as an apposite example of his meaning,
the curious results he obtained from the study of the development
of the Purpura lapillus (rock-whelk), which will be detailed in
their appropriate place (§§ 542, 543) ; for these were obtained
almost entirely by the aid of single lenses, the Compound Micro-
scope having been only occasionally applied-to, either for the verifi-
cation of what had been previously worked-out, or for the examina-
tion of such minute details as the power employed did not suffice
to reveal.
But it should be urged upon such as are anxious to render
service to Science, by the publication of discoveries which they
suppose themselves to have made with comparatively imperfect
instruments, that they will do well to refrain from bringing these
forward, until they shall have obtained the opportunity of verifying
them with better. It is, as already remarked, when an object is
least clearly seen, that there is most room for the exercise of the
imagination ; and there was sound sense in the reply once made
by a veteran observer, to one who had been telling him of won-
derful discoveries which another was said to have made "in spite
of the badness of his Microscope," — " ISTo, Sir, it was in conse-
quence of the badness of his Microscope." If those who observe,
with however humble an instrument, will but rigidly observe the
rule of recording only what they can clearly see, they can neither
go far astray themselves, nor seriously mislead others.
Among the erroneous tendencies which Microscopic enquiry
seems especially fitted to correct, is that which leads to the esti-
mation of things by their merely sensuous or material greatness,
instead of by their value in extending our ideas and elevating our
aspirations. For we cannot long scrutinize the " world of small"
* "I have seen," says Mr. Kingsley, "the cultivated man, craving for
travel and success in life, pent-up in the drudgery of London work, and yet
keeping his spirit calm, and his morals perhaps all the more righteous, by
spending over his Microscope evenings which would too probably have gene-
rally been wasted at the theatre."
MORAL INFLUENCE OF MICROSCOPIC STUDY. 29
to which, we thus find access, without having the conviction forced
upon us, that all size is but relative, and that mass has nothing to
do with real importance. There is something in the extreme of
minuteness, which is no less wonderful, — might it not almost be
said, no less majestic ? — than the extreme of vastness. If the mind
loses itself in the contemplation of the immeasurable depths of
space, and of the innumerable multitudes of stars and systems by
which they are peopled, it is equally lost in wonder and admira-
tion, when the eye is turned to those countless multitudes of living
beings which a single drop of water may contain, and when the
attention is given to the wondrous succession of phenomena which
lthe life-history of every individual among them exhibits, and to
the order and constancy which this presents. Still more is this
the case, when we direct our scrutiny to that universe which may
be said to be included in the body of Man, or of any one of the
higher forms of Organized being; and survey the innumerable
assemblage of elementary parts, each having its own independent
action, yet each working in perfect harmony with the rest, for the
completion of the wondrous aggregate which the Life of the whole
presents. In the study of the one class of phenomena, no less than
in the survey of the other, we are led towards that Infinity, in
comparison with which the greatest and the least among the
objects of Man's regard are equally insignificant; and in that
Infinity alone can we seek for a Wisdom to design, or a Power to
execute, results so vast and so varied, by the orderly co-operation
of the most simple means.
CHAPTEE I.
OPTICAL PRINCIPLES OF THE MICROSCOPE.
1. Laws of Refraction : — Spherical and Chromatic Aberration.
1. All Microscopes in ordinary use, whether Simple or Com-
pound, depend for their magnifying power on that influence exerted
by Lenses, in altering the conrse of the rays of light passing
through them, which is termed Refraction. This influence takes
place in accordance with the two following laws, which are fnlly
explained and illustrated in every elementary treatise on Optics.*
i. A ray of light passing from a rarer into a denser medium, is
refracted towards a line drawn perpendicularly to the plane which
divides them ; and vice versa.
ii. The sines of the angles of incidence and refraction (that is,
of the angles which the ray makes with the perpendicular before
and after its refraction) bear to one another a constant ratio for
each substance, which is known as its index of refraction.
Thus the ray e o (Fig. 1) passing from Air into Water, will not
go-on to f, but will be refracted towards the line c c' drawn per-
pendicularly to the surface a b of the water, so as to take the
direction o w. If it pass into Glass, it will undergo a greater
refraction, so as to take the direction o g. And if it pass into
Diamond, the chauge in its course will be so much greater, that it
will take the direction o d. The angle e o c is termed the ' angle
of incidence ;' whilst the angles woe', &oc' and doc' are the
8 angles of refraction.' And whether the angle of incidence be
large or small, its sine e e' bears a constant ratio in each case to
the sine iv w' or g g' or d d', of the angle of refraction ; and this
ratio is what is termed the f index of refraction.'
The ' index of refraction' is determined for different media, by
the amount of the refractive influence which they exert upon rays
passing into them, not from air, but from a vacuum ; and in ex-
pressing it, the sine of the angle of refraction is considered as the
unit, to which that of the angle of incidence bears a fixed relation.
Thus when we say that the ' index of refraction' of Water is 1'336,
we mean that the sine e e' of the angle of incidence e o c of a ray
passing into water from a vacuum, is to the sine w w' of the angle
* See especially "Brooke's Elements of Natural Philosophy," Sixth Edition,
Chaps, xvii.-xx.
LAWS OF EEFEACTION.
31
of refraction w o c', as 1*336 to 1, or almost exactly as If to 1, or
as 4 to 3. So, again, the index of refraction for (flint) Glass, being
about 1*6, we mean that the sine e e' of the angle of incidence of a
ray e o c passing into glass from a vacuum, is to the sine of g g'
Fig. 1.
c
A-TB*
the angle of refraction g o c', as 1*6 to 1, or as 8 to 5. So in the
case of Diamond, the sine e e' is to the sine d d' as 2 '439 to 1, or
almost exactly as 2^ to 1, or as 5 to 2. Thus, the angle of inci-
dence being given, the angle of refraction may be always found by
dividing the sine of the former by the ' index of refraction,' which
will give the sine of the latter. In accordance with these laws, a
ray of light passing from one medium to another perpendicularly,
undergoes no refraction ; and of several rays at different angles,
those nearer the perpendicular are refracted less than those more
inclined to the refracting surface. When a pencil of rays, however,
impinges on the surface of a denser medium (as when rays passing
through Air fall upon Water or Glass), some of the incident rays
are reflected from that surface, instead of entering it and under-
going refraction ; and the proportion of these rays increases with
the increase of their obliquity. Hence there is a loss of light in
every case in which pencils of rays are made to pass through
lenses or prisms : and this diminution in the brightness of the
image formed by refraction will bear a proportion, on the one hand,
to the number of surfaces through which the rays have had to pass ;
and on the other, to the degree of obliquity of the incident rays,
32 OPTICAL PRINCIPLES OF THE MICROSCOPE.
and to the difference of the refractive powers of the two media.
Hence in the passage of a pencil of rays out of Glass into Air, and
then from Air into Glass again, the loss of light is much greater
than it is when some medium of higher refractive power than
air is interposed between the two glass surfaces ; and advantage is
taken of this principle in the construction of Achromatic combina-
tions for the Microscope, the component lenses of each pair or
triplet (§ 14) being cemented together by Canada Balsam ; whilst
it is also applied in another mode in the ' immersion lenses' now
in common use (§ 19). On the other hand, advantage is taken of
the partial reflection of rays passing from air to glass at an oblique
angle to the surface of the latter, in the construction of the in-
genious (non-stereoscopic) Binocular of Messrs. Powell and Lea-
land (§ 67).
2. On the other hand, when a ray w o emerges from a dense
medium into a rare one, instead of following the straight course, it
is bent from the perpendicular according to the same ratio ; and
to find the course of the emergent ray, the sine of the angle of
incidence must be multiplied by the ' index of refraction,' which
will give the sine of the angle of refraction. Now when an
emergent ray falls very obliquely upon the surface of the denser
medium, the refraction which it would sustain in passing forth
into the rarer medium, tending as it does to deflect it still farther
from the perpendicular, becomes so great that the ray cannot pass
out at all, and is reflected back from the plane which separates the
two media, into the one from which it was emerging. This internal
reflection will take place, whenever the product of the sine of the
angle of incidence, multiplied by the index of refraction, exceeds
the sine of 90°, which is the radius of the circle ; and therefore the
' limiting angle,' beyond which an oblique ray suffers internal
reflection, varies for different substances in proportion to their
respective indices of refraction. Thus, the index of refraction of
"Water being 1*336, no ray can pass out of it into a vacuum,* if its
angle of incidence exceed 48° 28', since the sine h h' of that angle,
h o c', multiplied by 1"336 equals the radius ; and in like manner,
the ' limiting angle' for Flint-glass, its index of refraction being
1'60, is 38° 41'. — This fact imposes certain limits upon the per-
formance of microscopic lenses, since of the rays which would
otherwise pass out from glass into air, all the more oblique are
kept back ; whilst, on the other hand, it enables the Optician to
make most advantageous use of glass prisms for the purpose of
reflection, the proportion of the light which they throw back being
* The reader may easily make evident to himself the internal reflection of
Water, by nearly filling a wine-glass with water, and holding it at a higher
level than his eye, so that he sees the surface of the fluid obliquely from
beneath ; no object beld above the water will then be visible through it, if the
eye be placed beyond the limiting angle ; whilst the surface itself will appear
as if silvered, through its reflecting back to the eye the light which falls upon
it from beneath.
REFRACTION THROUGH CONVEX LENSES; -33
much larger than that returned from the best polished metallic
surfaces, and the brilliancy of the reflected image being consequently
greater. Such Prisms are of great value to the Microscopist for
particular purposes, as will hereafter appear. (§§ 31-35.)
3. The Lenses employed in the construction of Microscopes
are chiefly convex ; those of the opposite kind, or concave, being
only used to make certain modifications in the course of the rays
passing through convex lenses, whereby their performance is ren-
dered more exact (§§ 11, 13). — It is easily shown to be in accor-
dance with the laws of refraction already cited, that when a ' pencil'
of parallel rays, passing through air, impinges upon a convex _ sur-
face of glass, the rays will be made to converge ; for they will be
bent towards the centre of the circle, the radius being the perpen-
dicular to each point of curvature. The central or axial ray, as it
coincides with the perpendicular, will undergo no refraction ; the
others will be bent from their original course in an increasing
degree, in proportion as they fall at a distance from the centre of
the lens ; and the effect upon the whole will be such, that they
will be caused to meet at a point, called the Focus,- some distance
beyond the centre of curvature. — This effect will not be materially
changed by allowing the rays to pass into air again through a
plane surface of glass, perpendicular to the axial ray (Fig. 2) ; a
Fig. 2.
Parallel rays, falling on a plano-convex Lens, brought to
a focus at the distance of the diameter of its sphere of
curvature ; and conversely, rays diverging from that
point, rendered parallel.
lens of this description is called a plano-convex lens, and will here-
after be shown to possess properties which render it very useful in
the construction of microscopes. But if, instead of passing through
a plane surface, the rays re-enter the air through a second convex
surface, turned in the opposite direction, as in a double-convex
lens, they will be made to converge still more. This will be readily
comprehended, when it is borne in mind that the contrary direc-
tion of the second surface, and the contrary direction of its refrac-
tion (this being from the denser medium, instead of into it),
D
34 OPTICAL PRINCIPLES OF THE MICEOSCOPE.
antagonize each other ; so that the second convex surface exerts an
influence on the course of the rays passing through it, which is
almost exactly equivalent to that of the first. Hence the focus of a
double-convex lens will be at just half the distance, or (as com-
monly expressed) will be half the length, of the focus of a piano-
convex lens having the same curvature on one side (Fig. 3).
4. The distance of the Focus from the Lens will depend not
merely upon its degree of curvature, but also upon the refracting
Parallel rays, lalliDg on a double-convex Lens, brought
to a focus in the centre of its sphere of curvature ; con-
versely, rays diverging from that point rendered parallel.
power of the substance of which it may be formed ; since the lower
the index of refraction, the less will the oblique rays be deflected
towards the axial ray, and the more remote will be their point of
meeting ; and conversely, the greater the refractive index, the more
will the oblique rays be deflected towards the axial ray, and the
nearer will be their point of convergence. A lens made of any sub-
stance whose index of refraction is 1*5, will bring parallel rays to a
focus at the distance of its diameter of curvature, after they have
passed through one convex surface (Fig. 2), and at the distance of
its radius of curvature, after they have passed through two convex
surfaces (Fig. 3) ; and as this ratio almost exactly expresses the
refractive power of ordinary Crown or plate Glass, we may for all
practical purposes consider the ' principal focus' (as the focus for
parallel rays is termed) of a double-convex lens to be at the distance
of its Radius, that is, in the Centre of curvature, and that of a
jjlano -convex lens to be at the distance of twice its radius, that is,
at the other end of the Diameter of its sphere of curvature.
5. It is evident from what has preceded, that as a Double-convex
Lens brings parallel rays to a focus in its Centre of curvature, it
will on the other hand cause those rays to assume a parallel direc-
tion, which are diverging from that centre before they impinge
ur>on it (Fig. 3) ; so that, if a luminous body be placed in the prin-
REFRACTION THROUGH CONVEX LENSES. 35
cipal focus of a double-convex lens, its divergent rays, falling on
one surface of the lens as a cone, will pass forth from its other side
as a cylinder. If, however, the rays which fall upon a double-convex
lens be diverging from the farther extremity of the Diameter of its
sphere of curvature, they will be brought to a focus at an equal
distance on the other side of the lens (Fig. 4) ; but the more the
Fm. 4.
Rays diverging irom the farther extremity of one din meter
of curvature of a double-convex Lens, brought to a focus at
the same distance on the other side.
point of divergence is approximated to the centre or principal focus,
the farther removed from the other side will be the point of con-
vergence (Fig. 5), until, the point of divergence being at the centre,
Fig. 5
Rays diverging from points more distant than the principal
focus of a double-convex Lens on either side, brought to a focus
beyond it ; if the point of divergence be within the diameter
of curvature, the focus of convergence will be beyond it ; and
vice versa.
there is no convergence at all, the rays being merely rendered pa-
rallel (Fig. 3) ; whilst if the point of divergence be beyond the diu-
d 2
36 OPTICAL PRINCIPLES OF THE MICROSCOPE.
meter of the sphere of curvature, the point of convergence will be
within it (Fig. 5). The farther removed the point of divergence,
the more nearly will the rays approach the parallel direction : nntil,
at length, when the object is very distant, its rays in effect become
parallel, and are brought together in the principal focns (Fig. 3).
If, on the other hand, the point of divergence be with/m the prin-
cipal focns, they will neither be brought to converge, nor be ren-
dered parallel, but will diverge in a diminished degree (Fig. 6). And
conversely, if rays already converging fall upon a double-convex lens,
Fig. 6.
Rays already converging, brougut togetner by a double-
convex Lens at a point nearer than its principal focus ; and
rays diverging from a point within its principal focus, still
divergent, though in a diminished degree.
they will be brought together at a point nearer to it than its centre
of curvature (Fig. 6). — The same principles apply equally to a Plano-
convex lens ; allowance being made for the double distance of its
principal focus. They also apply to a lens whose surfaces have
different curvatures ; the principal focus of such a lens being found
by multiplying the radius of one surface by the radius of the other,
and dividing this product by half the sum of the same radii. — The
rules by which the foci of convex lenses may be found, for rays of
different degrees of convergence and divergence, will be found in
works on Optics.
6. The refracting influence of concave Lenses will evidently be
precisely the opposite of that of convex. Bays which fall upon
them in a parallel direction, will be made to diverge as if from the
principal focus, which is here called the negative focus. This will
be, for a plano-concave lens, at the distance of the diameter of the
sphere of curvature ; and for a double-concave, in the centre of that
sphere. In the same manner, rays which are converging to such a
degree that, if uninterrupted, they would have met in the principal
focus, will be rendered parallel ; n converging more, they will still
meet, but at a greater distance ; and if converging less, they will
diverge as from a negative focus at a greater distance than that for
parallel rays. If already diverging, they will diverge still more, as
REFRACTION THROUGH CONVEX LENSES, 37
from a negative focus nearer than the principal fccns ; but this will
approach the principal focus, in proportion as the distance of the
point of divergence is such that the direction of the rays approaches
the parallel.
7. If a Lens be convex on one side and concave on the other,
forming what is called a meniscus, its effect will depend upon the
proportion between the two curvatures. If they are equal, as in a
Watch- Glass, scarcely any perceptible effect will be produced ; if
the Convex curvature be the greater, the effect will be that of a less
powerful convex lens ; and if the Concave curvature be the more
considerable, it will be that of a less powerful concave lens. The
focus of convergence for parallel rays in the first case, and of diver-
gence in the second, may be found by dividing the product of the
two radii by half their difference.
8. Hitherto we have considered only the effects of Lenses upon a
' pencil' of rays issuing from a single luminous point, and that point
situated in the line of its axis. If the point be situated above the
line of its axis, the focus will be below it, and vice versa. The sur-
face of every luminous body may be regarded as comprehending an
infinite number of such points, from every one of which a pencil
of rays proceeds, and is refracted according to the laws already spe-
cified ; so that a complete but inverted Image or picture of the ob-
ject is formed upon any surface placed in the Focus and adapted
to receive the rays. It will be evident from what has gone before,
that if the object be placed at twice the distance of the principal
focus, the Image, being formed at an equal distance on the other
side of the lens (§ 5), will be of the same dimensions with the Ob-
ject : whilst, on the other hand, if the object (Fig. 7, a b) be nearer
Fig. 7.
Formation of Images by Convex Lenses.
the lens, the image a b will be farther from it, and of larger dimen-
sions ; but if the object a b be farther from the lens, the image a b
will be nearer to it, and smaller than itself. Further, it is to be
remarked that the larger the Image in proportion to the Object,
the less bright will it be, because the same amount of light has to be
38 OPTICAL PRINCIPLES OF THE MICROSCOPE.
spread over a greater surface ; whilst an image that is smaller than
the object will be more brilliant in the same proportion.
9. A knowledge of these general facts will enable the learner to
understand the ordinary operation of the Microscope ; but the
instrument is subject to certain optical imperfections, the mode of
remedying which cannot be comprehended without an acquaintance
with their nature. One of these imperfections results from the
unequal refraction of the rays which pass through Lenses whose
curvatures are equal over their whole surfaces. If the course of
the rays passing through an ordinary Convex Lens be carefully
laid down (Fig. 8), it will be found that they do not all meet
Fig. 8.
Diagram illustrating Spheric it Aberration.
exactly in the foci already stated ; but that the focus f of the rays
ab, ab, which have passed through the marginal portion of the
lens, is much closer to it than that of the rays ab, ab, which are
nearer the line of its axis. Hence, if a screen be held in the focus
f of the marginal portion of the lens, the rays which have passed
through its central portion will be stopped by it before they have
come to a focus ; and if the screen be carried back into the focus /
of the latter, the rays which were most distant from the axis will
have previously met and crossed, so that they will come to it in a
state of divergence, and will pass to c and d. In either case, there-
fore, the image will have a certain degree of indistinctness ; and
there is no one point to which all the rays can be brought by a
single Lens of Spherical curvature. The distance f/, between the
focal points of the central and of the peripheral rays of any lens,
is termed its Spherical Aberration. It is obvious that, to produce
the desired effect, the curvature requires to be increased around
the centre of the lens, so as to bring the rays which pass through
it more speedily to a focus ; and to be diminished towards the
circumference, so as to throw the focus of the rays influenced by it
to a greater distance. The requisite conditions may be theoretically
fulfilled by a single lens, one of whose surfaces, instead of being
spherical, should be a portion of an ellipsoid or hyperboloid
of certain proportions ; but the difficulties in the way of the
mechanical execution of lenses of this description are such, that
SPHERICAL ABERRATION. 39
for practical purposes this plan of construction is altogether
unavailable ; and their performance would only he perfectly
accurate for parallel rays.
10. Various means have been devised for reducing the Aberra-
tion of lenses of Spherical curvature. It may be considerably
diminished, by making the most advantageous use of ordinary
lenses. Thus the aberration of a Plano-convex Lens whose convex
side is turned towards parallel rays, is only l-^^ths of its thickness ;
whilst, if its plane side be turned towards them, the aberration is
4^ times the thickness of the lens. Hence, when a plano-convex
lens is used to form an image by bringing to a focus parallel or
slightly-diverging rays from a distant object, its convex surface
should be turned towards the object ; but, when it is used to render
parallel the rays which are diverging from a very near object, its
plane surface should be turned towards the object. The single lens
having the least spherical aberration, is a Double-convex whose
radii are as one to six : when its flattest face is turned towards
parallel rays, the aberration is nearly 3| times its thickness ; but
when its most convex side receives or transmits them, the aberra-
tion is only lyl^ths of its thickness. Spherical Aberration is
further diminished by reducing the aperture or working- surface of
the lens, so as to employ only the rays that pass through its central
part, which, if sufficiently small in proportion to the whole sphere,
will bring them all to nearly the same focus. Such a reduction is
made in the Object-glasses of common (non-achromatic) Micro-
scopes ; in which, whatever be the size of the lens itself, the greater
portion of its surface is rendered inoperative by a stop, which is a
plate with a circular aperture interposed between the lens and the
rest of the instrument. If this aperture be gradually enlarged, it
will be seen that, although the image becomes more and more
illuminated, it is at the same time becoming more and more
indistinct ; and that, in order to gain defining power, the aperture
must be reduced again. ISTow this reduction is attended with two
great inconveniences : in the first place, the loss of intensity of
light, the degree of which will depend upon the quantity transmitted
by the lens, and will vary therefore with its aperture ; and, secondly >
the diminution of the angle of aperture, that is, of the angle a b c
(Fig. 10) made by the most diverging of the rays of the pencil
issuing from any point of an object that can enter the lens ; on the
extent of which angle depend some of the most important qualities
of a Microscope (§ 145).
11. The Spherical Aberration may be approximately corrected,
however, by making use of combinations of lenses, so disposed
that their opposite aberrations shall correct each other, whilst
magnifying power is still gained. For it is easily seen that, as
the aberration of a concave lens is just the opposite of that of a
convex lens, the aberration of a convex lens placed in its most
favourable position may be corrected by that of a concave lens of
much less power in its most unfavourable position ; so that,
40 OPTICAL PRINCIPLES OF THE MICROSCOPE.
although the power of the convex lens is weakened, all the rays
which pass through this combination will he brought to one focus.
It is by a method of this kind, that the Optician aims to correct the
Spherical Aberration, in the construction of those combinations of
lenses which are now employed as Object-glasses in all Compound
Microscopes that are of any real value as instruments of observa-
tion. But it sometimes happens that this correction is not perfectly
made ; and the want of it becomes evident in the fog by which the
distinctness of the image, and especially the sharpness of its out-
lines, is impaired.
12. The researches of Dr. Eoyston-Pigott show that the very slight
residual errors, in the best Objectives hitherto made, are sufficient
to prevent some of the most difficult objects being distinctly seen.
For details of Dr. Pigott's method of detecting, and reducing these
optical errors, the reader must be referred to his paper " On a
Searcher for Aplanatic Images," read before the Royal Society,
April 28th, 1870 ; but we may here state his conclusion, "that when
any well-defined structure is viewed by the best microscopes, there
exist eidola or false images on each side of the best focal point."
These false images are liable to be confused with the true images,
and, as shown by Dr. Pigott's experiments, may lead to very
fallacious results. The object of his "Aplanatic Searcher" is to
provide further corrections. " It consists of a pair of slightly over-
corrected achromatic lenses, admitting of further correction by a
separating adjustment, mounted midway between a low eye-piece
and the objective, so as to admit of a traverse of 2 or 3 inches by
means of a graduated milled head. These lenses are conveniently
traversed within the draw tube, and can be brought to bear within
4 inches of the objective, or at a distance of 10 inches. The focal
length of the combination forming an Aplanatic Searcher may vary
from 1\ to f of an inch. The latter applies more effectively to low
objectives, when it is desirable to obtain extraordinary depth of focal
penetration and vision through very thick glass." — Dr. Pigott's
views have been met with much acrimonious discussion of theo-
retical points. The object is., however, essentially a 'practical
one. It can only be decided by a series of careful trials. Few
of his critics have taken the trouble to witness his exjDeriments.
Those who have done so, have found them well worthy of atten-
tion, but have been more or less impressed with the difficulty of
arranging all the optical combinations so as to yield the best
result. It is obvious that when, as is the case with the work
of the best makers, the errors of objectives are exceedingly small,
it must be a very delicate process to make them still smaller, and
demonstrate in a conclusive manner that this result has been
obtained.
13. But the spherical aberration is not the only imperfection with
which the Optician has to contend in the construction of Microscopes.
A difficulty equally serious arises from the unequal refrangibility
of the several Coloured rays which together make up White or
CHROMATIC ABERRATION. 41
colourless light,* so that they are not all brought to the same focus,
even by a lens free from spherical aberration. It is this difference
in their refrangibility, which causes their complete separation or
' dispersion' by the Prism into a Spectrum ; and it manifests itself,
though in a less degree, in the image formed by a convex Lens. For if
parallel rays of white light fall upon a Convex surface, the most
refrangible of its component rays, namely, the violet, will be brought
to a focus at a point somewhat nearer to the lens than the principal
focus, which is the mean of the whole ; and the converse will be
true of the red rays, which are the least refrangible, and whose focus
will therefore be more distant. Thus in Fig. 9 the rays of white
light, a e, a" b", which fall on the peripheral portion of the lens,
are so far decomposed, that the violet rays are brought to a focus
at c, and crossing there, diverge again and pass on towards F I ;
Fig. 9.
Diagram illustrating Chromatic Aberration.
whilst the red rays are not brought to a focus until d, crossing the
divergent violet rays at e e. The foci of the intermediate rays of
the spectrum (indigo, blue, green, yellow, and orange) are inter-
mediate between these two extremes. The distance c d between
the foci of the violet and of the red rays respectively is termed
Spherical Aberration. If the image be received upon a screen
placed at c — the focus of the violet rays, — violet will jDredominate
in its own colour, and it will be surrounded by a prismatic fringe
in which blue, green, yellow, orange, and red may be successively
distinguished. If, on the other hand, the screen be placed at d —
the focus of the red rays, — -the image will have a predominantly
red tint, and will be surrounded by a series of coloured fringes in
inverted order, formed by the other rays of the spectrum which
have met and crossed.f The line e e, which joins the points of
* It has been deemed better to adhere to the ordinary phraseology, when
speaking of this fact, as more generally intelligible than the language in which
it might be more scientifically described, and at the same time leading to no
practical error.
f This experiment is best tried with a Lens of long focus, of which the
central part is covered with an opaque stop, so that the light passes only
through a peripheral ring ; since, if its whole aperture be in use, the regular
formation of the fringes is interfered with by the spherical aberration, which
gives a different focus to the rays passing through each annular zone.
42 OPTICAL PRINCIPLES OF THE MICROSCOPE.
intersection between the red and the violet rays, marks the ' mean
focus,' that is, the situation in which the coloured fringes will be
narrowest, the ' dispersion' of the coloured rays being the least. As
the axial ray a' b' undergoes no refraction, neither does it sustain
any dispersion ; and the nearer the rays are to the axial ray, the
less dispersion do they suffer. Again, the more oblique the
direction of the rays, whether they pass through the central or the
peripheral portion of the lens, the greater will be the refraction
they undergo, and the greater also will be their dispersion ; and
thus it happens that when, by using only the central part of a
lens (§ 14), the chromatic aberration is reduced to its minimum,
the central part of a picture may be tolerably free from false
colours, whilst its marginal portion shall exhibit broad fringes*
14. The Chromatic Aberration of a lens, like the Spherical, may
be diminished by the contraction of its aperture, so that only its
central portion is employed. But the error cannot be got rid of
entirely by any such reduction, which, for the reasons already
mentioned, is in itself extremely undesirable. Hence it is of the
first importance in the construction of a really efficient Microscope,
that the chromatic aberration of its Object-glasses (in which the
principal dispersion is liable to occur) should be entirely corrected,
so that a large aperture may be given to these lenses without the
production of any false colours. No such correction can be accom-
plished, even theoretically, in a single lens ; but it may be effected
by the combination of two or more, advantage being taken of the
different relations which the refractive and the dispersive powers
bear to each other in different substances. For if we can unite
with a convex lens, whose dispersive power is low as compared to
its refractive power, a concave of lower curvature, whose dispersive
power is relatively high, it is obvious that the Dispersion of the
rays occasioned by the convex lens may be effectually neutralized
by the opposite dispersion of the concave (§ 6) ; whilst the Refract-
ing power of the convex is only lowered by the opposite refraction
of the concave, in virtue of the longer focus of the latter.— No
difficulty stands in the way of carrying this theoretical correction
into practice. For the ' dispersive' power of flint-glass bears so
much larger a ratio to its refractive power than does that of crown-
glass, that a convex lens of the former whose focal length is 7f
inches, will produce the same degree of colour as a convex lens of
crown-glass whose focal length is 4^ inches. Hence a concave lens
of the former material and curvature will fully correct the disper-
sion of a convex lens of the latter ; whilst it diminishes its refrac-
tive power to such an extent only as to make its focus 10 inches.
The correction for Chromatic Aberration in such a lens would be
perfect, if it were not that although the extreme rays — violet and
red — are thus brought to the same focus, the dispersion of the rest
is not equally compensated; so that what is termed a secondary
* This is well seen in the large pictures exhibited by ordinary Oxy-
hydrogen Microscopes.
CONSTRUCTION OF ACHROMATIC LENSES. 43
spectrum is produced, the images of objects seen through such a
lens being bordered on one side with a purple fringe, and on the
other with a green fringe. Moreover, such a lens is not corrected
for Spherical aberration ; and it must of course be rendered free
from this to be of any real service, however complete may be the
freedom of its image from false colours. This double correction
may be accomplished theoretically by the combination of three
lenses, namely, a double -concave of flint placed between two
double-convex of crown, ground to certain curvatures ; and this
method has long been employed in the construction of object-
glasses for Telescopes, which are, by means of it, rendered Achro-
matic,— that is, are enabled to exert their refractive power without
producing either Chromatic or Spherical aberration.
15. It has only been in comparatively recent times, however,
that the construction of Achromatic object-glasses for Microscopes
has been considered practicable ; their extremely minute size
having been thought to forbid the attainment of that accuracy
which is necessary in the adjustment of the several curvatures, in
order that the errors of each of the separate lenses which enters
into the combination, may be effectually balanced by the opposite
errors of the rest. The first successful
attempt was made in this direction, in Fig. 10.
the year 1823, by MM. Selligues and
Chevalier of Paris ; the plan which
they adopted being that of the com-
bination of two or more pairs of lenses,
each pair consisting of a double -
convex of crown-glass, and a plano-
concave of flint. In the next year,
Mr. Tulley, of London, without any
knowledge of what had been accom-
plished in Paris, applied himself (at
the suggestion of Dr. Goring) to the
construction of Achromatic object-
glasses for the Microscope ; and suc-
ceeded in producing a single combi-
nation of three lenses (on the tele- Section ot an AckromaticU bject-
t n , -i , -v £ -i • -I glass, composed of three pairs of
scopic plan), the corrections of which fense^ h £ 3^ each forn£d of a
were extremely complete. This com- double-convex of crown-glass and
bination, however, was not of high a plano-concave of flint ; «6c, its
power, nor of large angular aperture ; Angle of Aperture.
and it was found that these advan-
tages could not be gained without the addition of a second combi-
nation. Professor Amici at Modena, also, who had attempted the
construction of microscopic object-glasses as early as 1812, but,
despairing of success, had turned his attention to the application
of the reflecting principle to the Microscope, resumed his original
labours on hearing of the success of MM. Selligues and Chevalier ;
and, by working on their plan, he produced, in 1827, an Achro-
14
OPTICAL PRINCIPLES OF THE MICROSCOPE.
matic combination which surpassed anything of the same bkind
that had been previously executed.
16. It was in this country that the next important improvements
originated ; these being the result of the theoretical investigations
of Mr. J. J. Lister,* which led him to the discovery of certain
properties in Achromatic combinations that had not been pre-
viously detected. Acting upon the rules which he laid down,
practical Opticians at once succeeded in producing combinations
far superior to any which had been previously executed, both in
wideness of aperture, flatness of field, and completeness of correc-
tion ; and continued progress has been since made in the same
direction, by the like combination of theoretical acumen with
manipulative skill, — the subsequent investigations of Mr. Lister
having led him to suggest new combinations, which were speedily
carried into practical execution.
17. The enlargement of the Angle of Aperture, and the greater
completeness of the corrections, first obtained by the adoption of
Mr. Lister's principles, soon rendered sensible an imperfection in
the performance of these lenses under certain circumstances, which
had previously passed unnoticed ; and the important discovery was
made by Mr. A. Eoss, that a very obvious difference exists in the
precision of the image, according as the object is viewed, with or
without a covering of talc or thin glass ; an Object-glass which is
perfectly adapted to either of these conditions, being sensibly
defective under the other. The mode in which this difference
arises, is explained by Mr. Eoss as follows.f Let o, Fig. 11, be any
Fig. 11.
See his Memoir in the "Philosophical Transactions," for 1829.
f " Transactions of the Society of Arts," Vol. li.
* ADJUSTMENT OF COVERING-GLASS. 45
point of an object ; o p the axial ray of the pencil that diverges
from it ; and o t, o t', two diverging rays, the one near to, the
other remote from, the axial ray. Now if g g g g represent the
section of a piece of thin glass intervening between the object
and the object-glass, the rays o t and o t' will be refracted in their
passage through it, in the directions t r, t' b! ; and on emerging
from it again, they will pass on towards e and e'. Now if the
course of these emergent rays be traced backwards, as by the
dotted lines, the ray e r will seem to have issued from x, and the
ray e' r/ from y ; and the distance x y is an aberration quite
sufficient to disturb the previous balance of the aberrations of the
lens composing the object-glass. The requisite correction may be
effected, as Mr. Ross pointed out, by giving to the front pair
(Fig. 10, i) of the three of which the Objective is composed, an
excess of positive aberration (i.e., by under-correcting it), and by
giving to the other two pairs (2, 3) an excess of negative aberration
(i.e., by over-correcting them), and by mating the distance between
the former and the latter susceptible of alteration. For when the
front pair is approximated most nearly to the other two, and its
distance from the object is increased, its positive aberration is
more strongly exerted upon the other pairs, than it is when the
distance between the lenses is increased, and the distance between
the front pair and the object is diminished. Consequently, if the
lenses be so adjusted that their correction is perfect for an uncovered
object, the front pair being removed to a certain distance from the
others, its approximation to them will give to the whole combi-
nation an excess of positive aberration, which will neutralize the
negative aberration occasioned by covering the object with a thin
plate of glass.* This correction will obviously be more important
to the perfect performance of the combination, the larger is its
angle of aperture ; since the wider the divergence of the oblique
rays from the axial ray, the greater will be the refraction which
they will sustain in passing through a plate of glass, and the
greater therefore will be the negative aberration produced, which,
if uncorrected, will seriously impair the distinctness of the image.
It is consequently not required for low powers, whose angle of aper-
ture is comparatively small, nor for medium powers, so long as
their angle of aperture does not exceed 50° ; and even objectives of
l-4th of an inch focus, whose angle of aperture does not exceed 70c,
may be made to perform very well without adjustment, if their
corrections be originally made perfect for a thickness of glass of
1 -100th of an inch (which is about an average of that with which
objects of the finer kind are usually covered), being not much
deranged by a difference of a few lOOOths of an inch, more or less,
in that amount.
18. For many years the best Objectives contained three sets of
lenses ; and in Objectives of great merit and of high powers, as
* The mode in which this Adjustment is effected will be more fitly de-
scribed hereafter (§§ 127, 128).
46 OPTICAL PRINCIPLES OF THE MICROSCOPE.
many as eight distinct lenses have been combined, the front and
back being triplet combinations, with a doublet between. In this
manner an Angular Aperture of no less than 170° has been
obtained with an Objective of l-12th inch focns ; and it is obvious
that as an increase of divergence of no more than 10° would bring
the extreme rays into a straight line with each other, they would
not enter the lens at all ; so that no further enlargement of the
aperture can be practically useful. Some Opticians, however,
preferred a single front lens,— a plan which Mr. T. Ross stated to
have been followed by Amici, and which was recommended by
Mr. Wenham. In 1863 Messrs. Smith and Beck brought out an
objective of l-20th of an inch focus with a single front lens, which
was remarkable for its working distance from the object ; and
Messrs. Powell and Lealand now use a triplet, a doublet, and a
single front.
19. A principle of construction for Objectives of high power,
first devised by Amici, has of late years been carried out by
M. Hartnack (the successor of Oberhauser) of Paris, and also by
MM. Nachet, with great success ; that, namely, which is known as
the immersion system. English opticians were not very prompt
in adopting this method; but it was ultimately taken up by
Messrs. Powell and Lealand, Ross, Beck, and other London
makers. The l-8ths and l-16ths of the first named artists have
won especial praise; and excellent immersion l-10ths of high
merit and moderate price have been constructed by Messrs.
Beck. Mr. Ross has applied the immersion plan to his 1-Sths
and l-12ths. In America Mr. Tolles has achieved considerable
success ; and amongst German opticians may be mentioned ISTobert,
Schiek, Gundlach, &c. The immersion system consists in the
interposition of a drop of water between the front lens of the objec-
tive and either the object itself or its covering-glass ; so that the
rays which leave it to enter the objective do not pass through air,
but through water. It is easily shown that the loss of light de-
pendent on the reflexion of a portion of the oblique rays from a
surface of glass, whether they are entering or are quitting that
surface, is much less when they pass from water into glass than
from air into glass ; or vice versa, from glass into water than from
glass into air. Consequently when the object (the frustule of a
Diatom for example) is covered with a drop of water into which
the objective dips, there is a much diminished loss of light, alike at
the surface of the object and at that of the lens ; and in the same
manner, when a drop of water is interposed between the front lens
of the objective and the covering-glass of an object mounted in
balsam or in fluid, there is a much diminished loss of light at each
of the glass surfaces. It is of course requisite that the corrections
of the Objectives should be specially adapted to the course of the
rays which enter it from water, instead of from air ; and those
" immersion -lenses" which can only be used as such, are not
universally applicable. One great advantage they possess over
IMMEESION SYSTEM.— WENHAM'S OBJECTIVES. 47
dry objectives, is a considerable increase of working distance and
penetration; and a less exact adjustment for the thickness of
the covering glass is needed for their satisfactory performance.
Messrs. Powell and Lealand, and some other makers, supply dry
fronts to their immersion lenses. Mr. Wenham's latest pattern
will work either wet or dry, with variation of the corrections by
the screw collar.
20. Mr. Wenham's New Object-glasses. — In January, 1873, Mr.
Wenham read a paper before the Eoyal Society on " A New For-
mula for a Microscope Object-glass,"* in which he explained the con-
struction of objectives recently made under his direction by Messrs.
Boss and Co., upon a plan which greatly diminishes the labour and
cost. He observes, that " a pencil of rays exceeding an angle of 40°
from a luminous point cannot be secured with less than three super-
posed lenses of increasing focus and diameter • by the use of which
combination, rays beyond this angle are transmitted with successive
refraction in their course towards the posterior conjugate focus.
Until quite recently, each of these separate lenses has been partly
achromatized by its own .concave lens of flint glass, the surfaces in
contact with the crown glass being of the same radius united with
Canada balsam. The front lens has been made a triple, the middle
a double, and the back again a triple achromatic. This com-
bination therefore consists of eight lenses, and the rays in their
passage are subject to -the errors of sixteen surfaces of glass.
In the new form there are but ten surfaces ; and only one concave
lens of dense flint is employed for correcting four surfaces of crown
glass." — Describing a new l-8th of this combination, Mr. Wenham
says : " The single front is of the usual form, as this is much
alike in all cases. The radius, or focus, of the single plano-
convex bach is about 4| times that of the front, and the focus of
the middle triple three times. "f Very good results have been
obtained with various powers from \ inch upwards, made upon
this plan ; and, besides cheapness, it has the advantage, that the
same front will act in the dry, or in the immersion manner, by
altering the adjustment.
21. We are now prepared to enter upon the application of the
Optical principles which have been explained and illustrated in
the foregoing pages, to the construction of Microscopes. These
are distinguished as Simple and Compound ; each kind having its
peculiar advantages to the Student of Nature. Their essential
difference consists in this : that in the former, the rays of light
which enter the eye of the observer proceed directly from the
object itself, after having been subjected only to a change in their
course; whilst in the latter, an enlarged image of the object is
* "Proc. Eoy. Soc," Vol. xxi. No. 141, p. 111.
t Although Messrs. Eoss have patented these lenses, it is understood that
they have no wish to place unreasonable obstacles in the way of their manu-
facture by other houses.
48 OPTICAL PRINCIPLES OF THE MICEOSCOPE.
formed by a Lens, which image is viewed by the observer through
a simple microscope, as if it were the object itself. The Simple
Microscope may consist of one Lens ; but (as will be presently
shown) it may be formed of two, or even three; these, however,
being so disposed as to produce an action upon the rays of light
corresponding to that of a single lens. In the Compound Micro-
scope, on the other hand, not less than two Lenses must be em-
ployed : one to form the enlarged image of the Object, and this,
being nearest to it, is called the Object-glass ; whilst the other
again magnifies that image, being interposed between it and the
Eye of the observer, and is hence called the Eye-glass. A perfect
Object-glass, as we have seen, must consist of a combination of
lenses ; and the Eye-glass is best combined with another lens in-
terposed between itself and the object-glass, the two together
forming what is termed an Eye-piece (§ 26). — These two kinds of
instrument need to be separately considered in detail.
2. Simple Microscope.
22. In order to gain a clear notion of the mode in which a Single
Lens serves to ' magnify ' minute objects, it is necessary to revert
to the phenomena of ordinary Vision. An Eye free from any
defect has a considerable power of adjusting itself, in such a
manner as to gain a distinct view of objects placed at extremely
varying distances ; but the image formed upon the retina will of
course vary in size with the distance of the object ; and the
amount of detail perceptible in it will follow the same proportion.
To ordinary eyes, however, there is a limit within which no dis-
tinct image can be formed, on account of the too great divergence
of the rays of the different pencils which then enter the eye ;
since the eye is usually adapted to receive, and to bring to a focus,
rays which are parallel or but slightly divergent. This limit is
variously stated at from 5 to 10 inches : but though there are
doubtless many persons whose vision is good at the shorter range,
yet the longer is probably the real limit for persons of ordinary
vision ; who, though they may see objects much nearer the eye, see
little if any more of their details, since what is gained in size is lost
in distinctness. Now the utility of a convex lens interposed between
a near object and the eye, consists in its reducing the divergence of
the rays forming the several pencils which issue from it ; so that
they enter the eye in a state of moderate divergence, as if they
had issued from an object beyond the nearest limit of distinct
vision ; and a well-defined picture is consequently formed upon the
retina. Not only, however, is the course of the several rays in
each pencil altered as regards the rest by this refracting process,
but the course of the pencils themselves is changed, so that they
enter the eye under an angle corresponding with that at which
they would have arrived from a larger object situated at a greater
distance. The picture formed upon the retina, therefore, by any
PRINCIPLES OF SIMPLE MICROSCOPE. 49
object (Fig. 12), corresponds in all respects with one which would
have been made by the same object a b increased in its dimensions
to a b, and viewed at the smallest ordinary distance of distinct
vision. A ' short-sighted ' person, however, who can only see objects
Fig. 12.
Diagram illustrating the action of the Simple Microscope ; a b object ;
A B its magnified image.
distinctly at a distance of two or three inches, has the same power
in his eye alone by reason of its greater convexity, as that which
the person of ordinary vision gains by the assistance of a convex
lens which shall enable him to see at the same distance with equal
distinctness. It is evident, therefore, that the magnifying power
of a single lens, depending as it does upon the proportion between
the distance at which it renders the object visible, and the nearest
distance of unaided distinct vision, must be different to different
eyes. It is usually estimated, however, by finding how many times
the focal length of the lens is contained in ten inches ; since, in
order to render the rays from the object nearly parallel, it must be
placed nearly in the focus of the lens (Fig. 3) ; and the picture is
referred by the mind to an object at the ordinary distance. Thus,
if the focal length of a lens be one inch, its magnifying power for
each dimension will be 10 times, and consequently 100 superficial ;
if its focal distance be only one-tenth of an inch, its magnifying
power will be 100 linear, or 10,000 superficial. The use of the
convex lens has the further advantage of bringing to the eye a
much greater amount of light than would have entered the pupil
from the enlarged object at the ordinary distance, provided its own
diameter be greater than that of the pupil ; but this can only be
the case when its magnifying power is low.
23. It might seem desirable, especially when Lenses of very
high magnifying power are being employed, that their aperture
should be large ; since the light issuing from a minute " object
has then to be diffused over a large picture, and will be propor-
tionally diminished in intensity. But the shorter the focus, the
E
' QPTICAL^P&INCIPLES OF THE MICROSCOPE.
Jess pmst.be., the idiameier of tlie sphere of which the lens forms a
jyyjiS; %rk Unlets the ,4perture be proportionally diminished, the
Spherical and Chromatic aberrations will interfere so mnch with
the distinctness o£ the picture, that the advantages which might
be anticipated from the use of such lenses will be also negatived.
Nevertheless, the Simple Microscope has been an instrument of
extreme value in anatomical research, owing to its freedom from
those errors to which the Compound Microscope, as originally con-
structed, was necessarily subject ; the greater certainty of its indi-
cations being evident from the fact, that the eye of the observer
receives the rays sent forth by the object itself, instead of those
which proceed from an image of that object. — A history of the
means employed by different individuals for procuring Lenses of
extremely short focus, though possessing much interest in itself,
would be misplaced here ; since recent improvements, as will pre-
sently be shown, have superseded the necessity of all these. It may
be stated, however, that Leeuwenhoeck, De la Torre, and others
among the older Microscopists, made great use of small globules
procured by fusion of threads or particles of glass. The most
important suggestion for the improvement of the Simple micro-
scope composed of a single lens, proceeded some years ago from
Sir D. Brewster ; who proposed to substitute diamond, sapphire,
garnet, and other precious stones of high refractive power, for
glass, as the material of single lenses. A lens of much longer
radius of curvature might thus be employed to gain an equal mag-
nifying power ; and the aperture would admit of great extension,
without a proportional increase in the spherical and chromatic
aberrations. This suggestion was carried into practice by Mr.
Pritchard with complete success, as regards the performance of
lenses executed on this plan ; but independently of the costliness
of their material, the difficulties of various kinds in the way of
their execution are such as to render them very expensive ; and as
they are not superior to the combination now to be described, they
have latterly been quite superseded by it. — This combination, first
proposed by Dr. Wollaston, and known as his Doublet, consists of
two plano-convex lenses, whose focal lengths are in the proportion
of one to three, or nearly so, having their convex sides directed
towards the eye, and the lens of shortest focal length nearest the
object. In Dr. Wollaston' s original combination, no perforated
diaphragm (or ' stop ') was interposed ; and the distance between
the lenses was left to be determined by experiment in each case.
A great improvement was subsequently made, however, by the
introduction of a ' stop ' between the lenses, and, by the division
of the power of the smaller lens between two (especially when a
very short focus is required) so as to form a Triplet, as was first
suggested by Mr. Holland.* When combinations of this kind are
well constructed, both the spherical and the chromatic aberrations
* " Transactions of the Society of Arts," Vol. xlix.
SIMPLE MICROSCOPE. — CODDINGTON LENS. 51
are so much, reduced, that the angle of aperture may be conside-
rably enlarged without much sacrifice of distinctness ; and hence
for all powers above l-4th inch focus, Doublets and Triplets are
far superior to Single Lenses. The performance of even the best
of these forms of Simple microscope, however, is so far inferior to
that of a good Compound microscope, as now constructed upon the
Achromatic principle, that no one who has the command of the
latter form of instrument would ever use the higher powers of
the former. It is for the prosecution of observations and for the
carrying on of dissections which only require low powers, that the
Simple microscope is to be preferred ; and consequently, although
doublets and triplets afforded the best means of obtaining a high
magnifying power, before Achromatic lenses were brought to their
present perfection, they are now comparatively little employed.
24. Another form of Simple magnifier, possessing certain ad-
vantages over the ordinary double-convex lens, is that commonly
known by the name of the ' Coddington' lens.* The first idea of
it was given by Dr. "Wollaston, who proposed to apply two plano-
convex or hemispherical lenses by their plane sides, with a ' stop'
interposed, the central aperture of which should be equal to 1-oth
of the focal length. The great advantage of such a lens is, that
the oblique pencils pass, like the central ones, at right angles to
the surface ; and that they are consequently but little subject to
aberration. The idea was further improved upon by Sir D.
Brewster, who pointed out that the same end would be much
better answered by taking a sphere of glass, and grinding a deep
groove in its equatorial part, which should be then filled with
opaque matter, so as to limit the central aperture. Such a lens
gives a large field of view, admits a considerable amount of light,
and is equally good in all directions ; but its power of definition is
by no means equal to that of an achromatic lens, or even of a
doublet. This form is chiefly useful, therefore, as a Hand-
Magnifier, in which neither high power nor perfect definition is re-
quired ; its peculiar qualities rendering it superior to an ordinary
lens, for the class of objects for which a hand-magnifier of medium
power is required. It should be stated, however, that many of the
magnifiers sold as ' Coddington' lenses are not really portions of
spheres, but are manufactured out of ordinary double-convex
lenses, and are destitute, therefore, of many of the above advan-
tages.— The ' Stanhope' lens somewhat resembles the ' Coddington'
in appearance, but differs from it essentially in properties.— It is
nothing more than a double-convex lens, having two surfaces of
unequal curvatures, separated from each other by a considerable
thickness of glass ; the distance of the two surfaces from each
other being so adjusted that when the most convex is turned
towards the eye, minute objects placed on the other surface shall
* This name, however, is most inappropriate ; since Mr. Coddington neither
was, nor ever claimed to be, the inventor of the mode of construction by which
this lens is distinguished.
E 2
52 OPTICAL PBINCIPLES OF THE MICROSCOPE.
be in the focus of the lens. This is an easy mode of applying a
rather high magnifying power to scales of butterflies' wings, and
other similar flat and minute objects, which will readily adhere to
the surface of the glass ; and it also serves to detect the presence
of the larger Animalcules or of crystals in minute drops of fluid,
to exhibit the ' eels' in paste or vinegar, &c. &c. — A modified form
of the ' Stanhope' lens, in which the surface remote from the eye
is plane instead of convex, has been brought out in France under
the name of ' Stanhoscope,' and has been especially applied to the
enlargement of minute pictures photograjmed on its plane surface
in the focus of its convex surface. A good ' Stanhoscope,' magni-
fying from 100 to 150 diameters, is the most convenient form of
Hand-magnifier for the recognition of Diatoms, Infusoria, &c. ;
all that is required being to place a minute drop of the liquid to
be examined on the plane surface of the lens, and then to hold it
up to the light.*
3. Compound Microscope.
25. In its most simple form, this instrument consists of only two
lenses, the Object-glass and the Eye-glass : the former, c d
(Fig. 13), receiving the rays of light direct from the object, a b,
which is brought into near proximity to it, forms an enlarged and
inverted image a' b' at a greater distance on the other side ; whilst
the latter, l m, receives the rays which are diverging from this
image, as if they proceeded from an object actually occupying its
position and enlarged to its dimensions, and these it brings to the
eye at e, so altering their course as to make that image appear far
larger to the eye, precisely as is the case of the Simple microscope
(§ 22). — It is obvious that, by the use of the very same Lenses, a
considerable variety of magnifying power may be obtained, by
merely altering their position in regard to each other and to the
object ; for if the Eye-glass be carried farther from the Object-glass,
whilst the object is approximated nearer to the latter, the image
a' b' will be formed at a greater distance from it, and its dimensions
will consequently be augmented. If, on the other hand, the Eye-
glass be brought nearer to the Object-glass, whilst the object is re-
moved farther from it, the distance of the image will be shortened,
and its dimensions proportionably diminished. We shall hereafter
see that this mode of varying the magnifying power of Compound
Microscopes may be turned to good account in more than one
mode (§§ 68, 69) ; but there are limits to the use which can be ad-
vantageously made of it. The amplification may also be varied by
altering the magnifying power of the Eye-glass ; but here, too,
there are limits to the increase ; since defects of the object-glass
which are not perceptible when its image is but moderately
enlarged, are brought into injurious prominence when the imperfect
* See " Quart. Journ. of Microsc. Science," Vol. vii., N.S., p. 263.— Of the
Stanhoscopes sold by Toy-dealers at a very low price, only a part are really
serviceable ; care is requisite, therefore, in the selection.
PRINCIPLES OF COMPOUND MICROSCOPE.
53
image is amplified to a much greater extent. In practice, it is
generally found much better to vary the power by employing
Fig. 13.
Fig. 14.
Diagram of simplest form of
Compound Microscope*
Diagram of complete
Compound Microscope.
Object-glasses of different foci ; an object-glass of long focus form-
ing an image which is not at many times the distance of the
object from the other side of the lens, and which, therefore, is not
of many times its dimension ; whilst an object-glass of short
U OPTICAL PRINCIPLES OF THE MICROSCOPE.
focus requires that the object should be so nearly approximated to
it, that the distance of the image is a much higher multiple of that
of the object, and its dimensions are proportionably larger (§ 8). —
In whatever mode increased amplification may be obtained, two
things must always result from the change : the portion of the
surface of the object of which an image can be formed must be
diminished; and the quantity of light spread over that image
must be proportionably lessened.
26. In addition to the two lenses of which the Compound
Microscope essentially consists, another (Fig. 14, p r) is usually
introduced between the Object-glass and the image formed by it.
The purpose of this lens is to change the course of the rays in
such a manner, that the image may be formed of dimensions not
too great for the whole of it to come within the range of the Eye-
glass ; and as it thus allows more of the object to be seen at once,
it is called the Field-glass. It is now usually considered, however,
as belonging to the ocular end of the instrument, — the eye-glass
and the field-glass being together termed the Eye-piece. "Various
forms of this Eye-piece have been proposed by different Opticians ;
and one or another will be preferred, according to the purpose for
which it may be required. That which it is most advantageous to
employ with Achromatic Object-glasses, to the performance of
which it is desired to give the greatest possible effect, is termed the
Huyghenian ; having been employed by Huyghens for his tele-
scopes, although without the knowledge of all the advantages
which its best construction renders it capable of affording. It
consists of two plano-convex lenses (e e and f f, Fig. 14), with
their plane sides towards the eye ; these are placed at a distance
equal to half the sum of their focal lengths ; or, to speak with
more precision, at half the sum of the focal length of the eye-glass,
and of the distance from the field-glass at which an image of the
object-glass would be formed by it. A ' stop' or diaphragm, b b,
must be placed between the two lenses, in the visual focus of the
Eye-glass, which is, of course, the position wherein the image of
the object will be formed by the rays brought into convergence by
their passage through the field-glass. — Huyghens devised this
arrangement merely to diminish the Spherical aberration ; but it
was subsequently shown by Boscovich that the Chromatic disper-
sion was also in great part corrected by it. Since the introduction
of Achromatic Object-glasses for Compound Microscopes, it has
been further shown that nearly all error may be avoided by a
slight over-correction of these ; so that the blue and red rays may
be caused to enter the eye in a parallel direction (though not
actually coincident), and thus to produce a colourless image.
Thus let l m n (Fig. 15) represent the two extreme rays of three
pencils, which, without the field-glass, would form a blue image
convex to the eye-glass at b b, and a red one at n r ; then, by the
intervention of the field-glass, a blue image, concave to the eye-
glass, is formed at t' b', and a red one at b! k'. As the focus of
HUYGHENIAN EYE-PIECE.
Fig
the Eye-glaiss is shorter for blue rays than for red rays by just the
difference in the place of these images, their rays, after refraction
by it, enter the eye in a parallel
direction, and produce a picture
free from false colour. If the object-
glass had been rendered perfectly
achromatic, the blue rays, after
passing through the field-glass,
would have been brought to a focus
at 6, and the red at r ; so that an
error would be produced, which
would have been increased instead
of being corrected by the eye-glass.
Another advantage of a well-con-
structed Huyghenian eye-piece is,
that the image produced by the
meeting of the rays after passing
through the field-glass, is by it
rendered concave towards the eye-
glass, instead of convex, so that
every part of it may be in focus
at the same time, and the field of
view thereby rendered flat.* — Two
or more Huyghenian Eye-pieces, of
different magnifying powers, known adapted to over-corrected Achro-
as A, B, C, &c, are usually sup- matic Objectives,
plied with a Compound Microscope.
The utility of the higher powers will mainly depend upon the ex-
cellence of the Objectives ; for when an Achromatic combination of
small aperture, which is sufficiently well corrected to perform very
tolerably with a low eye-piece, is used with an Eye-piece of higher
magnifying power (commonly spoken of as a 'deeper' one), the
image may lose more in brightness and in definition than is gained
by its amplification ; whilst the image given by an Objective of
large angular aperture and very perfect correction, shall sustain so
little loss of light or of definition by ' deep eye-piecing,' that the in-
crease of magnifying power shall be almost clear gain. Hence the
modes in which different Objectives of the same power, whose per-
formance with shallow eye-pieces is nearly the same, are re-
spectively affected by deep eye-pieces, afford a good test of their
respective merits ; since any defect in the corrections is sure to be
brought out by the higher amplification of the image, whilst a defi-
ciency of aperture is manifested by the want of light. — The work-
* Those who desire to gain more information npon this subject than they
can from the above notice of it, may be referred to Mr. Varley's investigation
of the properties of the Huyghenian Eye-piece, in the 51st volume of the
" Transactions of the Society of Arts ;" and to the article " Microscope," by
Mr. Boss, in the "Penny Cyclopgedia," reprinted, with additions, in the
" English Cyclopaedia."
53 OPTICAL PRINCIPLES OF THE MICROSCOPE.
ing Microscopist will generally find the A eye-piece the most
suitable, B being occasionally employed when a greater power is
required to separate details, whilst C and others still deeper are
useful for the purpose of testing the goodness of Objectives, or for
special purposes with those of the finest quality. When great
penetration or " focal depth" is required, low objectives and deep
eye-pieces will often be found convenient.
27. An Eye-piece is sometimes furnished with Achromatic Micro-
scopes, especially for micrometric purposes, which, though com-
posed of only two plano-convex lenses, differs essentially in its
construction from the Huyghenian ; the field-glass having its con-
vex side upwards, and being so much nearer to the eye-glass that
the image formed by the object-glass does not he above (as at B B,
Fig. 14), but below it. This 'positive' eye-piece, which is known
as Bamsden's, gives a very distinct view in the central portion of
the field ; but, as it does not, like the Huyghenian, correct the con-
vexity of the image formed by the object-glass, but rather increases
it, the marginal portions of the field of view, when the centre is in
focus, are quite indistinct. Hence this Eye-piece cannot be recom-
mended for ordinary use ; and its chief value to the Microscopist
has resulted from its adaptation to receive a divided glass-micro-
meter, which may be fitted into the exact plane wherein the image
is formed by the object-glass, so that its scale and that image are
both magnified together by the lenses interposed between them and
the eye. We shall hereafter see, however, that the same end may
be so readily attained with the Huyghenian eye-piece (§ 77), that
no essential advantage is gained by the use of that of Ramsden. —
For viewing large flat objects, such as transverse sections of Wood
(Plate xu.) or of Echinus-spines (Plate n. Fig. 1), under low mag-
nifying powers, the Eye-piece known as Kellners may be employed
with advantage. In this construction the Field-glass, which is a
double-convex lens, is placed in the focus of the Eye-glass, without
the interposition of a diaphragm ; and the Eye-glass is an achromatic
combination of a plano-concave of flint with a double-convex of
crown, which is slightly under-corrected, so as to neutralize the
over-correction given to the Objectives that are ordinarily used
with Huyghenian eye-pieces (§ 26). A flat well-illuminated field
of as much as fourteen inches in diameter may thus be obtained
with very little loss of light ; but, on the other hand, there is a
certain impairment of defining power, which renders the Kellner
eye-piece unsuitable for objects presenting minute structural de-
tails ; and it is an additional objection that the smallest speck or
smear upon the surface of the field-glass is made so unpleasantly
obvious, that the most careful cleansing of that surface is required
every time that this Eye-piece is used. Hence it is better fitted
for the occasional display of objects of the character already
specified, than it is for the ordinary wants of the working Micro-
scopist.
PRINCIPLES OF STEEEOSCOPIC VISION. ' 57
4. Stereoscopic Binocular Microscope.
28. The admirable invention of the Stereoscope by Professor
AVheatstone, has led to a general appreciation of the valne of the
conjoint use of both eyes in conveying to the mind a notion of the
solid forms of objects, snch as the nse of either eye singly does not
generate with the like certainty or effectiveness. And after several
attempts, which were attended with various degrees of success, the
principle of the Stereoscope has now been applied to the Micro-
scope, with an advantage which those only can truly estimate, who
(like the Author) have been for some time accustomed to work with
the Stereoscopic Binocular* upon objects that are peculiarly
adapted to its powers. As the result of this application cannot be
rightly understood without some knowledge of one of the funda-
mental principles of Binocular vision, a brief account of this will
be here introduced. — All vision depends in the first instance on the
formation of a picture of the object upon the retina of the Eye, just
as the Camera Obscura forms a picture upon the ground glass
placed in the focus of its lens. But the two images that are formed
by the two Eyes respectively, of any solid object that is placed at
no great distance in front of them, are far from being identical ; the
perspective projection of the object varying with the point of view
from which it is seen. Of this the reader may easily convince him-
self by holding up a thin book in such a position that its back shall
be at a moderate distance in front of the nose, and by looking at
the book, first with one eye and then with the other ; for he will
find that the two views he thus obtains are essentially different, so
that if he were to represent the book as he actually sees it with
each eye, the two pictures would by no means correspond. Yet on
looking at the object with the two eyes conjointly, there is no
confusion between the images, nor does the mind dwell on either
of them singly; but from the union of the two a conception is
gained of a solid projecting body, such as could only be otherwise
acquired by the sense of Touch. Wow if, instead of looking at the
solid object itself, we look with the right and left eyes respectively
at pictures of the object, corresponding to those which would be
formed by it on the retinae of the two eyes if it were placed at a
moderate distance in front of them, and these visual pictures are
brought into coincidence, the same conception of a solid projecting
form is generated in the mind, as if the object itself were there.
The Stereoscope — -whether in the forms originally devised by Pro-
fessor Wheatstone, or in the popular modification long subsequently
introduced by Sir D. Brewster — simply serves to bring to the two
Eyes, either by reflexion from mirrors, or by refraction through
* It has become necessary to distinguish the Binocular Microscope which
gives true Stereoscopic effects by the combination of two dissimilar pictures,
from a Binocular which simply enables us to look with both eyes at images
which are essentially identical (§ 67).
58 OPTICAL PRINCIPLES OF THE MICROSCOPE.
prisms or lenses, the two dissimilar Pictures which would accurately
represent the solid object as seen by the two eyes respectively ;
throwing these on the two retinas in the precise positions they would
have occupied if they had been formed there direct from the solid
Object, of which the Mental Image (if the pictures have been cor-
rectly taken) is the precise counterpart.* Thus in Fig. 16 the
upper pair of pictures, a, b, when combined in the Stereoscope,f
Fig. 16.
A
C
suggest the idea of a projecting truncated Pyramid, with the
small square in the centre, and the four sides sloping equally
away from it ; whilst the lower pair, c, d (which are identical with
the upper, but are transferred to opposite sides), no less vividly
"bring to the mind the visual conception of a receding Pyramid, still
with the small square in the centre, but the four sides sloping
equally towards it.
29. Thus we see that by simply crossing the Pictures in the
Stereoscope, so as to bring before each eye the picture taken for
the other, a * Conversion of Relief ' is produced in the resulting
solid image ; the projecting parts being made to recede, and the re-
ceding parts brought into relief. In like manner when several objects
* Although it is a comparatively easy matter to draw in outline two dif-
ferent perspective projections of a Geometrical Solid, such as those which are
represented in Fig. 16, it would have been quite impossible to delineate land-
scapes, buildings, figures, &c, with the same precision ; and the Stereoscope
would never have obtained the appreciation it now enjoys, but for the ready
means supplied by Photography of obtaining simultaneous pictures, perfect in
their perspective, and truthful in their lights and shades, from two different
points of view so selected as to give an effective Stereoscopic combination.
t This combination may be made without the Stereoscope, by looking at
these figures with the axes of the eyes brought into convergence upon a some-
what nearer point, so that A is made to fall on B, and c on p.
STEREOSCOPIC AND PSEUDOSCOPIC VISION. 59
are combined in the same picture, their apparent relative distances
are reversed; the remoter being brought nearer, and the nearer
carried backwards ; so that (for example) a Stereoscopic photo-
graph, representing a man standing in irorit of a mass of ice, shall,
by the crossing of the pictures, make the figure appear as if im-
bedded in the ice. A like conversion of relief may also be made
in the case of actual solid objects by the use of the Pseudoscope, an
instrument devised by Professor Wheatstone, which has the effect
of reversing the perspective projections of objects seen through it
by the two eyes respectively ; so that the interior of a basin or
jelly-mould is made to appear as a projecting solid, whilst the
exterior is made to appear hollow. Hence it is now customary to
speak of Stereoscopic Vision as that in which the conception of the
true natural relief of an object is called-up in the mind by the
normal combination of the two perspective projections formed of it
by the right and left eyes respectively ; whilst by Pseudoscopic
Vision, we mean that conversion of relief which is produced by the
combination of two reversed perspective projections, whether these
be obtained directly from the Object (as by the Pseudoscope), or
from ' crossed ' Pictures (as in the Stereoscope). It is by no means
every Solid Object, however, or every pair of Stereoscopic Pictures,
which can become the subject of this conversion. The degree of-
facility with which the ' converted' form can be apprehended by the
Mind, appears to have great influence on the readiness with which
the change is produced. And while there are some objects— the
interior of a plaster mask of a face, for example — which can always
be ' converted ' (or turned inside-out) at once, there are others
which resist such conversion with more or less of persistence.
30. ISTow it is easily shown theoretically, that the Picture of any
projecting Object seen through the Microscope with only the right -
hand half of an objective having an even moderate angle of aper-
ture, must differ sensibly from the picture of the same object
received through the left-hand of the same objective ; and further,
that the difference between such picture must increase with the
Angle of Aperture of the objective. This difference may be prac-
tically made apparent by adapting a ' stop' to the objective, in
such a manner as to cover either the right or the left half of its
aperture ; and by then carefully tracing the outline of the object as
seen through each half. But it is more satisfactorily brought into
view by taking two Photographic pictures of the object, one through
each lateral half of the Objective; for these pictures when properly
paired in the Stereoscope, give a magnified image in relief, bringing
out on a large scale the solid form of the object from which they
were taken. What is needed, therefore, to give the true Stereo-
scopic power to the Microscope, is a means of so bisecting the cone
of rays transmitted by the objective, that of its two lateral halves
one shall be transmitted to the right and the other to the left eye.
If, however, the image thus formed by the right half of the objective
of a Compound Microscope were seen by the right eye, and that
60
OPTICAL PRINCIPLES OF THE MICROSCOPE.
formed by the left half were seen by the left eye, the resultant
conception would be not stereoscopic but pseudoscopic ; the pro-
jecting parts being made to appear receding, and vice versa. The
reason of this is, that as the Microscope itself reverses the picture
(§ 25), the rays proceeding through the right and the left hand
halves of the Objective must be made to cross to the left and the
right Eyes respectively, in order to correspond with the direct view
of the object from the two sides ; for if this second reversal does
not take place, the effect of the first reversal of the images produced
by the Microscope exactly corresponds with that produced by the
' crossing' of the Pictures in the Stereoscope, or by that reversal of
the two perspective projections formed direct from the Object which
is effected by the Pseudoscope (§ 29). From want of a due appre-
ciation of this principle (the truth of which can now be practically
demonstrated, § 34), the earlier attempts at producing a Stereo-
scopic Binocular Microscope tended rather to produce a ' Pseudo-
scopic conversion' of the objects viewed by it, than to represent
them in their true relief.
31. Nachet's Stereoscopic Binocular. — The first really satisfactory
solution of the problem was that worked out by MM. Nachet ;
whose original Binocular was constructed on the method shown in
Fig. 17. The cone of rays issuing from the upper end of the Ob-
Fig. 17.
Arrangement of Prisms in Nachet's Stereoscopic
Binocular Microscope.
jective meets the flat surface of a Prism p whose section is an equi-
lateral triangle ; and is divided by reflexion within this prism into
NACHET'S STEREOSCOPIC BINOCULAR.
61
two lateral halves, which, cross each other in its interior. For the
rays of ab forming the right half of the cone, impinging very
obliquely on the internal face of the prism, suffer total reflexion
(§ 2), emerging through its left side at right angles to its surface,
and therefore undergoing no refraction ; whilst the rays a b' form-
ing the left half of the cone, are reflected in like manner towards
the right. Each of these pencils is received by a lateral Prism,
which again changes its direc-
tion, so as to render it parallel
to its original course ; and thus
the two halves a b and a V of
the original pencil are com-
pletely separated from each
other, the former being re-
ceived into the left-hand body of
the Microscope (Fig. 18), and the
latter into its right-hand body.
These two bodies are parallel ;
and, by means of an adjusting
screw at their base, which alters
the distance between the cen-
tral and the lateral Prisms, they
can be separated-from or ap-
proximated-towards each other,
so that the distance between
their axes can be brought into
exact coincidence with the dis-
tance between the axes of the
Eyes of the individual observer.
This instrument sdves true Ste-
reoscopic projection to the con-
joint image formed by the men-
tal fusion of the two distinct
pictures ; and with low powers
of moderate angular aperture
its performance is highly satis-
factory. There are, however,
certain drawbacks to its general utility. First, every ray of each
pencil suffers two reflexions, and has to pass through four surfaces ;
this necessarily involves a considerable loss of light, with a further
liability to the impairment of the image by the smallest want of
exactness in the form of either of the prisms. Second, the mecha-
nical arrangements requisite for varying the distance of the bodies,
involve an additional liability to derangement in the adjustment
of the prisms. Third, the instrument can only be used for its
own special purpose ; so that the observer must also be provided
with an ordinary Monocular Microscope, for the examination of
objects unsuited to the powers of his Binocular. Fourth, the
parallelism of the bodies involves parallelism of the axes of the
Nachet's Stereoscopic Binocular.
02
OPTICAL PRINCIPLES OF THE MICROSCOPE.
Eyes,
any
the maintenance of
of time is
length
Fig. 19. observer's
which for
fatigning.
32. Wenham's Stereoscopic Bino-
cular.— All these objections are over-
come in the admirable arrangement
devised by the ingenuity of Mr. Wen-
ham. In Mr. Wenhanvs Binocular
the cone of rays proceeding upwards
from the objective is divided by the
interposition of a prism of the peculiar
form shown in Fig. 19 ; this is so
placed in the tube which carries the
objective (Figs. 20, 21, a), as only to
interrupt one half, a c, of the cone, the
other half, a b, going on continuously
Wenham's Prism. to the eye-piece of the principal body
it, in the axis of which the objective
is placed. The interrupted half of the cone (Fig. 19, a), on its
entrance into the Prism, is scarcely subjected to any refraction,
Fig. 20.
Fig. 21.
Wenham's Stereoscopic Binocular Microscope.
WENHAM'S STEREOSCOPIC BINOCULAR. 63
since its axial ray is perpendicular to the surface it meets ; within
the prism it is subjected to two reflexions at b and c, which send
it forth again obliquely in the line d towards the eye-piece of
the secondary body l; and since at its emergence its axial ray
is again perpendicular to the surface of the glass, it suffers no
more refraction on passing out of the prism than on entering it.
By this arrangement the image received by the right Eye is formed
by the rays which have passed through the left half of the Ob-
jective, and which have come on without any disturbance what-
ever ; whilst the image received by the left Eye is formed by the
rays which have passed through the rightlaaM. of the Objective, and
which have been subjected to two reflexions within the prism, pass-
ing through only two surfaces of glass. The adjustment for the
variation of distance between the axes of the eyes in different indi-
viduals, is made by drawing-out or pushing-in the Eye-pieces, which
are moved consentaneously by means of a milled-head, as shown in
Fig. 21. — ~Now although it may be objected to Mr. Wenham's
method (1), that as the rays which pass through the prism and are
obliquely reflected into the secondary body, traverse a longer dis-
tance than those which pass on uninterruptedly into the principal
body, the picture formed by them will be somewhat larger than
that which is formed by the other set ; and (2) that the picture
formed by the rays which have been subjected to the action of the
prism must be inferior in distinctness to that formed by the unin-
terrupted half of the cone of rays, — these objections are found to have
no practical weight. For it is well known to those who have experi-
mented upon the phenomena of Stereoscopic vision, (1) that a slight
difference in the size of the two pictures is no bar to their perfect
combination ; and (2) that if one of the pictures be good, the full
effect of relief is given to the image, even though the other picture
be faint and imperfect, provided that the outlines of the latter are
sufficiently distinct to represent its perspective projection. Hence
if, instead of the two equally half-good pictures which are obtain-
able by MM. JSTachet's original construction, we had in Mr. Wen-
ham's one good and one indifferent picture, the latter would be de-
cidedly preferable. But, in point of fact, the deterioration of the
second picture in Mr. Wenham's arrangement is less consider-
able than that of both pictures in the original arrangement of
MM. Cachet; so that the optical performance of the "Wenham
Binocular is in every way superior. It has, in addition, these fur-
ther advantages over the preceding : — First, the greater comfort in
using it (especially for some length of time together), which results
from the convergence of the axes of the Eyes at their usual angle
for moderately-near objects; second, that this Binocular arrange-
ment does not necessitate a special instrument, but may be applied
to any Microscope which is capable of carrying the weight of the
secondary body ; for the prism is so fixed in a moveable frame that
it may in a moment be taken out of the tube or replaced therein,
so that when it has been removed, the principal body acts in every
64
OPTICAL PRINCIPLES OF THE MICROSCOPE.
Fig. 22.
respect as an ordinary Microscope, the entire cone of rays passing
uninterruptedly into it ; and third, that the simplicity of its con-
struction renders its derangement almost impossible .*
33. Stephenson's Binocular Microscope. — A new form of Stereo-
scopic Binocular has been recently introduced by Mr. Stephenson ;
the plan of which will be readily understood from the subjoined
figures, a a are two prisms which are fixed to a cell projecting below
the female screw of the Microscope, so that when the objective is
attached they are brought close to its pos-
terior combination, and catch the light-
rays very soon after their emergence.
The prisms " are each "68 of an inch in
length, '412 of an inch in width, and "2 of
an inch in thickness. They are inclined
to each other at an angle of 4f°; this
makes the angle between the bodies 9i°,
and the imaginary point towards which
the eyes converge nearly 15 inches. "f
The two pencils of rays b b diverging a.t
an equal angle on each side of a line
perpendicular to the optical axis of the
instrument, pass upwards through the
two bodies to the eye-pieces ; the light
is thus equally divided between the two
images, which is not the case with Mr.
Wenham's construction ; and the recep-
tion of the rays by the prism placed close
to the back combination of the objective,
enables high power to be used with per-
fect definition. In the Wenham con-
struction one tube of the Microscope is upright, and the other
slanting. This is frequently a source of inconvenience, espe-
cially to persons whose eyes are wide apart, as they are compelled
to squint more or less with one eye. In Mr. Stephenson's pattern
both eyes are directed so that their optic axes converge equally
towards the object, as in natural vision, and fatigue is avoided.
Difficult test objects are well shown by this arrangement with
objectives of l-8th and l-16th inch focus ; but it is essential that
the prisms should be of the mott perfect workmanship, as very
slight errors in the accuracy of their angles and surfaces would in-
troduce intolerable confusion. The first instrument of this kind
was made for Mr. Stephenson by the late Mr. Thomas Boss ;
Mr. Browning subsequently undertook its construction, and has
carried it to complete success. — While, however, the preceding
* The Author cannot allow this opportunity to pass without expressing his
sense of the liberality with which Mr. Wenham freely presented to the Public
this important invention, by which there can be no doubt that he might have
largely profited if he had chosen to retain the exclusive right to it.
| " Monthly Microscopical Journal," April, 1872.
STEPHENSON'S ERECTING BINOCULAR.
remarks indicate points of superiority in Mr. Stephenson's plan
to Mr. Wenham's, the latter possesses the advantage of not in-
terfering with the monocular nse of the instrument. By sliding
his prism out or in, either Monocular or Binocular vision is imme-
diately attainable, and in the former case with the whole cone of
rays. Of course it is easy to look down one tube only of the
Stephenson Microscope ; but then only half the cone of rays reaches
the eye, and that half must partake
of the error — however trifling — which
every prism introduces. For Mono-
cular vision it would be desirable to
have a separate body.
Erecting Arrangement. — When the
rays passing through the two prisms
a a are suffered to enter the tubes of
the Microscope without deflexion, the
general arrangement of the Stephen-
son Microscope is the same as of
Mr. Wenham's ; but by interposing a
prism or plane mirror, as shown in
Fig. 23, each half of the cone is de-
flected, so that rays entering it at c b
strike against a b, and being reflected,
pass out through c a in the direction
of the dotted lines. They are then
able to enter the tubes in the position
shown in Fig. 24, which are inclined at an angle convenient for
observation when the stage is horizontal. This arrangement is ex-
tremely convenient when dis-
sections have to be prepared,
or objects viewed in uncovered
fluids. A plane silvered mir-
ror may be substituted for the
prism, and with some advan-
tage, when the instrument is
not likely to be exposed to in-
jurious vapours ; but, which-
ever is employed, the finest
workmanship is indispensable.
The result of the second re-
flexion occasioned by the plane
mirror, or prism, is to erect
the object. — Mr. Stephenson's
arrangement is obviously most
complete when adapted to the
Ross model ; and rf provided
with a separate tube for mo-
nocular vision, this might carry
Fig. 24.
needful for using Dr. Pigott's Searcher.
the drawtube, rackwork,
66
OPTICAL PRINCIPLES OF THE MICROSCOPE.
Fig. 25.
Polariscope Arrangement. — If the tubes, as shown in Fig. 24,
are inclined at an angle of 66^°, or twice the complement of the
polarizing angle, the reflexion from the plane mirror takes place at
the polarizing angle 56|°. When, therefore, the plane mirror or
prism is withdrawn, and a highly polished mirror of black glass
substituted, it acts as an analyser, with some decided advantages
over the JSTicol-prisms, but without being capable of rotation.
Condenser for Stephenson's Binocular. — On reference to Fig. 22,
representing the Stephenson prism in the cell of the objective, it
will be seen that the lower edges of the prism
are, so to speak, in the way of the central por-
tion of the cone of rays emerging from the ob-
jective. To remedy slight errors occasioned by
this condition, Mr. Stephenson has contrived a
condenser consisting of two deep cylindrical
lenses a and b, whose focal lengths are as 2*3 to 1,
with their curved faces opposed to each other,
as shown in section a c, that with the lesser
convexity having its plane side downwards to-
wards the stage mirror. Under this combina-
tion slides a moveable stop, with two circular
openings, as shown in Fig. 26. The light passes
in two pencils, one through each aperture ; and if the lamp em-
ployed is placed in front of the instrument, each eye receives a
completely equal illumination, and no confusion can occur from
rays impinging on the lower
Ftg. 26. ends of the prisms. With
this arrangement the Po-
dura markings are shown as
figured by the late Richard
Beck ; but the curvatures
of the scale come out with
the distinctness peculiar to
Binocular vision. This con-
denser is made by Mr.
Browning.
34. Stereoscopic Binocu-
lar Eye-piece. — An ordinary
Microscope may be con-
verted into a Stereoscopic
Binocular, by an arrange-
ment of prisms devised by
Professor Smith, of Kenyon
College, U.S. ; which corresponds in principle with that originally
adopted by MM. Nachet (Fig. 17), but is made on a larger scale,
and is inserted into the upper part of the body instead of into the
lower, so as to divide the pencils of rays near the plane at which
they would form the image into two lateral halves, according as they
have proceeded from the opposite lateral halves of the Objective.
NACHET'S STEKEO-PSEUDOSCOPIC BINOCULAR. 67
These pencils are reflected back to their own sides by the median
Prism ; and each set, received and reflected upwards by one of the
lateral prisms, forms its image in its own Eye-piece, the two images
combining Stereoscopically, just as if the pencils which form them
had been separated at the lower end of the body. — This arrange-
ment has the advantage of being capable of nse with high powers ;
but it involves a decided loss of light and of definition.
35. Nacliefs Stereo-pseudoscopic Binocular. — An ingenious
modification of Mr. Wenham's arrangement has since been intro-
duced by MM. Nachet, which has the attribute altogether peculiar
to itself, of giving to the image either its true Stereoscopic pro-
jection, or a Pseudoscopic ' conversion of relief,' at the will of the
observer. This is accomplished by the use of two Prisms, one of
them (Fig. 27, a) placed over the cone of rays proceeding upwards
Fig. '27.
Arrangement of Prisms in Nachet's Stereo-pseudoscopic Binocular : —
1, for Stereoscopic ; 2, for Pseudoscopic effect.
from the objective, and the other (b) at the base of the secondary
or additional body, which is here placed on the right (Fig. 28).
The Prism a has its upper and lower surfaces parallel ; one of its
lateral faces inclines at an angle of 45c, whilst the other is vertical.
When this is placed in the position 1, so that its inclined surface
lies over the left half (I) of the cone of rays, these rays, entering
the prism perpendicularly (or nearly so) to its inferior plane sur-
face, undergo total reflexion at its oblique face, and being thus
turned into the horizontal direction, emerge through the vertical
surface at right angles to it. They then enter the vertical face of
the other Prism b ; and after suffering reflexion within it, are
transmitted upwards into the right-ha>n& body r', passing out of
the prism perpendicularly to the plane of emersion, which has
such an inclination that the right-hand or secondary body
(r, Fig. 28) may diverge from the left or principal body at a
suitable angle. On the other hand, the right half (r) of the cone
of rays passes upwards, without essential interruption, through
i 2
63
OPTICAL PRINCIPLES OF THE MICROSCOPE.
Fig. 28.
the two parallel surfaces of the prism a, into the left-hand body
(V), and is thns crossed by the other in the interior of the prism.
But if the Prism a be pushed over towards the right (by pressing
the button a, Fig. 28), so as to leave the left half of the objective
uncovered (as in Mr. Wenham's arrangement), that half (I) of the
cone of rays will go on without any interruption into the left-
hand body {V), whilst the right half (r r' will be reflected by the
oblique face of the prism into the horizontal direction) will emerge
at its vertical face, and being received hy the second prism, b, will
be directed by it into the right-hand, body
(r'). The adjustment for the distance be-
tween the axes of the Eyes is made by
turning the milled-head b, Fig. 28, which,
by means of a screw-movement, acts upon a
moveable chariot that carries the prism b
and the secondary body b, the base of which
is implanted upon it. — ISTow in the^rs^ posi-
tion, the two halves of the cone of rays
being made to cross into the opposite bodies,
true Stereoscopic relief is given to the image
formed by their recombination, just as in the
arrangements previously described. But
when, in the second position, each half of
the cone passes into the body of its own
side, so that the reversal of the images pro-
duced by the Microscope itself (§ 25) is no
longer corrected by the crossing of the two
pencils separated by the Prism a, a Pseu-
doscopic effect, or ' conversion of relief,' is
produced, the projections of the surface of
the object being represented as hollows,
and its concavities turned into convexities.
The suddenness with which this conversion
is brought about, without any alteration
in the position either of the Object or of the Observer, is a pheno-
menon which no intelligent person can witness without interest ;
whilst it has a very special value for those who study the Physiology
and Psychology of Binocular vision* M. ISTachet, after introducing
this instrument in the form just described, modified it to remedy
* The result of the numerous applications which the Author has made of
this instrument to a great variety of Microscopic objects, has led to a con-
firmation of the principle of Pseudoscopic vision, stated at the conclusion of
§ 29. — Where, as in the case of the saucer-like disks of the Arachnoidiscus
(Plate x.), the real and the converted forms are equally familiar, the ' conver-
sion' either of the convex exterior or the concave interior is made both sud-
denly and completely. In more complex and less familiar forms, on the other
hand, the conversion frequently requires time ; being often partial in the first
instance, and only gradually becoming complete. And there are some objects
which resist conversion altogether, the only effect being a confusion of the
two images.
Nachet's Stereo-pseudo-
scopic Microscope.
NACHET'S STEREOSCOPIC BINOCULAR. 69
two defects pointed out by Mr. Heisch. In the newer form, the
distance between the Eye-pieces is changed to meet the require-
ments of different individuals, by an alteration in the inclination in
the tube R ; which is effected by a screw furnished with two threads
of different speeds, whereby an inclination is given to the prism
equal to half the angular displacement of the tube. " This ar-
rangement is necessitated by the fact that the displacement of the
rays reflected by a rotating surface is double the angle described by
this surface."* Alluding to the observation of Mr. Heisch, that
many persons use this form of binocular with greater ease than
that of Mr. Wenham, Mr. Nachet remarks, " that there is a certain
difficulty in combining the strongly convergent images of the
Wenham Binocular ; and also as a second source of uneasiness,
that an apparent diminution of the size of the image results from
the great convergence of the pencils." He considers it desirable for
these reasons, that all binocular arrangements should be less con-
vergent.— As an ordinary working instrument, however, this im-
proved Nachet Binocular can scarcely be equal to that of Wenham
or Stephenson ; whilst it must be regarded as inferior to the former
in the following particulars : First, that as the uninterrupted half of
the cone of rays (when the interposed prism is adjusted for Stereo-
scopic vision) has to pass through the two plane surfaces of the
prism, a certain loss of light and deterioration of the picture are
necessarily involved ; whilst, as the interrupted half of the cone of
rays has to pass through four surfaces, the picture formed by it is
yet more unfavourably affected ; second, that as power of motion
must be given to both prisms — to a, for the reversal of the images,
and to b for the adjustment of the distance between the two bodies
— there is a greater liability to derangement.f It does not give
the equal illumination of Mr. Stephenson's, is less free from optical
error, and cannot, like his, be used with high powers.
36. The Stereoscopic Binocular is put to its most advantageous
use, when applied either to opaque objects of whose solid forms we
are desirous of gaining an exact appreciation, or to transparent ob-
jects which have such a thickness as to make the accurate distinc-
tion between their nearer and their more remote planes a matter of
importance. That its best and truest effects can only be obtained by
Objectives not exceeding 40° of angular Aperture, may be shown
both theoretically and practically. Taking the average distance
between the pupils of the two Eyes as the base of a triangle, and
* See paper by M. Nachet, "Monthly Micros. Journ.," Vol. i. p. 31.
t This arrangement, like Mr. "Wenham's, can be adapted to any existing
Microscope ; and it seems peculiarly suitable to those of French or German
construction, in which the body is much shorter than in the ordinary English
models. For in the application of the Wenham arrangement to a short Micro-
scope, the requisite distance between the Eye-glasses of its two bodies can
only be obtained by making those bodies diverge at an angle so wide as to
produce great discomfort in the use of the instrument, from the necessity of
maintaining an unusual degree of convergence between the axes of the Eyes.
70 OPTICAL PRINCIPLES OF THE MICROSCOPE.
any point of an object placed at the ordinary reading distance as
its apex, the vertical angle enclosed between its two sides will be
from 12° to 15° ; which, in other words, is the angle of divergence
between the rays proceeding from any point of an object at the
ordinary reading distance to the two Eyes respectively. This angle,
therefore, represents that at which the two pictures of an object
should be taken in the Photographic Camera, in order to produce
the effect of ordinary Binocnlar vision without exaggeration ; and it
is the one which is adopted by Portrait-photographers, who have
found by experience that a smaller angle makes the image formed
by the combination of the pictures appear too flat, whilst a larger
angle exaggerates its projection. Now,
FlG- 29- in applying this principle to the Mi-
■ croscope, we have to treat the two late-
ral halves (l, n, Fig. 29) of the Objective
BM as the two separate lenses of a double
mSl Portrait Camera ; and to consider at
— EzrBI what angle each half should be entered
g^M by the rays passing through it to form
HHBM its picture. 'To any one acquainted
H |p with the principles of Optics, it must
HIP be obvious that the picture formed by
Km each half of the Objective must be (so
to speak) an average or general resul-
■HHH tant of the dissimilar pictures formed
by its different parts. Thus, if we could
divide the lateral halves or Semi-lenses l, k, of the Objective by
vertical lines into the three bands a b c and a' U c', and could stop-off
the two corresponding bands on either side, so as only to allow the
light to pass through the remaining pair, we should find that the
two pictures we should receive of the object would vary sensibly,
according as they are formed by the bands a a', b V , or c c. For
supposing the pictures taken through the bands b b' to be sufficiently
dissimilar in their perspective projections, to give, when combined
in the Microscope, a sufficient but unexaggerated Stereoscopic relief,
those taken through the bands a a' on either side of the centre
would be no more dissimilar than two portraits taken at a very
small angle between the Cameras, and their combinations would
very inadequately bring out the effect of relief ; whilst, on the other
hand, the two pictures taken through the extreme lateral bands c c',
would differ as widely as portraits taken at too great an angle of
divergence between the Cameras, and their combination would
exaggerate the actual relief of the object. Now, in each of the
bands b b', a spot v v' may be found by mathematical computation,
which may be designated the visual centre of the whole Semi-lens ;
that is, the spot which, if all the rest of the semi-lens were
stopped-off , would form a picture most nearly corresponding to that
given by the whole of it. This having been determined, it is easy
to ascertain what should be the Angle of Aperture (o p g, Fig. 30) of
LIMIT OF APERTUBE EOE BINOCULAR OBJECTIVES. 71
Fig. 30.
the entire Lens, in order that the angles v p v' between the ' visual
centres' of its two halves should be 15°. The investigation of
this question having been kindly undertaken for the Author by
his friend Dr. Hirst, the conclusion at which he has arrived is,
that the angle of aperture of the
entire Lens should be about 36"6°.
This, which he gives as an approxi-
mate result only (the requisite data
for a complete Mathematical solu-
tion of the question not having yet
been obtained), harmonizes most
remarkably with the results of ex-
perimental observations made upon
objects of known shape, with Objec-
tives of different angular apertures ;
so that the Stereoscopic images pro-
duced by the several objectives may
be compared, not only with each
other, but with the actual forms
which they ought to present. No
better objects can be selected for this
purpose, than those which are per-
fectly spherical; such as various
globular forms of the- Poly cyst in a
(Plate xix.), or the Pollen-grains of
the Malvacece and many other
Flowering-plants. Now when either of these is placed under a
Stereoscopic Binocular, provided with an Objective of one-half or
four-tenths of an inch focus having an angular aperture of 80°
or 90°, the effect of projection is so greatly exaggerated, that
the side next the eye, instead of resembling a hemisphere, looks
like the small end of an egg. If then the aperture of such
an Objective be reduced to 60° by a diaphragm placed behind
its back lens, the exaggeration is diminished, though not removed ;
the hemispherical surface now looking like the large end of an
egg. But if the aperture be further reduced to 40° by the same
means, it is at once seen that the hemispheres turned towards the
eye are truly represented ; the effect of projection being quite
adequate, without being in the least exaggerated. Hence it
may be confidently affirmed — alike on theoretical and on prac-
tical grounds— that when an Objective of wider angle than 40°
is used with the Stereoscopic Binocular, the object viewed by it is
represented in exaggerated relief, so that its apparent form must
be more or less distorted. — -There are other substantial reasons,
moreover, why Objectives of limited Angle of Aperture should be
preferred (save in particular cases) for use with the Stereoscopic
Binocular. As the special value of this instrument is to convey
to the mind a notion of the solid forms of objects, and of the
relations of their parts to each other, not merely on the same but
72 OPTICAL PEINOIPLES OF THE MICEOSCOPE.
on different planes, it is obvions that those Objectives are most
suitable to produce this effect, which possess the greatest amount
of penetration or focal depth, that is, which most distinctly show,
not merely what is precisely in the focal plane, but what lies nearer
to or more remote from the Objective. Now, as will be explained
hereafter (§ 145, n.), increase of the Angle of Aperture is neces-
sarily attended with diminution of Penetrating power ; so that an
Objective of 60° or 80° of aperture, though exhibiting minute
surface -details which an Objective of 40° cannot show, is much
inferior to it in suitability to convey a true conception of the general
form of any object, the parts of which project considerably above
the focal plane or recede below it.*
37. In concluding these general observations upon the use of the
Stereoscopic Binocular, the Author would draw attention to two
important advantages he has found it to possess ; his own expe-
rience on these points being fully confirmed by that of others. In
the first place, the Penetrating power or Focal Depth of the Bino-
cular is greatly superior to that of the Monocular Microscope ; so
that an object whose surface presents considerable inequalities, is
very much more distinctly seen with the former than with the
latter. The difference may in part be attributed to the practical
reduction in the Angle of Aperture of the Objective, which is
produced by the division of the cone of rays transmitted through
it into two halves ; so that the picture received through each half
of an Objective of 60° is formed by rays diverging at an angle
of only 30°. But that this optical explanation does not go far to
account for the fact, is easily proved by the simple experiment of
looking at the object in the first instance through each eye sepa-
rately (the prism being in place), and then with both eyes together;
the distinctness of the parts which lie above and beneath the focal
plane being found to be much greater when the two pictures are
combined, than it is in either of them separately. In the absence
of any adequate Optical explanation of the greater range of focal
depth thus shown to be possessed by the Stereoscopic Binocular,
the Author is inclined to attribute it to an allowance for the rela-
tive distances of the parts which seems to be unconsciously made
by the Mind of the observer, when the solid image is shaped out in
it by the combination of the two pictures. This seems the more
likely from the second fact to be now mentioned : namely, that when
the Binocular is employed upon objects suited to its powers, the
* Irr-accordance with these principles, the Author has caused Messrs.
Powell and Lealand to construct for him an Objective of Half-inch focus with
an Angular aperture of 40° ; and he has found it to answer most admirably
the purpose for which it was intended, — the examination of Opaque objects
with the Stereoscopic Binocular. For not only are these represented in their
true forms, but the relations of their different parts are seen with a complete-
ness not otherwise attainable. And an Objective so constructed has this great
advantage over one whose originally large aperture has been reduced by a
diaphragm, — that the distance between its front lens and the object is so much
greater, as to admit far more conveniently of side illumination.
ADVANTAGES OF STEEEOSCOPIC BINOCULAR. 73
prolonged use of it is attended with very mtich less fatigue than is
that of the Monocular Microscope. This, again, may be in some
degree attributed to the division of the work between the two
eyes ; but the Author is satisfied that, unless there is a feeling of
discomfort in the Eye itself, the sense of fatigue is rather mental
than visual, and that it proceeds from the constructive effort which
the observer has to make, who aims at realizing the solid form of
the object he is examining, by an interpretation based on the fiat
picture of it presented by his vision, aided only by the use of
the Focal Adjustment, which enables him to determine what are
its near and what its remote parts, and to form an estimate of
their difference of distance (§ 126). ISTow, a great part of this con-
structive effort is saved by the use of the Binocular, which at once
brings before the Mind's eye the solid image of the object, and thus
gives to the observer a conception of its form usually more complete
and accurate than he could derive from any amount of study of a
Monocular picture.*
* It has happened to the Author to be frequently called on to explain the
advantages of the Binocular to Continental (especially German) Savans who
had not been previously acquainted with the instrument. And he has been
struck with finding that when he exhibited to them objects with which they
had already become familiar by careful study, and of whose solid forms they
had attained an accurate conception, they perceived no advantage in the Ste-
reoscopic combination, seeing such objects with it (visually) just as they had
been previously accustomed to see them (mentally) without it. But when he
has exhibited to them suitable objects with which they had not been previously
familiarized, and has caused them to look at. these in the first instance Monocu-
larly, and then Stereoscopically, he has never failed to satisfy them of the value
of the latter method, except when some visual imperfection has prevented
them from properly appreciating it. He may mention that he has found the
wing of the Moth known as Zenzera (Esculi, which has an undulating surface,
whereon the scales are set at various angles, instead of having the usual im-
bricated arrangement, a peculiarly appropriate object for this demonstration;
the general inequality of its surface, and the individual obliquities of its scales,
being at once shown by the Binocular, with a force and completeness which
could not be attained by the most prolonged and careful Monocular study.
CHAPTEK II.
CONSTRUCTION OP THE MICROSCOPE.
38. The Optical principles whereon the operation of the Micro-
scope depends having now been explained, we have nest to consider
the Mechanical provisions whereby they are bronght to bear npon
the different purposes which the instrument is destined to serve.
And first, it will be desirable to state those general principles which
have now received the sanction of universal experience, in regard to
the best arrangement of its constituent parts. — Every complete
Microscope, whether Simple or Compound, must possess, in addi-
tion to the Lens or combination of lenses which affords its magni-
fying power, a Stage whereon the Object may securely rest, a
Concave Mirror for the illumination of Transparent objects from
beneath, and a Condensing -lens for the illumination of Opaque
objects from above.
I. Now, in whatever mode these may be connected with each
other, it is essential that the Optical part and the Stage should be
so disposed, as either to be altogether free from tendency to vibration,
or to vibrate together ; since it is obvious that any movement of one,
in which the other does not partake, will be augmented to the eye
of the observer in proportion to the magnifying power employed.
In a badly-constructed instrument, even though placed upon a
steady table resting upon the firm floor of a well-built house, when
high powers are used, the object is seen to oscillate so rapidly at
the slightest tremor — such as that caused by a person walking
across the room, or by a carriage rolling-by in the street — as to be
frequently almost indistinguishable : whereas in a well-constructed
microscope, scarcely any perceptible effect will be produced by even
greater disturbances. Hence, in the choice of a Microscope, it
should always be subjected to this test, and should be unhesitatingly
rejected if the result be unfavourable. If the instrument should be
found free from fault when thus tested with high powers, its
steadiness with loiv powers may be assumed ; but, on the other
hand, though a Microscope may give an image free from perceptible
tremor when the lower powers only are employed, it may be quite
unfit for use with the higher. — The Author has found no test for
steadiness so crucial as the vibration of a paddle-steamer going at
full speed against a head-sea ; and the result of his comparison
between the two principal ' models' in use in this country will be
stated hereafter (§ 44).
MECHANICAL ARRANGEMENTS OF MICROSCOPE. 75
II. The next requisite is a capability of accurate adjustment to
every variety of focal distance, ivitlwut movement of the object. It
is a principle universally recognised in the construction of good
Microscopes, that the Stage whereon the object is placed should be
a fixture ; the movement by which the Focus is to be adjusted being
given to the Optical portion. This movement should be such as to
allow free range from a minute fraction of an inch to three or four
inches, with equal power of obtaining a delicate adjustment at any
part. It should also be so accurate, that the optic axis of the
instrument should not be in the least altered by any movement in
a vertical direction ; so that if an object be brought into the centre
of the field with a low power, and a higher power be then substi-
tuted, it should be found in the centre of its field, notwithstanding
the great alteration in the focus. In this way much time may often
be saved by employing a low power as a finder for an object to be
examined by a higher one ; and when an object is being viewed by a
succession of powers, little or no readjustment of its place on the
stage should be required. For the Simple Microscope, in which it
is seldom advantageous to use lenses of shorter focus than l-4th inch
(save where Doublets are employed, § 23), a rack-and-pinion adjust-
ment, if it be made to work both tightly and smoothly, answers
sufficiently well ; and this is quite adequate also for the focal adjust-
ment of the Compound body, when Objectives of low power only are
employed. But for any lenses whose focus is less than half an inch,
a ' fine adjustment,' or ' slow motion,' by means of a screiv-move-
ment operating either on the object-glass alone or on the entire
body, is of great value ; and for the highest powers it is quite
indispensable. In some Microscopes, indeed, which are provided
with a 'fine adjustment,' the rack-and-pinion movement is dis-
pensed with, the ' coarse adjustment ' being given by merely
sliding the body up and down in the socket which grasps it ; but
this plan is only admissible where, for the sake of extreme cheap-
ness or portability, the instrument has to be reduced to the form of
utmost simplicity.
in. Scarcely less important than the preceding requisite, in the
case of the Compound Microscope, though it does not add much to
the utility of the Simple, is the capability of being placed in either
a vertical or a horizontal position, or at any angle with the horizon,
without deranging the adjustment of its parts to each other, and
without placing the eye-piece in such a position as to be incon-
venient to the observer. It is certainly a matter of surprise, that
some Opticians, especially on the Continent, should still neglect the
very simple means of giving an inclined position to Microscopes ;
since it is now generally acknowledged that the vertical position is,
of all that can be adopted, the very worst, — excepting, of course, in
cases which necessitate its use. There are some objects which can
only be seen in a vertical microscope, as they require to be viewed
in a position nearly or entirely horizontal ; such are dissections in
water, urinary deposits, saline solutions undergoing crystalliza-
76 CONSTEUCTION OF THE MICROSCOPE.
tion, &c. In Dr. Laurence Smith's microscope, and in the Chemical
Microscope of Chevalier, this inconvenience is avoided by the intro-
duction of a prism : the stage is horizontal and the tube sloping.
In this form the objective is placed below the object, so that fumes
from it do not affect the glasses. In Stephenson's Binocular the
stage is horizontal and the tubes slanting. — In ordinary cases an
inclination of about 55° to the horizon will usually be found most
convenient for unconstrained observation ; and the instrument
should be so constructed, as, when thus inclined, to give to the Stage
such an elevation above the table, that when the hands are employed
at it, the arms may rest conveniently upon the table. In this manner
a degree of support is attained, which gives such free play to the
muscles of the hands, that movements of the greatest nicety may
be executed by them ; and the fatigue of long-continued observation
is greatly diminished. Such minutiae may appear too trivial to
deserve mention ; but no practised Microscopist will be slow to
acknowledge their value. — For other purposes, again, it is requisite
that the Microscope should be placed horizontally, as when the
Camera Lucida is used for drawing or measuring. It ought, there-
fore, to be made capable of every such variety of position ; and the
Stage must of course be provided with some means of holding the
object, when it is itself placed in a position so inclined that the
object would slip down unless sustained.
iv. The last principle on which we shall here dwell, is sh^li-
city in the construction and adjustment of every part. Many in-
genious mechanical devices have been invented and executed, for
the purpose of overcoming difficulties which are in themselves really
trivial. A moderate amount of dexterity in the use of the hands is
sufficient to render most of these superfluous ; and without such
dexterity, no one, even with the most complete mechanical facilities,
will ever become a good microscopist. Among the conveniences of
simplicity, the practised Microscopist will not fail to recognise the
saving of time effected by being able quickly to set up and put
away his instrument. Where a number of parts are to be screwed
together before it can be brought into use, interesting objects (as
well as time) are not unfrequently lost ; and the same cause will
often occasion the instrument to be left exposed to the air and dust,
to its great detriment, because time is required to put it away ; so
that a slight advantage on the side of simplicity of arrangement
often causes an inferior instrument to be preferred by the working
Microscopist to a superior one. Yet there is, of course, a limit to
this simplification ; and no arrangement can be objected to on this
score, which gives advantages in the examination of difficult objects,
or in the determination of doubtful questions, such as no simpler
means can afford. — The meaning of this distinction will become
apparent, if it be applied to the cases of the Mechanical Stage
and the Achromatic Condenser. For although the Mechanical
Stage may be considered a valuable aid in observation, as facili-
tating the finding of a minute object, or the examination of the
MECHANICAL ARRANGEMENTS OF MICROSCOPE. 77
entire surface of a large one, yet it adds nothing to the clearness of
our view of either ; and its place may in great degree be supplied
by the fingers of a good manipulator. On the other hand, the use
of the Achromatic Condenser not only contributes very materially,
but is absolutely indispensable, to the formation of a perfect image,
in the case of many objects of a difficult class ; the want of it
cannot be compensated by the most dexterous use of the ordinary
appliances ; and consequently, although it may fairly be considered
superfluous as regards a large proportion of the purposes to which
the Microscope is directed, whether for investigation or for display,
yet as regards the particular objects just alluded to, it must be
considered as no less necessary a part of the instrument than the
Achromatic Objective itself. Where expense is not an object, the
Microscope should doubtless be fitted with both these valuable
accessories ; where, on the other hand, the cost is so limited that
only one can be afforded, that one should be selected which will
make the instrument most useful for the purposes to which it is
likely to be applied.
In the account now to be given of the principal forms of Micro-
scope readily procurable in this country, it will be the Author's
object, not so much to enumerate and describe the various patterns
which the several Makers of the instrument have produced ; as, by
selecting from among them those examples which it seems to him
most desirable to make known, and by specifying the peculiar
advantages which each of these presents, to guide his readers in
the choice of the hind of Microscope best suited, on the one hand,
to the class of investigations they may be desirous of following
out, and, on the other, to their pecuniary ability. He is anxious,
however, that he should not be supposed to mark any preference
for the particular instruments he has selected, over those con-
structed upon the same general plan by other Makers. To have
enumerated them all, would obviously be quite incompatible with
the plan of his Treatise ; but he has considered it fair (save in one
or two special cases) to give the preference to those Makers who
have worked out their own plans of construction, and have thus
furnished (to say the least) the general designs which have been
adopted with more or less of modification by others.
Simple Microscopes.
39. Under this head, the common Hand-Magnifier or pocket-
lens first claims our attention ; being in reality a Simple Micro-
scope, although not commonly accounted as such. Although this
little instrument is- in every one's hands, and is indispensable to the
Naturalist, — furnishing him with the means of at once making
such preliminary examinations as often afford him most important
guidance, — yet there are comparatively few who know how to
handle it to the best advantage. The chief difficulty lies in the
78 CONSTRUCTION OF THE MICROSCOPE.
steady fixation of it at the requisite distance from the object ; espe-
cially when the lens employed is of such short focus, that the
slightest want of exactness in this adjustment produces evident
indistinctness of the image. By carefully resting the hand which
carries the glass, however, against that which carries the object, so
that both, whenever they move, shall move together, the observer,
after a little practice, will be able to employ even high powers with
comparative facility. The lenses most generally serviceable for
Hand-Magnifiers range in focal length from two inches to half an
inch ; and a combination of two or three such in the same handle,
with an intervening perforated plate of tortoiseshell (which serves
as a diaphragm when they are used together), will be found very
useful. When such a magnifying power is desired as would re-
quire a lens of a quarter of an inch focus, it is best obtained by the
substitution of a ' Coddington' (§ 24) for the ordinary double-convex
lens. The handle of the magnifier may be pierced with a hole at
the end most distant from the joint by which the lenses are
attached to it ; and through this may be passed a wire, which,
being fitted vertically into a stand or foot, serves for the support
of the magnifying lenses in a horizontal position, at any height at
which it may be convenient to fix them. Such a little apparatus
is a rudimentary form (so to speak) of what is commonly under-
stood as a Simple Microscope ; the term being usually applied to
those instruments in which the magnifying powers are supported
otherwise than in the hand, or in which, if the whole apparatus
be supported by the hand, the lenses have a fixed bearing upon the
object.
40. Boss's Simple Microscope. — This instrument holds an inter-
mediate place between the Hand-Magnifier and the complete Micro-
scope ; being, in fact, nothing more than a lens supported in such a
manner as to be capable of being readily fixed in a variety of
positions suitable for dissecting and for other manipulations. It
consists of a circular brass foot, wherein is screwed a short tubular
pillar (Fig. 31), which is ' sprung' at its upper end, so as to grasp
a second tube, also ' sprung,' by the drawing-out of which the
pillar may be elongated to about 3 inches. This carries at its upper
end a jointed socket, through which a square bar about 3-| inches
long slides rather stiffly ; and one end of this bar carries another
joint, to which is attached a ring for holding the lenses. By
lengthening or shortening the pillar, by varying the angle which
the square bar makes with its summit, and by sliding that bar
through the socket, almost any position and elevation may be given
to the lens, that can be required for the purposes to which it may
be most usefully applied ; care being taken in all instances, that
the ring which carries the lens should (by means of its joint) be
placed horizontally. At a is seen the position which adapts it
best for picking out minute shells, or for other similar manijmla-
tions ; the sand or dredgings to be examined being spread upon a
piece of black paper, and raised upon a book, a box, or some other
BOSS'S SIMPLE MICROSCOPE.
79
support, to such a height that when the lens is adjusted thereto,
the eye may be applied to it continuously without unnecessary
fatigue. It will be
found advantageous Fig. 31.
that the foot of the
microscope should
not stand upon the
paper over which the
objects are spread,
as it is desirable to
shake this from time
to time in order to
bring a fresh por-
tion of the matters
to be examined into
view ; and generally
speaking, it will be
found convenient to
place it on the oppo-
site side of the ob-
ject, rather than on
the same side with
the observer. At b
is shown the posi-
tion in which it may
be most conveniently
set for the dissection
of objects contained in
a plate or trough, the
sides of which, being
higher than the lens,
would prevent the use
of any magnifier
mounted on a horizon-
tal arm. — The powers
usually supplied with
this instrument are
one Lens of an inch
focus, and a second of
either a half or a quar-
ter of an inch. By un-
screwing the pillar, the
whole is made to pack
into a small flat case,
the extreme portability
of which is a great Boss's Simple Microscope,
recommendation. Al-
though the uses of this little instrument are greatly limited by
its want of stage, mirror, &c, yet, for the class of purposes to
CONSTRUCTION OF THE MICROSCOPE.
which it is suited, it has advantages over jDerhaps every other
form that has been devised.
41. Queketfs Dissecting Microscope. — To the Scientific investi-
gator, however, it is generally more convenient to have a larger
Stage than the preceding instrument affords ; and in this respect
an arrangement devised by the late Mr. Quekett (Fig. 32) will be
found extremely convenient. The Stage, which constitutes the
principal part of the apparatus, is a plate of brass (bronzed*) nearly
six inches square, screwed to a piece of mahogany of the same size,
and about 5-8ths of
-Fl&- 32- an inch thick ; un-
derneath this a fold-
ing flap four inches
broad is attached on
each side by hinges ;
and the two flaps are
so shaped that, when
folded together, one
lies closely upon the
other, as shown at b,
Fig. 32, whilst, when
opened, as shown at a,
they give a firm sup-
port to the stage at
a convenient height.
At the back of the
Stage-plate is a
round hole, through
which a tubular
Stem works verti-
cally with a rack-
and-pinion move-
ment, carrying at
its summit the hori-
zontal Arm for the
magnifying powers ;
and into the under-
side of the stage -
plate there screws a
stem which carries
the Mirror - frame .
From this frame the Mirror may be removed, and its place sup-
plied by a convex lens, which serves as a Condenser for opaque
objects, its stem being then fitted into a hole in the stage, at one
side or in front of _ its central perforation. The instrument is
usually furnished with three Magnifiers — namely, an inch and a
half-inch ordinary lenses, and a quarter-inch Coddington (§ 24) ;
* The Stage-plate is sometimes made of a piece of plate-glass ; and this is
decidedly advantageous where Sea- water or Acids are used.
Quekett's Dissecting Microscope, set up for use
at A, and packed together at B.
QUEKETT'S AND FIELD'S DISSECTING MICROSCOPES. 81
and these will be found to be the powers most useful for the pur-
poses to which it is specially adapted. As a black background
is often required in dissecting objects which are not transparent,
this may be most readily provided by attaching a disk of dead-
black paper to the back of the Mirror. The lenses, mirror, con-
denser, vertical stems, and milled-head, all fit into a drawer which
shuts into the under-side of the Stage, and is then covered and
kept in place by the side-flaps ; so that, when packed together, and
the flaps kept down by an elastic band, as shown at B, Fig. 32, the
instrument is extremely portable, furnishing (so to speak) a case
for itself. It may be easily made to serve as a Compound Micro-
scope, by means of an additional stem and horizontal arm, carrying
a light Body. — The principal disadvantages of this very ingenious
and otherwise most convenient arrangement, are that it must
always be used with the light in front of the observer, or nearly so,
since the side-flaps interfere with the access of side-light to the
mirror ; and that the obstruction of the side-flaps also prevents the
hands from having that ready access to the mirror, which is con-
venient in making its adjustments.* These inconveniences, how-
ever, are trifling, when compared with the great facilities afforded
for scientific investigation by the size and firmness of the Stage,
combined with its extreme portability ; and the Author can con-
fidently recommend the instrument for all such purposes, from
much personal experience of its utility.
42. Field's Dissecting and Mounting Microscope. — This instru-
ment, constructed on the plan of Mr. W. P. Marshall, is a combina>
tion of a Dissecting Microscope, with a set of apparatus and
materials for the preparation and mounting of microscopic objects ;
and the whole is packed in a small cubical case about seven inches
each way, convenient both for general use, but more particularly as
a travelling case for carrying the several requisites for the examina-
tion and mounting of objects when in the country, or at the seaside.
— The Microscope can be used either Simple or Compound, as
shown in the Figure ; and is fitted with a mirror, side-condenser,
and stage-forceps, and with metal and glass stage-plates ; a dissect-
ing-trough, lined with cork, also fits into the opening of the stage.
The Simple microscope, as used for dissecting and mounting, is
shown in the lower figure ; it has two powers, used singly or in
combination, which are carried by the smaller arm of the stand.
The Compound body, as shown in the upper figure, screws into the
larger arm of the stand, and has a divided objective, giving a range
of three powers ; the nose is made with the standard screw, so as
to fit any first-class objectives. A telescopic sliding arm, fitting
into a socket on either side of the stage, can also be used to carry
the simple-microscope powers, as well as a larger low-power lens,
that serves also as a hand-magnifier ; and the arm can be readily fixed
* Another form of this instrument, supported by brass folding legs instead
of by wooden flaps, so as to allow the light to fall on the mirror from either
side as well as from the front, is made by Messrs. Parkes of Birmingham.
G
32
CONSTRUCTION OF THE MICROSCOPE.
in any desired position for examining objects away from the instru-
ment. A watch-glass holder, used upon the glass stage-plate,
gives the means of sliding steadily in any direction upon the stage
objects that are under examination in a watch-glass. A turn-table
for mounting purposes is carried upon a long spindle that works
through the corner of the stage (as shown in the lower figure),
Fig. 33.
Field's Dissecting and Mounting Microscope.
the arm of the stand serving as a support for the hand, whilst
using the turn-table ; the top is made of the size - of an ordinary
glass slide, and the slide is held upon it by an india-rubber band.
A hot plate fits into the opening of the stage, and is heated by a
spirit-lamp placed in the position of the mirror, which is then
turned to one side ; and the larger arm serves also as a watch-
BINOCULAE DISSECTING MICROSCOPES. 83
glass holder for preparing crystals by evaporation over the spirit-
lamp. A selection of materials required in preparing and mount-
ing objects is supplied in a rack of bottles sliding in the case ;
and a set of instruments — dissecting-needles, knife, forceps, dipping-
tubes, brushes, &c— with a supply of cover-glasses, cells, &c, are
carried in the three drawers ; all the different contents of the case
being readily accessible when it is set open,, as shown in the
engraving.*
43. Beck's and Nachetfs Binocular Dissecting Microscopes. — A
more substantial and elaborate form of Dissecting Microscope,
devised by the late Mr. R. Beck, is represented in Fig. 33. From
the angles of a square mahogany base, there rise four strong brass
pillars, which support, at a height of 4 inches, a brass plate
<6\ inches square, having a central aperture of 1 inch across ; upon
this rests a circular brass plate, of which the diameter is equal to
the side of the preceding, and which is attached to it by a revolving
fitting that surrounds the central aperture, and can be tightened
by a large milled-head beneath ; whilst above this is a third plate,
which slides easily over the second, being held down upon it by
springs which allow a movement of 1^ inch in any direction. The
top-plate has an aperture of 1^ inch for the reception of various
glasses and troughs suitable for containing objects for dissection ;
and into it can also be fitted a spring holder, suitable to receive
and secure a glass slide .of the ordinary size. By turning the
large circular plate, the object under observation may be easily
made to rotate, without disturbing its relation to the optical por-
tions of the instrument ; whilst a traversing movement may be
given to it in any direction, by acting upon the smaller plate. The
left-hand back pillar contains a triangular bar with rack-and-
pinion movement for focal adjustment, which carries the horizontal
arm for the support of the magnifiers ; this arm can be turned
away towards the left side, but it is provided with a stop which
checks it in the opposite direction, when the Magnifier is exactly
over the centre of the Stage-aperture. Beneath this aperture is a
concave Mirror, which, when not in use, lies in a recess in the
mahogany base, so as to leave the space beneath the stage entirely
free to receive a box containing apparatus ; whilst from the right-
hand back corner there can be raised a stem carrying a side Con-
densing-lens, with a ball-and-socket movement. In addition to
the single Lenses and Coddington ordinarily used for the purposes
of dissection, a Binocular arrangement was devised by Mr. R.
Beck,f on the principle applied by MM. Nachet, about the same
date, in their Stereo-pseudoscopic Microscope (§ 35). For adopt-
ing Mr. Wenham's method of allowing half the cone of rays to
proceed to one eye without interruption, he caused the other half
* The whole of the above -de scribed apparatus is supplied complete at the
moderate cost of £4 ; or without the Compound body and inclined movement
of the stand, at £2 10-s.
f " Transactions of the Microscopical Sooietv." N. S. Vol, xii. p. 3.
g2
Si
CONSTRUCTION OF THE MICROSCOPE.
to be intercepted by a pair of Prisms disposed as in Fig. 22, 2, and
to be by them transmitted to the other eye. It will be readily
understood that this arrangement, though psetidoscopic for the
Compound Microscope, is Stereoscopic for the Simple Microscope,
in which there is no reversal of the pictures ; and the Author can
Beck's Dissecting Microscope, with Nachet's Binocular Microscope.
testify to the fidelity of the effect of relief obtainable by Mr. E.
Beck's apparatus, which, being carried on an arm superposed upon
that which bears the magnifier, can be turned aside at pleasure.
But he has found its utility to be practically limited by the narrow-
ness of its field of view, by its deficiency of light and of magnify -
ino- power, and by the inconvenience of the manner in which the
eyes have to be applied to it.— An arrangement greatly superior in all
these particulars having been since worked out by MM. JSIachet,
the Author has combined the Optical part of their Dissecting Micro-
scope with Mr. R. Beck's Stand, and finds every reason to be satis-
fied with the result ; the solidity of the stand giving great firmness,
whilst the size of the Stage-plate affords ample room for the hands
to rest upon it. The Objective in Nachet's arrangement is an
Achromatic combination of three pairs, having a clear aperture of
nearlv o-4ths of an inch, and a power about equal to that of a
single lens of one-inch focus ; and immediately over this is a pair
of Prisms, each resembling a, Pig. 27, having their inclined sur-
faces opposed to each other, so as to divide the pencil of rays
COMPOUND MICEOSCOPES. 85
passing upwards from the Objective into two halves. These are
reflected horizontally, the one to the right and the other to the
left ; each to be received by a lateral Prism corresponding to b,
and to be reflected upwards to its own Eye, at snch a slight diver-
gence from the perpendicular as to give a natural convergence to
the axes when the eyes are applied to the Eye-tubes superposed on
the lateral prisms, — the distance between these and the central
prisms being made capable of variation, as in the Compound
Binocular of the same makers (§ 35). The magnifying power of
this instrument may be augmented to 35 or 40 diameters, by
inserting a concave lens in each Eye-piece, which converts the
combination into the likeness of a Galilean Telescope (or Opera -
glass) ; and this arrangement (originally suggested by Prof.
Briicke of Vienna) has the additional advantage of increasing the
distance between the object and the object-glass, so as to give
more room for the use of dissecting instruments. — To all who are
engaged in investigations requiring very minute and delicate dis-
section, the Author can most strongly recommend MM. Nachet's
instrument. ISFo one who has not had experience of it can estimate
the immense advantage given by the Stereoscopic view, not merely
in appreciating the solid form of the object under dissection, but
also in precisely estimating the relation of the instrument to it in
the vertical direction. This is especially important when hori-
zontal sections are being made with fine Scissors ; since the course
of the section can thus be so regulated as to pass through the plane
desired, with an exactness totally unattainable by the use of any
Monocular Magnifier.
Compound Microscopes.
44. The various forms of Compound Microscope may be grouped
with tolerable definiteness into three principal Classes : the First
consisting of those instruments in which the greatest possible per-
fection and completeness are aimed at, without regard to cost ; the
Second including those which are adapted to all the ordinary re-
quirements of the observer, and which can be fitted with the most
important of those Accessories,* whose use enables him not only to
work with more facility and certainty, but, in some instances, to
gain information with regard to the objects of his examination
which he could not obtain without them ; whilst to the Third belong
those in which simplicity and cheapness are made the primary con-
siderations. Besides these, there is a class of Microscopes devised
for Special purposes, but not suited for ordinary use. — In all, save
the last, the same basis of support is adopted — namely, a triangular
* It is true that the most important of these Accessories may be applied to
some of the smaller and lighter kind of Microscopes ; but when it is desired
to render the instrument complete by the addition of them, it is far preferable
to adopt one of those larger and more substantial models, which have been
devised with express reference to their most advantageous and most conve-
nient employment.
86 CONSTRUCTION OF THE MICROSCOPE.
• foot,' from which arise two uprights ; and between these the
Microscope itself is swung in such a manner, that the weight of its
different parts may be as nearly as possible balanced above and
below the centres of suspension in all the ordinary positions of
the instrument. This double support was first introduced by
Mr. George Jackson, who substituted two pillars (a form which
Messrs. E. and J. Beck still retain in their Large Compound Micro-
scope, Plate vii.) for the single pillar connected with the Microscope
itself by a ' cradle- joint' which was previously in use; but in
place of pillars screwed into the tripod base, a pair of flattened
uprights, cast in one piece with it, is now generally adopted; with
a view both to greater solidity and to facility of construction.
Messrs. Powell and Lealand, it will be observed, adopt a tripod
support of a different kind (Plates v., vi.) ; still, however, carrying
out the same fundamental principle of swinging the Microscope
itself between two centres ; and the same general arrangement is
adopted in the very ingenious form devised by Mr. Ladd (Fig. 38). —
Two different modes of giving support and motion to the ' Body'
will be found to prevail. One consists in its attachment at its base
to a transverse 'Arm,' which is borne on the summit of the moveable
Stem, whose rack is acted on by the pinion of the milled-head, as
in Plates rv\, v., vi. ; whilst in the other, the body is supported
along a great part of its length by means of a solid ' Limb,' to
which is attached the pinion that acts on a rack fixed to the body
itself, as in Plate vn. The former, which may be described as
the Boss model, is subject to the disadvantage that unless the
transverse arm and the body are constructed with great solidity, the
absence of support along the length of the latter leaves it subject to
vibration, which may become unpleasantly apparent when high
powers are used, giving a dancing motion to the objects. With a view
of preventing this vibration, the top of the ' body' is sometimes con-
nected with the back of the transverse arm by a pair of oblique
' stays' (Plate v.) ; but the usual plan is to obtain the requisite firm-
ness by the thickness and weight of the several parts. When
strong enough, there is less chance than in the Jackson model of
the hand communicating a vibration to the tube when using the
coarse or fine adjustments, which are detached from it. The second,
which may be designated the Jackson model, attains steadiness with
much less solidity, and therefore with less cumbrousness ; the mode
in which the rack is applied, moreover, in the microscopes of Messrs.
Beck (most of which are constructed upon this plan) gives to it a
great easiness of working ; but the traversing movement of the
body is sacrificed, Although some attach considerable impor-
tance to this movement, the Author's experience of instruments
constructed upon both plans leads him on the whole to give a pre-
ference to the second. The Jackson model is used by many English
makers, and by most American. It is certain that greater freedom
from vibration can be obtained in light instruments constructed
on this pattern, than in instruments of the same weight con-
THIRD-CLASS MICROSCOPES. 87
stracted on the old Eoss model ;* and Messrs. Eoss have recently
adopted it for one of their instruments (§ 57).
In describing the instruments which he has selected as typical
of the Classes above enumerated, the Author wishes not to be un-
derstood as giving any special preference to these, above what may
be the equally good instruments of other makers. The number of
Opticians who now construct really excellent Microscopes has of
late years increased greatly ; but their models are for the most
part copied more or less closely from those previously adopted for
their First-Class Microscopes by the three principal firms which
long had exclusive possession of the field. Where any individual
maker has introduced a real novelty, either in plan of construction,
or in simplification leading to reduction of price, the Author has
thought this worthy of special notice ; whilst the limits within
which he is restricted oblige him to content himself with a bare
mention of other Makers whose productions are favourably known
to him. It will be found most advantageous to commence with the
Third Class Microscopes, as the most simple in construction ; and
to rise from these, through the Second, to the First Class, — reserv-
ing the Special Class for the conclusion.
Third- Glass Microscopes.
45. Microscopes in which simplicity and cheapness are the primary
-considerations, are rather suited for Educational purposes than for
Scientific observation. Yet it is unquestionable that very important
-contributions to our knowledge of nature have been made by the
assistance of instruments not surpassing the least perfect of those
now to be described. And there is this advantage in commencing
Microscope- work with a Third- Class instrument, that the risk of
injury to a more costly Microscope, which necessarily arises from
want of experience in its use, is avoided ; whilst the inferior instru-
ment will still be found serviceable for many purposes, after a better
one has been acquired. Microscopes, of whatever class, should be
provided with the ' universal screw,' to which objectives of any
quality can be fitted.
46. Field's Educational Microscope. — This instrument is known
as the l Society of Arts Microscope,' in consequence of its having
gained the medal awarded by that society in 1855 (at the suggestion
of the Author) for the best three-guinea Compound Microscope that
was then produced. It has two eye-pieces, and two achromatic
objectives, condenser, live-box, &c, and retains its place amongst
Tiseful instruments of low price. — It is inferior in general utility.,
however, to the Compound Microscope supplied by the same
Makers with their Dissecting and Mounting apparatus (Fig. 33).
47. Crouch's Educational Microscope. — The instrument now to-
be mentioned may be recommended to those who think it well to
* See the Author's experience in "Monthly Mierosc. Journ.,"' VoL iiL p. 183.
83 CONSTRUCTION OF THE MICROSCOPE.
provide themselves in the first instance with a Microscope that is
capable of being improved by progressive additions. It is con-
structed (Fig. 35) on the Jackson model, and is not only very light
and portable, but very free from tremor. The rack-movement is
so good that an Objective of l-4th inch may be focussed by it with
Eig. 35.
Crouch's Educational Microscope.
great exactness ; additional facility in this adjustment being given
(as in Mr. Ladd's Microscope, Fig. 38) by the use of a Lever-
handle, which ordinarily hangs quite freely from the axis of the
milled-head, so as not to tnrn with it, but which can be made to
'grip' it by a slight lateral pressure. It then acts as a 'slow-
motion.' The Stage is furnished with a pair of springs for hold-
CROUCH'S AND PILLISCHER'S SMALL MICROSCOPES.
89
ing down the object ; a simple method which is very suitable for
ordinary purposes, but which requires special care in its use when
a slide carrying a drop of fluid beneath a covering-glass is being
moved about under the objective, since, if the slide be carried
too far towards either side, the covering-glass is displaced by im-
pinging against the spring. This instrument is provided with two
Objectives, each consisting of a good triplet combination, of two
inches and one inch focus respectively ; and when to these is added
a l-4th Objective of moderate angular aperture, it is rendered a
very serviceable Student's Microscope. The aperture of the Stage
being carefully centered to the axis of the Body, a tube can be
screwed into it which will carry a Diaphragm-plate, a Polariscope,
or a Paraboloid ; and
thus by additions which
may be made at any
time, either simulta-
neously or successive-
ly, this instrument, of
which the first cost is
no greater than that of
the preceding, may be
rendered quite com-
plete enough for the
ordinary wants of the *
Scientific investigator.
48. Pillischer's Small
Student's Microscope.—
The instrument repre-
sented in Fig. 36 de-
serves special mention,
as having been the first
really good Microscope
brought out in this
country at the price of
hi. ; and as having
gained for its con-
structor the award of
a Medal at the Inter-
national Exhibition of
1862 ' for cheapness
combined with excel-
lence.' This Microscope
is framed upon the :
Ross model, and is pro-
vided with a fine ad- Pillischer's Small Student's Microscope,
justment as well as
with the rack-and-pinion movement. The Body is furnished with a
sliding tube, by pushing-in which it may be shortened for packing ;
thus enabling the instrument to be put away in a very small com-
90 CONSTRUCTION OF THE MICROSCOPE.
pass. The Stage carries a simple but very convenient Object-holder,
consisting of a back-and-front piece pivoted to the npper left-band
corner of tbe stage, and of a transverse bar, of which the left-hand
extremity is pivoted to the lower end of the preceding, whilst its
right-hand extremity, which projects beyond the stage, is kept
down npon it by a spring applied to its under surface. From this
transverse bar there project forward two tongues, on which the
slide bearing the object is laid ; and these tongues are furnished
with springs for keeping the slide in place. By applying the right
hand to a pin which projects upwards from the free end of the
transverse bar, motion may be readily given to the object in any
direction ; whilst if it should be desired to clear the stage for the
reception of large objects, the traversing apparatus may be at once
detached by unscrewing the pivot, which is furnished with a milled-
head. This instrument is furnished with a dividing set of achro-
matic Objectives, giving a power of l-4th inch when complete, of
\ inch when the front lens is removed, and of 1 inch when the middle
lens has also been taken off ; and these, as in the preceding instance,
may be replaced by superior objectives if desired. An additional
Eye-piece, 2-inch Objective, Polarizing apparatus, and other Acces-
sories, are furnished at a very moderate price.
Second-Class Microscopes.
49. Under this head may be ranked those instruments which com-
bine first-rate workmanship with simplicity in the plan of con-
struction ; and which may be consequently designated as ' Superior
Student's Microscopes.' The value of Stereoscopic binocular vision
in Scientific investigation being now admitted by all who have
really worked with it upon suitable objects, the Author would
earnestly recommend every one about to provide himself with
even a Second-class Microscope, to incur the small expense of the
Binocular addition. This addition, however, will lose an important
element of its value, if the Stage of the instrument be not adapted
to rotate in the optic axis of the Body ; so that objects which are
being viewed by incident light may be presented to the illumi-
nating rays in every direction. This rotation not only gives most
valuable aid in the appreciation of the solid form of the object, by
the play of light and shade among the inequalities of its surface ;
but also frequently brings into view features that would other-
wise have escaped notice, either from having been previously
thrown into shadow by some neighbouring prominence, or from
not receiving their light at the angle at which they could most
advantageously reflect it. And as it may be readily introduced
into the construction of any Microscope, either on the plan of
MM. ISTachet (§ 51), or on that of Beck's 'Popular' Microscope
(§ 54), the Author anticipates that it will ere long be adopted
in almost every form of Stereoscopic Binocular.
50. Messrs. -Beck's and Ladd's Student's Microscopes, Figs. 37,
BECK'S STUDENT'S MICROSCOPE.
91
and 38, may be had in either form. The first needs no explanation
beyond that which can be obtained by inspection of the figure,
Fig. 37.
Messrs. E. and J. Beck's Student's Microscope.
It will be seen that the fine adjustment is placed behind the
pillar carrying the body. It can also be placed in front, on the
body, as in their larger instruments, which is better, as a lateral
motion occurs with the former plan after it has been for some
time in use, owing to the wear of the sliding-piece and the slot
in which it moves. — -Mr. Ladd's pattern is remarkable for its light-
ness, obtained without sacrifice of steadiness, by an ingenious
92 CONSTRUCTION OF THE MICROSCOPE.
framework of tubes screwed together at a convenient angle. The
line adjustment is worked by a lever, shown in the figure, and
the coarse adjustment is effected by a chain and spindle instead
Fig. 38.
Ladd's Student's Microscope.
of a rack and pinion. The manner in which the body is supported
along a great part of its length, gives it the advantage of the
Jackson model.
51. Nachetfs Student's Microscope. — Although the Author has
abstained from noticing any Continental Microscope of the Third
Class, as on the whole inferior to those of English makers, yet he
feels it due to MM. Nachet to make special mention of their
form of Student's Microscope, as possessing excellences which dis-
NACHET'S STUDENT'S MICROSCOPE.
93
Fig. 39.
tinguish it from all constructions previously devised. The general
build of this instrument corresponds with that of the Student's
Microscope of Messrs. Beck, except that it is upon a smaller scale,
and is supported on a single pillar with a cradle- joint, instead of
being swung between two uprights. The Body is furnished with a
draw -tube, by which it is shortened for packing ; and instead of
being itself attached to the rack, its lower part is embraced by a tube
which carries the rack, so that this Single body may be readily drawn
out and replaced by the Binocular already described (§ 35, Fig. 28).
The ' slow motion' is given by a milied-head placed at the top of
the sliding-stem, so as to be near that which gives the rack-and-
pinion adjustment. This plan was formerly adopted by Smith and
Beck, but it tends to become
unsteady with use, by the wear
of the slot shown in the figure.
The chief peculiarity of this in-
strument, however, lies in its
Stage, which the Author has no
hesitation in pronouncing to the
most perfect of its kind that has
been yet devised. Its base is
formed of a thick plate, %\ inches
square, having a large circular
aperture ; and on this is super-
posed a circular plate of 3 inches
in diameter, to which a rotatory
movement, concentric with the
optic axis of the Microscope, can
be given with great facility. In
this circular plate a disk of thin
plate-glass is cemented with
black cement, the united thick-
ness of the two around the cen-
tral aperture being not more
than l-8th of an inch, so that
light of the greatest obliquity
can be transmitted to the object
from beneath. The rotating
plate is furnished with a pro-
jection at the back, to which is
attached a strong V-shaped pair
of springs, having their extremi-
ties armed beneath with small
ivory knobs, which press down
on the Object-carrier. This last
consists of a brass frame fur-
nished with tongues and springs
projecting forward for the reception of the slide, and also with
a pair of knobs, to which the fingers may be applied in giving
Nachet's Student's Microscope.
94 CONSTRUCTION OF THE MICROSCOPE.
motion to it; whilst the frame encloses a piece of plate-glass a
little thicker than itself. Thus the under surface of the glass
plate of the Object-carrier slides over the upper surface of the
circular glass stage-plate ; being held down upon it and retained
in any position by the pressure of the ivory knobs. In the perfect
facility with which the Object-carrier may be moved, and the stea-
diness with which it keeps its place when not unduly weighted,
this arrangement is at least equal to the Magnetic stage, whilst
superior to it in the essential particular of not being liable to de-
rangement from rust ; having also the further advantage of being
capable of ready readjustment in case the movement should become
too easy, nothing more being necessary to tighten it in any required
degree than bending down the V springs. The front portion of the
rotating plate bears a small projecting piece on either side, into
which may be screwed a pin that carries a sliding- spring ; this
arrangement is suited for securing a Zoophyte-trough or other piece
of apparatus not suitable to being received by the object-carrier,
which can be easily slipped away from beneath the ivory knobs,
thus leaving the stage free. To the under side of the stage is firmly
pivoted a broad bar, into which is screwed a short sprung tube,
that is exactly concentric with the optic axis of the instrument
when the bar (which is shown turned-away in the figure) is pushed
beneath the stage until checked by a firm stop ; and as this bar is
composed of two pieces, held together by a pair of screws working
through slots, the centering of the tube may be precisely readjusted
if it should at any time become faulty. Into this tube may be in-
serted another that carries either (1) a Diaphragm, which can be
slid up and down, so as to vary the proportion of the pencil of con-
vergent rays thrown upwards by the mirror ■ (2) a Polarizing
prism ; (3) a Ground-glass for diffusing the light, which may be
either plane or a plano-convex lens, ground on its flat side which is
directed upwards ; and (4) a Glass Cone, having its apex pointing
downwards, and a large black spot in the centre of its base which is
directed towards the object ; this serves the same purpose as the
Paraboloid now commonly applied to English Microscopes (§ 94).
Lastly, the Mirror is attached to a stem which is so jointed as to
enable it to reflect rays of very great obliquity. — To those who wish
a compact instrument of great completeness and capability, which
may be worked advantageously even with high powers (for which
an Achromatic condenser might easily be added if desired), the
Author can strongly recommend this Microscope, especially when
furnished with MM. Nachet's Stereo-pseudoscopic arrangement
(§ 34). The rotatory movement of the Stage has most of the ad-
vantages which are only obtained at a great increase of cost in
First-class instruments ; and it is so exact as to answer equally
well for all the purposes which this rotation is specially fitted to
serve. The traversing movement of the Object-holder is in some
respects (especially for following living objects) decidedly superior
to that of any Mechanical Stage; and those who have become
BROWNING'S ROTATING MICROSCOPE.
95
accustomed to its nse will seldom feel the need of the latter more
costly appliance. The Sub-stage fitting is so arranged as to carry
the most needful Accessories, without either interfering with ex-
tremely oblique illumination (as is done by the tube which is
screwed into the aperture of the stage of most English Student's
Microscopes), or requiring any complicated and therefore costly
provisions for the exact centering of its fittings with the optic axis
of the instrument. And the manner in which the Mirror is
mounted gives it a remarkable range of position. — The Objectives
ordinarily supplied with this instrument by MM. Nachet are of
excellent quality, and are quite adequate for the ordinary purposes
of scientific investigation ; but for the sake of purchasers who may
prefer Objectives of English or American make, MM. ISTachet now
provide it with the universal screw.
52. Browning's Rotating Microscope.— -The peculiarity of this in-
strument is that, as in many of the Continental models, the whole
of the Optical part, together
with the Stage, revolves in one Fig. 40.
mass ; so that no change can
take place either in the accu-
racy of the centering, or in the
correctness of the focus to which
it has been adjusted before the
rotation is made. The body is
supported, as in the Jackson
model, upon a limb, a, grooved
for the rack-movement ; and
this limb is firmly fixed to the
stage B, which rotates upon the
strong plate c. In the simplest
form of the instrument, shown
in the annexed sketch, the ro-
tation is effected by pressing a
finger on the projecting pins
attached to ~b ; but if required,
b can be made to move by a
pinion and toothed wheel, with
graduated scale attached ; and
a sub- stage for carrying illu-
minating apparatus can be fixed
to an arm below c. This Micro-
scope is further characterized
by the solidity of its several
parts, and the care taken in its
construction to secure it against
derangement from an accidental
strain. It is not capable of re-
ceiving the Binocular addition ;
but is particularly adapted to the use of those who work with high
Browning's Eotating Microscope.
96 CONSTRUCTION OF THE MICROSCOPE.
powers, upon objects requiring the varied illumination for which
the rotating arrangement gives special facilities.
53. Grouch's Student's Binocular. — This instrument was devised
at a time when the construction of the Binocular was still almost
exclusively confined to the makers of First-class instruments ; and
it had the great merit of bringing within reach of the Student a
convenient and well-constructed Binocular, at a cost not greater
than that originally charged for the addition of the Wenham prism
and Secondary body alone. With the improvements it has since
received, it still remains one of the best instruments of its class ;
and the Author, after considerable use of it, can strongly recom-
mend it to such as desire to possess a Binocular at once cheap,
good, and portable. Its general arrangement, as shown in Plate in.,
corresponds closely with that of the small Microscope of the same
maker already described; the double body being supported on a
' limb' on the Lister model. The adjustment of the Eye-pieces for
the distance of the eyes is made by a transverse bar which is at-
tached to one of them, and which works through a slot-piece fixed
to the other ; so that if by the application of the finger and thumb
to the projecting pin, the bar with the attached eye-piece be raised
or lowered, the other eye-piece also is moved accordingly. The
Stage resembles that of MM. ISTachet's Microscope (Fig. 39). It is
of black glass, of circular form, and works with the like freedom
and smoothness ; and rotates in a manner similar to that of
M. JSTachet, of which it is a modification. It has also a similar ob-
ject-holder.— An Achromatic Condenser, Polarizing apparatus, &c.
can be added to this instrument ; and it is then as well adapted to
all the ordinary purposes of scientific investigation as those of
much higher cost, while it has the advantage of lightness and
portability.
54. Beck's Popular Microscope.— For the general purposes of
Microscopists, and especially for such as work with low and
moderate powers upon objects for the study of which Binocular
vision is peculiarly advantageous, the instrument represented in
Plate iv., which was devised by the late Mr. R. Beck, will be found
especially suitable. Its chief peculiarity consists in the in-
genious mode in which it is framed and supported ; a mode which
particularly adapts it to the requirements of Travellers, as
enabling it to bear a good deal of rough usage without injury.
The Stem to which the stage d and the mirror e are attached, and
which contains the racked bar c that carries the arm b and the
Binocular body a, is itself attached by a pair of centres to the
broad stay g, which again is attached by a pair of centres at its
lower angles to the triangular base f. The lower end h of the
stem carries a stout projecting pin, which fits into various holes
along the medial line of the base ; whereby the instrument may
be steadied in positions more or less inclined, or may be fixed
upright. It may be also fixed in the horizontal position required
for drawing with the Camera Lucida (§ 81) ; for the pin at the
PLATE III.
Cbouch's Student's Binocular.
[To face p. 96.
PLATE IV.
Beck's Popular Microscope.
[To face p. 97.
BECK'S AND COLLINS'S STUDENTS' BINOCULARS. 97
bottom of the stem then enters the hole at the top of the stnd Kr
and the stay g falls flat down, resting on the top of the stout pin
l. The advantages of this construction are that it is strong, firm,
and yet light ; that the instrument rests securely at the particular
inclination desired, which is often not the case on the ordinary
construction when the joint has worked loose ; and that in every
position there is the needful preponderance of balance. The Stage
d is circular, and upon it fits a circular plate t, which rotates in the
optic axis of the Microscope ; the special advantage of this rotation
for Binocular study has been already pointed out (§ 49). On the
plate t there slides the Object-holder u, which is so attached to it
by a wire spring that bears against its under surface, as to be easily
moved by either or both hands ; and as access can be readily
gained to this spring by detaching the plate t from the stage, it
may either be removed altogether so as to leave the stage free, or
may be adjusted to any degree of stiffness desired by the observer.
The Object-holder has a ledge v for the support of the slide; and
it is also provided with a small spring w, attached to it by a
milled-head, by turning which the spring may be brought to bear
with any required pressure against the edge of the slide laid upon
the object-holder, so as to prevent it from shifting its place when
rotation is given to the stage, or when, the instrument being
placed in the horizontal position, the stage becomes vertical. The
central tube of the Stage, is adapted to receive fittings of various
kinds, such as Diaphragm-plate, Dark-well, Paraboloid, and Pola-
rizing prism ; and it can also carry either a Webster Condenser or
an ordinary Achromatic Condenser. This instrument may be fur-
nished either with First-class or with Second-class Objectives ; the
latter are well adapted for Educational use ; but the Scientific in-
vestigator will do well to provide himself with the former, bearing
in mind, however, the caution already given (§ 36) as to Angle of
Aperture*
55. Collins's Harley Binocular. — This instrument, represented
in Fig. 41, is substantially framed and well hung on the Koss
model; and can be furnished with all the Accessories usually
needed. The caps of the Eye-pieces are provided with shades,
which cut off the outside lights from each eye ; these can be adapted
to any instrument, and the Author can speak strongly of their
value from his own experience. The Wenham prism at the com-
mon base of the bodies is fitted into an oblong box, which slides
through the arm that carries them ; this contains, in addition, a
jSTicol analyzing prism, and is also pierced with a vacant Aperture ;
so that by merely sliding this box transversely until the Aperture
comes into the axis, the instrument may be used as an ordinary
* Thus the small-angled 4-10th Objective of Messrs. Smith and Beck is much
better adapted to Binocular use than the large-angled 4-10ths of the same
makers. On the other hand, as the l-4th inch Objective is t;nsuited to Bino-
cular use, the choice between a wide and a narrow angle will have to be
determined by other considerations (§ 145).
H
98 CONSTEUCTION OF THE MICEOSCOPE.
Monocular ; or, if the analyzing prism is made to take the place of
the Wenham, whilst the polarizing prism beneath the stage is
brought into position by rotating the Diaphragm-plate in which it
Collins' s Harley Binocular.
is fixed, it is at once converted into a Polarizing Microscope. The
chief drawback to the value of this instrument (in the Author's
opinion) is its not being furnished with a Stage-plate rotating
in the optic axis of the Microscope; it would not be difficult,
however, to substitute the Nachet stage for the Mechanical stage
represented in Fig. 41 ; and such substitution would not merely
diminish the cost of the instrument, but would be (in the Author's
opinion) a real improvement .*
* In addition to the Second-class instruments that have here been noticed,
others, alike Monocular and Binocular, may be mentioned as favourably known
FIKST-CLASS MICEOSCOPES.
First-class Microscopes.
56. "We now pass to an entirely different class of Instruments —
those of which the aim is, not simplicity, but perfection ; not the
production of the best effect compatible with limited means, but
the attainment of everything that the Microscope can accomplish,
without regard to cost or complexity. To such, of course, the
Stereoscopic Binocular is an indispensable addition; and the
Author regards it as not less essential that the Stage should have
a rotatory movement in the Optic axis of the instrument, — not only
for the due examination of opaque objects, as already mentioned
(§ 49), but also because this movement is requisite for the effective
examination of very delicate transparent objects by Oblique light,
allowing the effect of light and shadow to be seen in every direc-
tion (§ 133) ; and, in addition, because in the examination of
objects under Polarized light, a class of appearances is produced
by the rotation of the object between the prisms, which is not
developed by the rotation of either of the prisms themselves. It
is also important for the most advantageous use of the Illumi-
nating Apparatus, that the Sub -stage also should be furnished
with a rotatory movement.
57. Boss's First-class Microscopes. — Messrs. Eoss have recently
introduced a new first-class microscope, founded upon the Jackson
model, with important modifications suggested by Mr. Wenham :
but as what is known as the Ross model will continue to be made,
and may be preferred by some purchasers, we shall commence with
a description of the original form of the Instrument which has
gained so high a celebrity. — The general plan of this Microscope,
as shown in Fig. 42, is essentially the same as that which we have
already seen to be adopted in a simpler form by many other
makers ; but it is carried out with the greatest attention to solidity
of construction, in those parts especially which are most liable to
tremor, as also to the due balancing of the weight of the different
parts upon the horizontal axis. The ' coarse' adjustment is made
by the large milled-head situated just behind the summit of the
uprights, which turns a pinion working into a rack cut on the
back of a very strong flattened Stem that carries the transverse
arm at its summit ; a second milled-head (which is here concealed
by the stage-fittings) is attached to the other end of the axis of the
pinion so as to be worked with the left hand. The ' fine' adjust-
ment is effected by the milled-head on the transverse Arm just
behind the base of the ' body ;' this acts upon the ' nose' or tube
projecting below the arm, wherein the objectives are screwed,
The other milled-head, seen at the summit of the stem, serves to
secure the transverse arm to this, and may be tightened or
to the Author, which are constructed, not only by the makers of the above,
but by Messrs. Baker, Browning, How, Murray and Heath, Pillischer, Eoss,
Swift, and Wheeler, as also by Mr. Dancer, of Manchester.
i2
100 CONSTRUCTION OF THE MICROSCOPE.
slackened at pleasure, so as to regulate the traversing movement
of the arm ; this movement is only allowed to take place in one
direction, namely, towards the right side, being checked in the
opposite by a ' stop,' which secures the coincidence of the axis of
the Body with the centre of the Stage, and with the axis of the
Illuminating apparatus beneath it. — It is in the movements of the
Stage that the greatest contrivance is shown : these are three,
namely, a traversing movement from side to side, a traversing
movement from before backwards, and a rotatory movement.
The traversing movements, which allow the platform carrying the
object to be shifted about an inch in each direction, are effected by
the two milled-heads situated at the right of the stage ; and these
are placed side by side, in such a position that one may be con-
veniently acted-on by the forefinger, and the other by the middle-
finger, the thumb being readily passed from one to the other. The
traversing portion of the stage carries the Platform whereon the
object is laid, which has a ledge at the back for it to rest against ;
and this platform has a sliding movement of its own, from before
backwards, by which the object is first brought near to the axis of
the Microscope, its perfect adjustment being then obtained by the
traversing movement. To this platform, and to the traversing
slides which carry it, a rotatory movement is imparted by a milled-
head placed underneath the stage on the left-hand side ; for this
milled-head turns a pinion which works against the circular rack
(seen in the figure), whereby the whole apparatus above is carried
round about two-thirds of a revolution, without in the least dis-
turbing the place of the object, or removing it from the field of the
Microscope. The graduation of the circular rack, moreover,
enables it to be used as a Goniometer (§ 79). In the improved
form of this instrument here represented, the whole Stage-
apparatus is made so thin, and the opening beneath so large, as to
permit the employment of light of extreme obliquity ; and to
enable the Mirror to afford this, it is mounted upon an extending
arm, the socket of which slides upon a cylindrical stem. Below
the stage, and in front of the stem that carries the mirror, is a
dovetail sliding-bar, which is moved up and down by the milled-
head shown at its side ; this sliding-bar carries what is termed by
Mr. Ross the ' Secondary Stage' (shown separately at b), which
consists of a cylindrical tube for the reception of the Achromatic
Condenser, Polarizing prism, and other fittings ; it is here shown
as fitted with a Condenser specially devised by Mr. T. Ross for the
illumination of a large field under low magnifying powers. To this
Secondary Stage, also, a rotatory motion with a graduated circle is
communicated by the turning of a milled-head ; and a traversing
movement of limited extent is likewise given to it by means of two
screws, one on the front and the other on the left-hand side of
the frame which carries it, in order that its axis may be brought
into perfect coincidence with the axis of the body. — The special
advantages of this instrument consist in its steadiness, in the
ROSS'S FIRST-CLASS MICROSCOPE.
101
Fig. 42.
Ross's First-Class Microscope.
102 CONSTRUCTION OF THE MICROSCOPE.
admirable finish, of its workmanship, and in the variety of move-
ments which may be given both to the Object and to the fittings of
the Secondary Stage. Its disadvantages consist in the want of
portability that necessarily arises from the substantial mode of
its construction ; and in the multiplicity of its moveable parts,
which presents to the beginner an aspect of great complexity.
This complexity, however, is much more apparent than real ; for
each of these parts has an independent action of its own, the
nature of which is very soon learned ; and the various milled-heads
are so disposed that the hand readily (and at last almost in-
stinctively) finds its way from one to the other, so as to make any
required adjustment whilst the eye is steadily directed to the
object. To the practised observer, therefore, this multiplication of
adjustments is a real saving of time and labour, enabling him to
do perfectly and readily what might otherwise require much
trouble, besides affording him certain capabilities which he would
not otherwise possess at all.
58. New Boss-Jackson Model. — The modifications of the usual
Jackson type, introduced by Mr. Wenham's advice in the newer in-
strument, are shown in Plate v. The foot is extremely solid, cast in
one piece, and of a shape that insures extreme steadiness in all posi-
tions of the instrument. The curve of the arm sustaining the body
allows the large screws of the slow motion and the stage screws to be
brought nearer together. The body is attached to a firm frame
that carries the rack, and the rack fits into a ploughed groove, as in
the Jackson model, Plate vn. The fine adjustment works, as in the
Ross model, upon the lever principle, and is attached to the frame
that carries the body, in a position not likely to cause any vibration
when used with high powers. It is always within reach of one of
the fingers of the hand grasping the large milled-head. The Stage
has all the movements of that in Fig. 42, but its supports are
stronger. The arrangements of the Sub-stage are also very similar.
The under slide being set back to correspond with the upper one,
the space beneath the Stage is left quite clear when the Sub- stage
is removed. Like the original model, this one has a clamping
screw, worked by a short lever, by which the instrument can be
firmly fixed in any required position.
59. Poivell and Lealand's First-class Microscopes. — The earlier
form, represented in Fig. 43 * is light in its general ' build,' without
being at all deficient in steadiness. Its character is sufficiently
shown by the engraving. Though less complete than that exhibited
in Plate vi., it may be preferred by some purchasers on account of
its smaller cost and greater portability. Like the more perfect
pattern of the same makers, it is of admirable workmanship. This
later pattern (Plate vi.) resembles the preceding in its general plan
of construction, though much more massive ; but differs from it en-
tirely in the construction of the stage and sub-stage, both of which
rest on the foundation of a large solid brass ring, firmly attached to
* A smaller and lighter form of this instrument is made, in which the legs
fold together, so that it admits of being packed into a flat case.
PLATE V.
Boss's Lae&e Jackson-Model Microscope.
[To face p. 102.
POWELL AND LEALAND'S SMALLER MICROSCOPE. 103
Fig. 4a
Powell and Lealand's Smaller Microscope.
104 CONSTRUCTION OF THE MICROSCOPE.
the stem of tlie instrument. The upper side of this ring bears a sort
of carriage that supports the Stage ; and to this carriage a rotatory
movement is given by a milled-head, the amount of the movement
(which may be carried through an entire revolution) being exactly
measured by the graduation of a circle of gun-metal, which is borne
on the upper surface of the ring. The rotatory action of the Stage
being thus effected beneath the traversing movement, the centering
of an object brought into the axis of the Microscope is not disturbed
by it ; and the workmanship is so accurate, that the stage may be
made to go through its whole revolution without throwing out of
the field an object viewed even with the J -16th inch objective. The
Stage, which is furnished with the usual traversing movements, is
made thin enough to admit of the most oblique light being thrown
on the object. It is worked upon Turrell's plan, by two milled-
heads placed upon the same axis, instead of side by side, and it is
furnished with graduated scales, so that the place of any particular
object can be registered without the use of a ' finder' (§ 85). The
Sub-stage also is furnished with rotatory and rectangular, as well
as with vertical movements ; and, like that of Eoss and Beck, it is
mode in such a manner as to admit of the simultaneous use of the
Polarizing prism and of the Achromatic Condenser. The Mirror
has a doubly -extending arm ; and can be so placed as to reflect
light upon the object from outside the large brass ring that
supports the stage and sub-stage. Light of the greatest
obliquity, however, may be more conveniently obtained by an
Amici's prism (§ 91) placed above the supporting ring. — Not-
withstanding the weight of all this apparatus, the instrument is so
well balanced on its horizontal axis, that it remains perfectly stead}'-
without clamping, in whatever position it may be placed. And in
regard to the apparent complexity of its arrangements, the re-
marks already made upon Mr. Boss's instrument are equally
applicable to the one described.
60. Messrs. Becks' First-class Microscope. — It was by this Firm
that the Jackson model was first adopted, for which the Author has
already expressed his preference (§ 44) : the support of the Body
along a large proportion of its length, upon the substantial Limb
to which the Stage is securely attached, giving it a decided advan-
tage in steadiness over any form of instrument (not exceeding it in
massiveness) in which the Body is attached at its lower extremity
only to an Arm between which and the Stage there is no fixed
connexion; whilst the Back-and-pinion movement giving the
' coarse ' adjustment can be made to work more easily on this con-
struction, than where it is requisite that the stem moved by it
should be fitted as tightly as possible. On the other hand, it must
be admitted that the ' fine ' adjustment can be more effectually
made by the longer leverage provided in the Eoss model, than by
the attachment of the screw to the lower end of the body, as in the
instrument before us. The Stage of the older form of this instru-
ment was furnished with the usual traversing movements, and
was made (by an arrangement first devised by Messrs. Smith and
PLATE VI.
Powell and Lealand's Labge Micboscope.
[To face p. 104.
PLATE VJ1.
Messes. Beck's Large Microscope.
[To face p. 105.
BECK'S FIKST-CLASS MICEOSCOPE. 105
Beck, and since adopted by other makers) so thin as to allow of
extremely oblique illumination ; but although the platform which,
carries the object could be made to rotate upon the traversing
apparatus, yet the object was liable to be thrown out of centre by this
rotation. This has been completely remedied in the newer pattern
shown in Plate vn., the Stage of which has a nearly complete rota-
tion in the optic axis of the instrument. This rotation is effected by
a milled-head and pinion ; which, by a shifting movement can be
thrown out of gear, so as to allow the Stage to be rotated rapidly
by hand, which is often advantageous. This Stage is furnished
with a graduated circle, to which a Yernier can be attached when
desired for the measurement of angles. Below the stage is the
ingenious ' Iris Diaphragm.' The new concentric stage can be
added at a moderate cost to the first-class stands on the old
pattern. — Beneath the stage in either form is a continuation of the
gun -metal 'limb' which carries the body; and this is ploughed
out into a groove for the reception of a sliding-bar, which carries
what may be termed the Secondary Body — namely, a short tube
(seen beneath the stage) capable of being moved up and down by
a milled-head, which answers the same purpose as the ' secondary
stage' of Ross's Microscope. Being made to work in a groove
which is in perfect correspondence with that wherein the principal
' body ' works (this correspondence being secured by the action of
the planing-machine that ploughs both grooves), the ' secondary '
body always has its axis so perfectly continuous with that of
its principal, that no special adjustment is needed to 'centre'
the greater part of the illuminating apparatus. The ' secondary
body ' or ' cylindrical fitting ' is so constructed as to carry the
Achromatic Condenser at its upper end, the Polarizing prism at
its lower, and the Selenite plates between the two (§ 98) ; it has
not, however, any rotatory movement of its own ; but its fittings
may be turned in the tube which carries them. The Mirror is
hung in the usual way between two centres ; but the semicircle
that carries these, instead of being at once pivoted to the tube
which slides upon the cylindrical stem, is attached to an inter-
mediate arm ; and by means of this it may be placed in such a
position as to reflect light very obliquely upon the object.
Though the mode in which the body is supported has the dis-
advantage of separating the focal adjustments from each other and
from the stage-motions more widely than is the case in the three
preceding instruments, yet the difference is scarcely perceptible in
practice. The milled-heads acting on the former are both of them
in positions in which they are easily reached by the left hand,
when the elbow is resting on the table ; whilst the right hand
finds the milled-heads of the traversing stage and of the secondary
body in close proximity to each other *
* Several other Opticians may be named as makers of Microscopes which
deserve to rank in the First Class, on account both of their Optical and of their
Mechanical excellence ; such are the instruments constructed by Messrs.
Browning, Baker, Collins, Crouch, Dallmeyer, Ladd, Pillischer, Swift. These
106 CONSTRUCTION OF THE MICROSCOPE.
Microscopes for Special Purposes.
Of the large number of Instruments which have been inge-
niously devised, each for some particular use, it would be quite
foreign to the purpose of this Treatise to attempt to give an
account. A few forms, however, may be noticed, as distinguished
either by their special adaptiveness to very common wants, or by
the ingenious manner in which the requirements of particular
classes of investigators have been met.
61. Dr. BeaWs Pocket Microscope. — This instrument consists
of an ordinary Microscope-body, the Eye-piece of which is fitted
with a draw-tube, which slides smoothly and easily ; whilst its
lower end is fitted into an outer tube, of which the end projects
beyond the objective. Against this projecting end the Object-slide
is held by a spring, as shown in Fig. 44, being fixed (if necessary)
by a screw-clip. The coarse adjustment is made by sliding the
body through the outer tube which carries the object; and the
fine adjustment by sliding the eye-tube in or out. The object, if
transparent, is illuminated either by holding up the Microscope to
a window or lamp, from which the rays may pass directly through
it, or by directing it towards a mirror laid on the table at such an
angle as to reflect light from either of these sources : if opaque,
it is allowed to receive direct light through an aperture in the
outer tube. The extreme simplicity and portability of this instru-
ment (which when closed is only six inches long) constitute its
special recommendation. Being fitted with the Universal Screw
it may be worked with the Objectives of any British maker ; and
with due care even high powers may be used, the eye-piece adjust-
ment (first employed for this purpose by Mr. Highley) giving the
power of very exact focussing. Hence this Pocket Microscope may
be conveniently applied to the purposes of Clinical observation
(the examination of Urinary Deposits, Blood, Sputa, &c), either in
hospital or in private practice ; whilst it may also be advantage-
ously used by the Field Naturalist in examining specimens of
Water for Animalcules, Protophytes, &c.
62. Dr. Beetle's Demonstrating Microscope. — The same instru-
ment has been successfully employed by Dr. Beale for the pur-
poses of Class-demonstration, its outer tube being attached by
a wooden support to a horizontal board, which also carries a small
lamp attached to it in the required position (Fig. 44). The object
having been fixed in its place, and the coarse adjustment made by
sliding the body in the outer tube, these parts may then be im-
movably secured, and nothing need be left moveable except the
eye-tube, by sliding which in or out the fine adjustment may be
effected. Thus the whole apparatus may be passed from hand to
hand with the greatest facility, and without any probability of
are for the most part copied, with more or less of modification in detail, from
the models either of Mr. Ross, or of Messrs. Smith and Beck ; very little that
is original having been introduced.
DEMONSTRATING AND TRAVELLING MICROSCOPES. 107
disarrangement ; and every observer may readily ' focns ' for him-
self, without any risk of injuring the object*
Fig. 44.
Dr. Beale's Demonstrating Microscope.
63. Baker's Travelling Microscope. — An instrument has been
devised by Mr. Moginie, which is but little inferior in portability
Fig. 45.
Baker's Travelling Microscope.
* The price of Dr. Beale's Clinical Microscope, without Objectives, is only
£1 5s. That of the same instrument fitted up as a Demonstrating Micro-
108 CONSTKUCTION OF THE MICKOSCOPE.
to the Pocket Microscope of Prof. Beale, and has many advantages
over it. The Body (Fig. 45) slides in a tube which is attached
to a stem that carries at its lower end a small Stage and Mirror.
The Stem itself contains a fine adjustment that is worked by a
milled-head at its summit ; and near to this is attached by a pivot-
joint a pair of legs, which, when opened out, form with the stem a
firm tripod support, The coarse adjustment having been made by
sliding the body through the tube which grasps it, the fine adjust-
ment is made by the milled-head ; and thus even high powers may
be very conveniently worked. The legs being tubular, one of them
is made to hold glass dipping-tubes, whilst the other contains
needles set in handles, with three short legs of steel wire, by
screwing which into the stem and stage, the instrument may be
used (though not without risk of overturn) in the vertical position.
Where the extreme of portability, however, is not required, a
folding foot is supplied, which enables the Microscope to be used
in the vertical position with satisfactory security and steadiness :
and the instrument thus fitted can be packed into a small flat box,
in such a limited compass that space is still left for the Objectives
and Accessory apparatus most useful to the working Naturalist.
This instrument may be specially recommended to those who,
already possessing a superior Microscope, desire neither to en-
cumber themselves with it whilst travelling, nor to expose it to
the risk of injury, but wish to utilize its Objectives by means of
a simple and portable arrangement.*
64. King's Pneumatic Aquarium Microscope. — The purpose of
this instrument is to enable such as possess an Aquarium to apply
the Microscope to the examination of the structure and habits of
the living animals it may contain, without disturbing or interfering
with them in any way. It is simply a Microscope especially adapted
for use with very low powers (a 2-inch and a 4-inch combination
will be found most serviceable), which can be attached by a kind
of sucker to the glass of the Aquarium, whether round or flat ; the
needful exhaustion being made by turning a screw.f
65. Dr. Lawrence Smith's Inverted Microscope. — A very inge-
nious arrangement has been devised by Dr. J. Lawrence Smith, of
Louisiana, U.S., whereby objects may be viewed from their under
instead of from their upper surface ; and thus Heat or Eeagents
may be applied to them, without any risk of dimming or more
seriously injuring the object-glass by the vapours thus raised. The
general plan of this instrument, as constructed by MM. Nachet, is
scope, is £3. — An excellent Demonstrating Microscope is made also by Messrs.
Murray and Heath ; and Mr. Collins has recently devised a new pattern for
Hospital use, which may be used either as a Demonstrating or as an ordinary
Student's Microscope.
* An instrument nearly resembling the above is made by Messrs. Murray
and Heath, and a similar one by Mr. Browning.
t The Aquarium Microscope is made by Mr. Collins, at the price of
8 guineas.
INVERTED MICROSCOPE.
109
shown in Fig. 46, whilst Fig. 47 explains the principle of its action.
The Body is screwed obliquely into a kind of box which is attached
to the base of the instrument, and which contains a Prism of the
form shown in Fig. 46, its angles being respectively 55°, 107ic, 52^,
Fig. 46.
Fig. 47.
Dr. Lawrence Smith's Inverted Microscope.
Inverting Prism.
and 145°. The Objective is screwed erect into this box, pointing
upwards towards the lower side of the stage; and it is so at-
tached that the coarse focal adjustment may be made by sliding
it up and down, whilst the fine adjustment is made by means of a
milled-head just above the prism-box. The Illuminating apparatus
is of course placed above the stage, the light having to be sent
downwards instead of upwards. Besides the Mirror, there is an
arm which may carry Diaphragms, Polarizing prism, &c. When
it is desired to apply Heat to an object, this is effected by placing
the glass whereon it lies upon a plate of metal large enough to pro-
ject beyond the stage, and by applying to the projecting part of
this plate the flame of a spirit-lamp. The Optical part of the in-
strument is so fitted to the base, that it may be entirely drawn away
from beneath the stage, for the sake of changing the powers. Its
action will be readily understood from an inspection of the dia-
gram (Fig. 47). The luminous rays which pass downwards from
the object through the objective, impinge upon the prism at a per-
pendicularly to its surface ; when they meet its first oblique sur-
face at b they undergo total reflexion, by means of which they are
sent on to c, where they meet its second oblique surface, and are
again totally reflected, so as to pass forth at d perpendicularly to
its surface, and consequently without refraction. — This instrument
is extremely well adapted, not merely for Chemical investigations,
but also for the examination of any objects (such as Diatomacese)
no
CONSTRUCTION OF THE MICEOSCOPE.
that sink to the bottom of the liquid in which they are immersed ;
since, by coming into contact with the glass on which they lie, their
surfaces are seen more exactly in one plane than when viewed from
above. It is also well adapted for the purpose of Dissection ; the
hands and instruments being left much more free to work, when
the object-glass does not stand in their way.*
66. Racket's Double-Bodied Microscope. — The division of the
pencil of rays issuing from the object-glass by a separating Prism
placed in its course, first introduced for the production of Stereo-
scopic effects (§§ 31-34), has been applied by MM. Nachet to
another purpose, — that of enabling two or more observers to look
at the same object at once, which is often a matter not only of
considerable convenience, but also of great importance, especially
in the demonstration of dissections. The instrument, as arranged
for this purpose, is shown in
Fig. 48. Fig. 48. MM. Nachet have also
devised another arrangement, by
which the form of the separating
Prism is adapted to divide the
pencil into three or even into four
parts, each of which may be di-
rected into a different body, so
as to give to several observers at
one time a nearly identical image
of the same object. Of course,
the larger the number of secon-
dary pencils into which the pri-
mary pencil is thus divided, the
smaller will be the share of light
which each observer will receive ;
but this reduction does not inter-
fere with the distinctness of the
image, and may be in some de-
gree compensated by a greater
intensity of illumination.f
67. Powell and Lea-land* s Non- Stereoscopic Binocular. — The
great comfort which is experienced by the Microscopist from the
conjoint use of both Eyes, has led to the invention of more than
* The cost of this instrument, as made by MM. Nachet, and furnished with
four Objectives, Micrometer eye-piece, Goniometer, and other accessories, is
only 350 francs, or £14. — Dr. Leeson may fairly claim the credit of an inde-
pendent inventor as regards this form of instrument; one essentially the same
having been constructed for him by Messrs. Smith and Beck, at the same time
that Dr. J. L. Smith's pattern was being worked out by MM. Nachet. See
Mr. Highley's account of his Mineralogical Microscope, in " Quart. Journ. of
Micros. Sci.," Vol. iv. p. 281. A Microscope on the same principle was con-
structed, in 1834, by M. Charles Chevalier for M. Dumas. It is figured in
" L'Etudiant Micrographe," par Arthur Chevalier. Paris, 1864.
t The price of the Double-bodied Microscope, with three Objectives, is
300 francs, or about £12.
Nachet's Double-bodied Microscope.
NON-STEREOSCOPIC BINOCULAR.
Ill
Em. 49.
one arrangement by which this comfort can be secured, when
those high powers are required which cannot be employed with the
Stereoscopic Binocular. This is accomplished by Messrs. Powell
and Lealand by taking advantage of the fact
already adverted to (§ 1), that when a pencil of
rays falls obliquely upon the surface of a refract-
ing medium, a part of it is reflected without en-
tering that medium at all. In the place usually
occupied by the Wenham prism, they interpose
an inclined plate of glass with parallel sides,
through which one portion of the rays proceeding
upwards from the whole aperture of the Objective
passes into the principal Body with very little
change in its course, whilst another portion is
reflected from its surface into a rectangular prism
so placed to direct it obliquely upwards into the
secondary Body (Fig. 49). Although there is a
decided difference in brightness between the two
images,* that formed by the reflected rays being
the fainter, yet there is marvellously little loss
of definition in either, even when the 25th-inch
Objective is used. The disk and prism are fixed
in a short tube, which can be readily substituted
in any ordinary Binocular Microscope for the one
containing the Wenham prism. — The Author can
bear the most explicit testimony to the diminu-
tion of fatigue resulting from the use of this little apparatus : by
which a prolonged employment of high powers is permitted, that
would be prejudicial to the eye used singly; whilst it entirely
prevents that bad effect which is liable to proceed from the too
exclusive use of a single eye, the impairment of its power of focus-
sing consentaneously with the other eye in ordinary vision.
* An arrangement has been devised by Mr. "Wenham (" Transact, of Microsc.
Soc," Vol. xiv. p. 103), by which the brightness of the images is more nearly
equalized; but this involves difficulties of construction with which no one
save its ingenious inventor has successfully grappled.
Powell and Lea-
land's Non-Stereo-
scopic Binocular
Apparatus.
CHAPTER in.
ACCESSORY APPARATUS.
In describing the various pieces of Accessory Apparatus with which
the Microscope may be furnished, it will be convenient in the first
place to treat of those which form (when in use) part of the instru-
ment itself, being Appendages either to its Body or to its Stage,
or serving for the Illumination of the objects which are under
examination ; and secondly, to notice such as have for their function
to facilitate that examination, by enabling the Microscopist to bring
the Objects conveniently under his inspection.
Section 1. Appendages to the Microscope.
68. Draw-Tube. — It is advantageous for many purposes that
the Eye-piece should be fitted, not at once into the ' body' of the
Microscope, but into an intermediate Tube ; the drawing-out of
which, by augmenting the distance between the Objective and the
Image which it forms in the focus of the eye-glass, still further
augments the size of the image in relation to that of the object
(§ 25). For although as a general rule the magnifying power
cannot be thus increased with advantage to any considerable extent,
yet, if the corrections of low objectives have been well adjusted,
their performance is not seriously impaired by a moderate lengthen-
ing of the body ; and recourse may be conveniently had to this on
many occasions in which some amplification is desired, intermediate
between the powers furnished by any two Objectives, Thus if
one objective give a power of 80 diameters, and another a power
of 120, by using the first and drawing out the Eye-piece, its power
may be increased to 100. Again, it is often very useful to make
the Object fill up the whole, or nearly the whole, of the field of
view : thus if an object that is being viewed by transmitted rays
is so far from transparent as to require a strong light to render its
details visible, the distinctness of those details is very much inter-
fered with, if, through its not occupying the peripheral part of the
field, a glare of light enter the eye around its margin; and the
importance of this adjustment is even greater, if opaque objects
mounted on black disks are being viewed by the Lieberkiihn (§ 92),
since, if any light be transmitted to the eye direct from the mirror,
in consequence of the disk failing to occupy the entire field, it
greatly interferes with the vividness and distinctness of the image
DEAW-TUBB AND EEECTOE,
113
of the object. In the use of the Micrometric eye-pieces to be pre-
sently described (§§ 76, 77), very great advantage is to be derived
from the assistance of the Draw-tube ; as enabling ns to make
a precise adjustment between the divisions of the Stage-micrometer
and those of the Eye-piece micrometer; and as admitting the
establishment of a more convenient numerical relation between the
two than could be otherwise secured without far more elaborate
contrivances. Moreover, if, for the sake of saving room in packing,
it be desired to reduce the length of the body, the draw-tube affords
a ready means of doing so ; since the body may be made to ' shut-
up,' like a Telescope, to little more than half its length, without
any impairment of the optical performance of the instrument when
mounted for use (§48). — Deep objectives, however, require special
adjustment when any considerable length of draw-tube is used.
69. Lister's Erector. — It is only, however, in the use of the
Erector, that the value of the Draw-tube comes to be fully appre-
ciated. This instrument, first applied to the Com-
pound Microscope by Mr. Lister, consists of a tube Fig. 50.
about three inches long, having a meniscus at one
end and a plano-convex lens at the other (the
convex sides being upwards in each case), with a
diaphragm nearly half way between them; and
this is screwed into the lower end of the draw-
tube, as shown in Fig. 50". Its effect is (like the
corresponding erector of the Telescope), to antago-
nize the inversion of the image formed by the
object-glass, by producing a second inversion, so
as to make the Image presented to the eye corre-
spond in position with the Object — an arrange-
ment of great service in cases in which the object
has to be subjected to any kind of manipulation.
The passage of the rays through two additional
lenses of course occasions a certain loss of light by
reflexion from their surfaces, besides subjecting
them to aberrations whereby the distinctness of
the image is somewhat impaired ; but this need
not be an obstacle to its use for the class of pur-
poses for which it is especially adapted in other
respects, since these seldom require a very high
degree of defining power. By the position given
to the Erector, it is made subservient to another
purpose of great utility ; namely, the procuring a Draw-tube fitted
very extensive range of Magnifying power, without with Erector,
any change in the Objective. For when the draw-
tube, with the erector fitted to it, is completely pushed-in, the
acting length of the body (so to speak) is so greatly reduced by the
formation of the first image much nearer the objective, that, if a
lens of 2-3rds of an inch focus be employed, an object of the
diameter of 1^ inch can be taken in, and enlarged to no more
114
ACCESSORY APPARATUS.
than 4 diameters ; whilst, on the other hand, when the tnbe is
drawn-out 4| inches, the object is enlarged 100 diameters. Of
course every intermediate range can be obtained by drawing-ont
the tube more or less ; and the facility with which this can be
accomplished, especially when the Draw-tube is furnished with a
rack-and-pinion movement (as in Messrs. Becks' Compound Dissect-
ing Microscope), renders such an instrument very useful in various
kinds of research.
70. Nachet's Erecting Prism. — An extremely ingenious arrange-
ment has been made by MM. Nachet, on the basis of an idea first
carried into practice by Prof. Amici, by which the inverted image
given by the Compound Microscope is erected by a single rectan-
gular Prism placed over the Eye-piece. The mode in which this
prism is fitted up is shown in Fig. 51 (2) ; the rationale of its action
is explained by the diagram Fig. 51 (1). The Prism is interposed
Fig. 51.
between the two lenses of the Eye-piece, and has somewhat the form
of a double wedge, with two pentagonal sides, abode, and a b h g f,
which meet each other along the common edge a b, and two facets,
defg, and c D g h, which meet along the common edge d g, the
edges a b and d g being perpendicular to each other. The rays
emerging from the Field-glass enter this prism by its lower surface,
and are reflected at I upon the face abhgi, from which they are
again reflected upon the lower surface at the point k, and thence to
the point l upon the vertical face cbgh, and lastly at the point m
upon the other vertical face defg; from which the image, normally
and completely erected, is again sent back, to issue by the superior
surface upon which the Eye-glass is placed. All the reflexions are
total, except the first at i ; and the loss of light is far less than
would be anticipated. The obliquity which this Prism gives to the
visual rays, when the Microscope is placed vertically for dissecting
or for the examination of objects in fluid, is such as to bring them
to the eye at an angle very nearly corresponding with that at which
the Microscope may be most conveniently used in the inclined
MICEO-SPECTKOSCOPE. 115
position (§ 38, in.) ; so that, instead of being an" objection, it is a
real advantage.
71. Sorby-Broivning Micro- Spectroscope. — For general informa-
tion on the Spectroscope and its nses, the stndent can consult
Professor Roscoe's "Lectures on Spectrum Analysis," or the trans-
lation of Dr. Schellen's " Spectrum Analysis." It will suffice
here to indicate the special advantages to be derived from adapting
the Spectroscope to the Microscope according to the Sorby-Brown-
ing method ; other forms of the instrument being usually prefer-
able for viewing the spectra of incandescent bodies. — The Micro-
Spectroscope is not adapted for investigations in which a large
amount of dispersion is required ; but it is the most convenient
apparatus for the examination of the highly interesting and im-
portant phenomena of absorption bands, or the dark cloudy inter-
ruptions of the normal solar or daylight spectra, which occur when
light is made to pass through, or is reflected from, a variety of solid
or fluid bodies. The Micro- Spectroscope also furnishes the means
of viewing the spectra of exceedingly minute quantities of such
bodies ; so delicate is it, that a single Red Corpuscle of Human
Blood, or even a portion of it, will exhibit the characteristic bands.
In this case a high power must be employed ; but Objectives from
2 inches to 2-3rds inch will be found most convenient for general
use.
72. When the Solar Spectrum is viewed through a prism of suffi-
cient dispersion, to which the light is admitted by a narrow slit, a
multitude of black lines make their appearance. The existence of
these lines was originally noticed by Wollaston ; but Fraunhofer
first gave the subject a thorough investigation, and mapped them
out. Hence they are known as Fraunhofer lines.* The greater the
dispersion given by the spectroscope, the more of these lines are
seen ; and they bear considerable magnification. They result from
interruptions, or absorptions of certain rays ; the law, first stated
by Angstrom, being that " rays which a substance absorbs are pre-
cisely those which it emits when made self-luminous. "f Kirchhoff
showed that the incandescent vapours of Sodium, Potassium,
Lithium, &c, give a spectrum with characteristic bright lines ; and
that the same vapours intercept portions of white light, so as to
give dark lines in place of the bright ones, absorbing their own
special colour, and allowing rays of other colours to pass through.
Absorption-bands differ from the Fraunhofer lines, not only in
their greater breadth, but in being more or less nebulous or
cloudy. They cannot be resolved into distinct lines by magnifica-
tion, and too much dispersion thins them out to indistinctness.
The Micro- Spectroscope being specially intended to view such bands,
its dispersive powers are moderate, and the whole spectrum, from
* Mr. Browning has published a beautiful photograph of the original chart
drawn and engraved by Fraunhofer, which was presented to him by Lord
Lindsay.
t " Schellen Trans.," p. 204.
i2
116
ACCESSORY APPARATUS.
Fig. 52.
Micro -Spectroscope.
Fig. 53.
the red to the violet, comes into one field of view. The Sorby-
Browning Micro- Spectroscope can be applied as an Eye-piece to
any Microscope. This appa-
ratus, represented in Fig. 52,
fundamentally consists of an
ordinary Eye-piece, provided
with certain special modifica-
tions. Above its Eye-glass,
which is Achromatic, and ca-
pable of focal adjustment for
rays of different refrangibili-
ties, there is placed a tube con-
taining a series of five prisms,
two of Flint-glass (Fig. 53, f f)
interposed between three of
Crown (c c c), in such a man-
ner that the emergent rays r r,
which have been separated by
the dispersive action of the
flint-glass prisms, are parallel to the rays which enter the com-
bination. Below the eye-glass, in the place of the ordinary stop,
is a Diaphragm with a narrow slit, which limits the admission of
light. This, with an Objective of
suitable power, would be all that is
needed for the examination of the
Spectra of objects placed on the
stage of the Microscope, whether
opaque or transparent, solid or
liquid, provided that they transmit
a sufficient amount of light. But as
it is of great importance to make
exact comparisons of such Artificial spectra, alike with the
Ordinary or Natural spectrum, and with each other, provision is
made for the formation of a second spectrum, by the insertion of a
right-angled prism that covers one-half of this slit, and reflects
upwards the light transmitted through an aperture seen on the
right side of the eye-piece. For the production of the ordinary
spectrum, it is only requisite to reflect light into this aperture
from the small mirror i carried at the side ; whilst for the pro-
duction of the spectrum of any substance through which the light
reflected from the mirror can be transmitted, it is only necessary to
place the slide carrying the section or crystalline film, or the tube
containing the solution, in the frame d d adapted to receive it. In
either case, this second Spectrum is seen by the eye of the observer
alongside of that produced by the object viewed through the body
of the Microscope, so that the two can be exactly compared *
73. The exact position of Absorption-bands is as important as that
* See Mr. Sorby's description of this apparatus aud of the mode of using it,
in the " Popular Science Eeview" for Jan. 1866, p. 66.
Arrangement of prisms in Spectro-
scope Eye-piece.
BRIGHT-LINE SPECTRO-MICROMETER.
117
Fig. 54.
of the Fraunhofer lines ; and some of the most conspicuous of the
latter afford fixed points of reference, provided the same spectro-
scope is employed. The amount of dispersion determines whether
the Fraunhofer lines or
absorption bands are seen
nearer, or farther apart;
their actual positions in the
field of view varying accord-
ing to dispersion, while their
relative positions are in con-
stant proportions. — The best
contrivance for measuring
spectra of absorption bands
is Browning's Bright-Line
Micrometer, shown in Fig.
54. a is a small mirror by
which light from the lamp
employed can be reflected
through e d to the lens c,
which, by means of a perfo-
rated stop, forms a bright
pointed image on the sur-
face of the upper prism,
from whence it is reflected
to the eye of the observer.
m is a wheel and milled-
head. Its rotation carries
the bright point over the
spectrum, and the exact
amount of motion may be
read off to the 10-1000" on
the graduated circle of the
wheel. To use this appa-
ratus, the Fraunhofer lines
must be viewed by sending
Bright-line Spectro-Micrometer.
bright daylight through the spectroscope, and the positions of the
principal ones carefully measured, the reading on the micrometer-
wheel being noted down. A Spectrum-map may then be drawn
on cardboard, on a scale of equal parts, and the lines marked on
it, as shown in the upper half of Fig. 55. The lower half of the
same figure shows an absorption-spectrum, with its bands at
certain distances from the Fraunhofer lines. The cardboard Spec-
trum-map, when once drawn, should be kept for reference.*
74 A beginner with the Micro- Spectroscope should first hold it
up to the sky on a clear day, without the intervention of the micro-
scope, and note the effects of opening and closing the slit by rotating
* Mr. Browning has constructed an apparatus, attached to the Bright-Line
Micrometer, by which any spectrum can be accurately drawn on a definite
scale of enlargement by mechanical means.
118
ACCESSOEY APPAEATUS.
the screw c (Fig. 52) ; the lines can only be well seen when the
slit is rednced to a narrow opening. The screw h diminishes
the length of the slit, and causes the spectrum to be seen as a
broad or a narrow ribbon. The screw e (or in some patterns two
small sliding-knobs) regulates the quantity of light admitted
through the square aperture seen between the points of the springs
d d. — Water tinged with port wine, Madder, and Blood, are good
fluids with which to commence the study of absorption-bands.
They may be placed in small test tubes, in flat glass cells, or in
wedge-shaped cells. The following list of objects, kept for sale in
small tubes, by Browning, will be useful ; and the subjoined re-
marks from his catalogue should be carefully attended to.
Class I.
Specimens for Illustrating the application of the Micro-
Spectroscope to Chemistry.
1. Didymium Nitrate.
2. Uranous Sulphate.
3. Uranic Acetate.
4. Cobalt in Calcium.
5. Cobalt in Alcohol.
6. Chloride of Uranium.
7. Cyanide of Cobalt. No. 1.
8. Cyanide of Cobalt. No. 2.
9. Oxalate of Chromium and
Soda.
10. Chromic Sulphate.
11. Nitrophenic Acid.
12. Hofmann's Yiolet.
Class II.
Specimens for Illustrating the Applications of the Micro-
Spectroscope to Vegetable Chemistry.
1. Lobelia Speciosa.
2. Purple Cineraria.
3. Interior of Carrot.
4. Alkanet Root. No. 1.
5. „ „ „ 2.
6. „ „ „ 3.
7. Normal Chlorophyll.
8. Acid Chlorophyll.
9. Purpurine from Madder.
No. 1.
10. Purpurine from Madder.
No. 2.
11. Camwood.
12. Annatto.
STUDY OF ABSORPTION-BANDS. 119
Class III.
Specimens for Illustrating the Application of the Micro-
Spectroscope to Medicine.
1. Cochineal.
2. Sulphate of Cruentine ")
3. Alkaline Cruentine ! Blood
4. Deoxidized Ha3maglobin C Compounds.
5. Alkaline „ J
b\ Ox-bile Preparation.
Class IV.
Specimens to Illustrate the Application of the Micro-Spectroscope
to Blowpipe Chemistry and Mineralogy.
BLOWPIPE BEADS.
1. Uranium Oxide.
2. Chromium Oxide.
3. Copper Oxide.
4. Cobalt Oxide. _
5. Didymium Oxide.
CRYSTALS, ETC.
6. Native Phosphate of Uranium.
7. Acetate of Uranium (Crystals).
8. Binoxalate of Potash and Chromium.
9. Cobalt Chloride (Crystals).
Class V.
Byes.
aniline series.
1.
2.
3.
Aniline Violet.
Mauve.
Aniline Green.
4. Aniline Blue.
5. „ „
6. Magenta.
No. 1.
„ 2.
75. " Objects belonging to Class iv. should invariably have a small
cardboard diaphragm, l-8th inch diameter, placed beneath them ; the
spectrum is then much better denned. With a slide containing a
mass of small crystals, the object need merely be thrown a little out
of focus. When observing the spectra of liquids in experiment-cells^
or through small test-tubes, always slip over the tube containing the
\\ or 2 in. objective a cap with a hole l-16th of an inch diameter.
Slide the tube just sufficiently to bring the small hole a little
within the focus of the objective. By this arrangement all
extraneous light is prevented from passing up the body of the
microscope, except what passes through the object. Unless this
precaution be attended to, a false result is sometimes obtained.
Substances which give bands or lines in the red, are best seen
by gaslight, while those which give bands in the blue are brought
120
ACCESSORY APPARATUS.
out far better by daylight. Such a specimen as Oxalate of Chro-
mium and Soda is almost opaque by daylight, showing no bands ;
though when examined by a lamp, the spectrum exhibits three
beautifully fine lines in the red, two of which are exceedingly
delicate. Again, Uranic Acetate can only be seen to advantage by
strong daylight, since the band in the violet would be invisible by
lamplight." — As each colour varies in refrangibility, the focus must
be changed according to the part of the spectrum that is examined.
This is done by the screw b, Fig. 52. — When it is desired to see the
spectrum of an exceedingly minute object, or of a small portion
only of a larger one, the prisms can be removed by withdrawing the
tube containing them. The slides should then be opened wide, and
the object, or part of it, brought into the centre of the field ; the
vertical and horizontal slits can then be partly shut, so as to en-
close it. If the prisms are then replaced, and a suitable objective
employed, the required spectrum will be seen unaffected by adja-
cent objects. — The spectrum of an incandescent body can be shown
B
D
Fig. 56.
E Z
p
e
1
i II
mi
2
F
V
3
1
H
IIH
4
1 i
i i
IIH
1, Spectroscopic appearance of fresh Scarlet Blood ; 2, of Deoxydized
Blood (cruorine) ; 3, of Hsernatin, obtained by acting on cruorine with
an acid ; 4, of Hseniatin reoxydized.
by admitting its light through the side slit between the points of
the springs d ; and can be brought into comparison with any other
spectrum formed by an object on the stage. A spirit lamp,
Bunsen gas-burner, or coil-machine, will give the heat required,
and can easily be arranged at the proper height of the slit, or
MICEOMETEIC APPARATUS. 121
the light can be reflected through it by the mirror f. — As speci-
mens of absorption-bands, those obtained by Professor Stokes from
Human Blood in different conditions (Fig. 56), are very instruc-
tive.*— Slices of Minerals often form interesting objects. Mr.
Lettsom, for example, recently found that specimens of Cerite
gave the spectrum of the recently discovered metal Didymium.
76. Micrometric Apparatus. — Although some have applied their
micrometric apparatus to the Stage of the Microscope, yet it is to
the Eye-piece that it may be most advantageously adapted.f The
Cobweb Micrometer, invented by Eamsden for Telescopes, is pro-
bably, when well constructed, the most perfect instrument that
the Microscopist can employ. It is made by stretching across
the field of an Eye-piece two very delicate parallel Wires or
Cobwebs, one of which can be separated from the other by the
action of a fine-threaded screw, the head of which is divided at its
edge into a convenient number of parts, which successively pass by
an index as the milled-head is turned. A portion of the field of
view on one side is cut off at right angles to the cobweb-threads, by
a scale formed of a thin plate of brass having notches at its edge,
whose distance corresponds to that of the threads of the screw,
every fifth notch being made deeper than the rest for the sake of
ready enumeration. The object being brought into such a position
that one of its edges seems to touch the stationary thread, the
other thread is moved by the micrometer- screw until it appears to
lie in contact with the other edge of the object ; the number of
entire divisions on the scale shows how many complete turns of the
screw must have been made in thus separating the threads, while
the number to which the index points on the milled-head shows
what fraction of a turn may have been made in addition. It is
usual, by employing a screw of 100 threads to the inch, to give to
each division of the scale the value of l-100th of an inch, and to
divide the milled-head into 100 parts ; but the absolute value of the
divisions is of little consequence, since their micrometric value
depends upon the Objective with which the instrument may be
employed. This must be determined by means of a ruled slip of
glass laid upon the stage ; and as the distance of the divisions
even in the best-ruled slip is by no means uniform.^ it is advisable
to take an average of several measurements, both upon different
* For further information on "The Spectrum Method of Detecting Blood,"
see an important paper by Mr. Sorby, in "Monthly Microsc. Journ.," July,
1871, p. 9.
f The Stage-micrometer constructed by Fraunhofer is employed by many
Continental Microscopists ; but it is subject to this disadvantage, — that any
error in its performance is augmented by the lohole magnifying power em-
ployed ; whilst a like error in the Eye-piece Micrometer is increased by the
magnifying power of the eye-piece alone.
\ Of the degree of this inequality, some idea may be formed from the state-
ment of Hannover, that the value of the different divisions of a glass ruled by
Chevalier to l-100th of a millimetre, varied between the extreme ratios of
31 : 36, the mean of all being 34.
122 ACCESSOEY APPARATUS.
slips, and upon different parts of the same slip. Here the Draw-
tube will be of essential use, in enabling the Microscopist to bring
the value of the divisions of his Micrometer to even numbers.
Thus, suppose that with a l-4th inch Objective, the tube being
pushed in, a separation of the lines by one entire turn and
37-100ths of another were needed to take in the space between
two lines on the ruled slip whose actual distance is one 1-1 000th of
an inch, then it is obvious that 137 divisions on the milled-head
are equivalent with that power to a dimension of 1-1 000th of an
inch, or the value of each division is 1-1 37,000th of an inch. But
as this is an awkward number for calculation, the magnifying
power may be readily increased by means of the Draw-tube, until
the space of l-1000th of an inch shall be represented by a separa-
tion of the cobweb-threads to the extent of 150 divisions ; thus
giving to each division the much more convenient value of
1-1 50,000th of an inch. The Microscopist who applies himself to
researches requiring micrometric measurement, should determine
the value of his Micrometer with each of the Objectives he is likely
to use for the purpose ; and should keep a table of these determi-
nations, recording in each case the extent to which the Tube has
been drawn out, as marked by the graduated scale of inches which
it should possess. And he should also make an accurate estimate
of the thickness of the Cobweb-threads themselves ; since, if this
be not properly allowed for, a serious error will be introduced into
the measurements made by this instrument, especially when the
spaces measured are extremely minute. (See Mitchell, in " Transact.
Microsc. Soc." Yol. xiv. p. 71.)
77. The costliness of the Cobweb Micrometer being an important
obstacle to its general use, a simpler method is more commonly
adopted, which consists in the insertion of a transparent scale
into the focus of the Eye-piece, on which the image of the object
is seen to be projected. By Mr. Andrew Boss, who first devised
this method, the ' positive' Eye-piece (§ 27) was employed, and a
glass plate ruled in squares was attached beneath its field-glass, at
such a distance that it and the image of the object should be in
focus together ; and the value of these squares having been deter-
mined with each Objective, in the manner already described,
the size of the object was estimated by the proportion of the
square that might be occupied by its image. While the use of the
positive eye-piece, however, renders, the definition of the ruled
lines peculiarly distinct, it impairs the definition of the object;
and the ' negative' or common Huyghenian eye-piece is now gene-
rally preferred. — The arrangement devised by Mr. G. Jackson
allows the divided glass to be introduced into the ordinary Eye-
piece (thus dispensing with the necessity for one specially adapted
for micrometry), and greatly increases the facility and accuracy
with which the eye -piece scale may be used. This Scale is ruled
like that of an ordinary measure {i.e., with every tenth line long,
and every fifth line half its length) on a shp of glass, which is so
EYE-PIECE MICROMETER.
123
fitted into a brass frame (Fig. 57, b), as to have a slight motion
towards either end; one of its extremities is pressed upon by a
small fine milled-head screw which works through the frame, and
the other by a spring (concealed in the figure) which antagonizes
the screw. The scale thus mounted is introduced through a pair
of slits in the Eye-piece tube, immediately above the diaphragm
(Fig. 57, a), so as to occupy the centre of the field; and it is
brought accurately into focus by unscrewing the glass nearest to
Lialilakii
Jackson's Eye-piece Micrometer.
the eye, until the lines of the scale are clearly seen. The value
of the divisions of this scale must be determined by means of a
ruled Stage-micrometer, as in the former instance, for each Objec-
tive employed in micrometry (the drawing out of the eye-piece
tube enabling the proportions to be adjusted to even and convenient
numbers) ; and this having been accomplished, the Scale is brought
to bear upon the object to be measured, by moving the latter as
nearly as possible into the centre of the field, and then rotating
the Eye -piece in such a manner that the scale may lie across that
diameter which it is desired to measure. The pushing-screw at
the extremity of the scale being then turned until one edge of the
object appears to be in exact contact with one of the long lines, the
number of divisions which its diameter occupies is at once read-off
by directing the attention to the other edge, — the operation, as Mr.
Quekett justly remarks, being nothing more than laying a rule
across the body to be measured. This method of measurement
may be made quite exact enough for all ordinary purposes, pro-
124 ACCESSORY APPARATUS.
vided, in the first place, that the Eye-piece Scale be divided with a
fair degree of accuracy ; and secondly, that the value of its divi-
sions be ascertained (as in the case of the cobweb micrometer) by
several comparisons with the scale laid upon the Stage. Thus if,
by a mean of numerous observations, we establish the value of
each division of the eye-piece scale to be 1-1 2,500th of an inch,
then, if the image of an object be found to measure 3^ of those
divisions, its real diameter will be 3| X y^ioo or '0028 inch *
"With an Objective of l-12th-inch focus, the value of the divisions
of the Eye-piece Scale may be reduced to l-25,000th of an inch ;
and as the Eye can estimate a fourth part of one of the divi-
sions with tolerable accuracy, it follows that a magnitude of as
little as l-100,000th of an inch can be measured with a near
apj>roach to exactness. Even this exactness may be increased by
the application of the diagonal scale (Fig. 82) devised by M.
Hartnack. The vertical lines are crossed by two parallel lines, at
Hartnack's Eye-piece Micrometer.
a distance from each other of five divisions of the vertical scale ;
and the parallelogram thus formed is crossed by a diagonal. It is
obvious from this construction, that the lengths of the lower seg-
ments of the 50 vertical lines, cut off by the diagonal, will pro-
gressively increase from *1 to 5*0; so that when it is desired to
obtain an exact measurement of an object between these limits, it.
is only requisite to find out that one whose length precisely coin
cides with the diameter to be taken, which it will then give in
tenths of the value of the vertical divisions, whatever these may be.
Thus, at a, the length of the segment will be 1 '8 ; at b it will be 3'4.
Micrometric measurements may also be made with the Camera
Lucida, in the manner to be presently described, or with the neutral
tint reflector so much used by Dr. Beale (§ 82).- — Whatever method
be adopted, if the measurement be made in the Eye-piece and not
on the stage, it will be necessary to make allowance for the adjust-
* The calculation of the dimensions is much simplified by the adoption of a
Decimal scale ; the value of each division being made, by the use of the Draw-
tube adjustment, to correspond to some aliquot part of a ten-thousandth or a
hundred-thousandth of an inch, and the dimensions of the object being then
found by simple multiplication : — Thus (to take the above example) the value
of each division in the decimal scale is *00008, and the diameter of the object
is -00028.
MICEOMETEES. — GONIOMETERS. 125
ment of the Object-glass to the thickness of the glass that covers
the object, since its magnifying power is considerably affected by
the separation of the front pair of lenses from those behind it (§ 127).
It will be found convenient to compensate for this alteration by
altering the Draw -tube in such a manner as to neutralize the effect
produced by the .adjustment of the Objective ; thus giving one
uniform value to the divisions of the Eye-piece scale, whatever may
be the thickness of the covering glass : the amount of the alteration
required for each degree must of course be determined by a series
of measurements with the Stage -micrometer.
78. Dr. Pigott's Micrometers. — In the " Monthly Microsc. Journ."
Jan. 1873, Dr. Pigott describes a plan of engraving micrometric
lines on a long focus plano-convex lens of an eye-piece. This,
executed by Mr. Ackland, gave good results. In the same paper
he describes a simple method of forming an aerial image of the
spider-lines of a cobweb micrometer, adding much to the delicacy
of the instrument, and capable of easy use.
79. Goniometer. — When the Microscope is employed in researches
on minute Crystals, a means of measuring their angles is provided
by the adaptation of a Goniometer to the eye-piece. The simplest
form (contrived by Schmidt and made by Boss), which answers
sufficiently well for all ordinary purposes, essentially consists
merely of a ' positive' eye-piece, with a single cobweb-thread
stretched diametrically across it in a circular frame capable of
rotation ; the edges of this frame are graduated in degrees, and a
Vernier also is attached to the index, whereby fractional parts of
degrees may be read off. By rotating the frame carrying the
thread, so that it shall lie successively in the directions of the two
sides of the crystal, the angle which they form is at once measured
by the difference of the degree to which the index points on the
two occasions. For the cobweb-thread, a glass plate, ruled with
parallel lines at about the l-50th of an inch asunder, may be advan-
tageously substituted ; since it is not then necessary to bring the
crystal into such a position as to lie along the diametrical thread,
but its angle may be measured by means of any one of the lines to
which it happens to be nearest. — If a higher degree of precision be
required than either of these methods is fitted to afford, the Double-
refracting Goniometer, invented by Dr. Leeson, may be substituted.*
The graduated Botatory Stages described as attached to First-
class Microscopes are usually found sufficient for angular measure-
ments, provided the eye-pieces employed exhibit a fixed line. This
line is brought into coincidence with one of the lines forming the
angle to be measured, when the stage is at zero ; the stage is then
rotated until the fixed line coincides with the other line of the
angle, and the amount of movement is read off on the scale.
80. Diaphragm Eyepiece. — It is often useful to cut off the light
* For a description of this instrument see Dr. Leeson's description of it in
Part xxxiii. of the " Proceedings of the Chemical Society," and Mr. Richard
Beck's Treatise on the Microscope, p. 65.
126 ACCESSORY APPARATUS.
surrounding the object or part of the object to be examined ; for
the sake alike of avoiding glare that is injurious to the eye, and of
rendering the features of the object more distinct. This may be
accomplished on the plan of Mr. Slack, by the introduction, just
above the ordinary ' stop,' of four small shutters, worked by as
many milled-heads projecting slightly beyond the flange of the eye-
piece. By combining the movements of these shutters in various
ways, it is easy to form a series of symmetrical apertures, bounded
by straight lines, and of any dimensions required. As remarked by
its inventor, this Diaphragm Eye-piece may also be used to isolate
one out of many objects that may be on the same slide, and thus to
show that object alone to persons who might not otherwise distin-
guish it. — For this last purpose the Indicator of Mr. Quekett may
also be used ; which is a small steel hand placed just over the dia-
phragm, so as to point to nearly the centre of the field, whilst it
may be turned back when not required, leaving the field of view
quite free. The particular object or portion of the object to which
it is desired to direct attention, being brought to the extremity of
the hand, is thus at once ' indicated ' to any other observer.
81. Camera Lucida and other Draiving Apparatus. — Various
contrivances may be adapted to the Eye-piece, in order to enable
the observer to see the image projected upon a surface whereon
he may trace its outlines. The one most generally employed is
the Camera Lucida prism contrived by Dr. Wollaston for the
general purposes of delineation; this being fitted on the front
of the Eye-piece, in place of the ' cap' by which it is usually sur-
mounted. The Microscope being placed in a horizontal position,
as shown in Fig. 59, the rays which pass through the Eye-piece
into the Prism sustain such a total reflexion from its oblique
surface, that they come to its upper horizontal surface at right
angles to their previous direction ; and the eye being so placed
over the edge of this surface that it receives these rays from the
prism through part of the pupil, whilst it looks beyond the
prism down to a white paper surface on the table with the other
half, it sees the image so strongly and clearly projected upon
the surface, that the only difficulty in tracing it arises from a
certain incapacity which seems to exist in some individuals for
seeing the image and the tracing-point at the same time. This
difficulty (which is common to all instruments devised for this pur-
pose) is lessened by the interposition of a slightly convex lens in
the position shown in the figure, between the eye and the paper,
in order that the rays from the paper and tracing-point may diverge
at the same angle as those which are received from the prism ;
and it may be generally got- over altogether, by experimentally
modifying the relative degrees of light received from the object
and from the paper. If the image be too bright, the paper, the
tracing -point, and the outline it has made, are scarcely seen ; and
either less light may be allowed to come from the object, or more
light (as by a taper held near) may be thrown on the paper and
CAMERA LUCIDA.
127
tracing-point. Sometimes, on the other hand, measures of the
contrary kind must be taken. — Another instrument for the same
purpose is a flat Speculwm of polished Steel, of smaller diameter
than the ordinary pupil of the eye, fixed at an angle of 45° in
Fig. 59.
Microscope arranged with Camera Lucida, for Drawing or Micrometry.
front of the Eye-piece ; and this answers exactly the same end as
the preceding, since the rays from the eye-piece are reflected verti-
cally upwards to the central part of the pupil placed above the
mirror, whilst, as the eye also receives rays from the paper and
tracer, in the same direction, through the peripheral portion of the
pupil, the image formed by the Microscope is visually projected
downwards, as in the preceding case. This Disk, the invention of
the celebrated anatomist Soemmering, is preferred by some micro-
scopic delineators to the camera lucida. The fact is, however (as
the Author can testify from his own experience), that there is a
sort of ' knack' in the use of each instrument, which is commonly
acquired by practice alone ; and that a person habituated to the use
of either of them does not at first work well with another. — A dif-
ferent plan is preferred by some Microscopists, which consists in
the substitution of a plate of neutral -tint or darkened glass for the
oblique mirror ; the eye receiving at the same time the rays of the
microscopic image, which are obliquely reflected to it from the
surface of the glass, and those of the paper, tracing-point, &c,
which come to it through the glass. — In another very ingenious
arrangement, devised by Professor Amici, and adapted to the hori-
123
ACCESSORY APPARATUS.
zontal microscope by M. Chevalier, the eye looks through the
Microscope at the object (as in the ordinary view of it), instead of
looking at its projection npon the paper ; the image of the tracing-
point being projected npon
the field, which is in many
respects much more advan-
tageous. This is effected by
combining a perforated steel
mirror with a reflecting
prism ; it is fitted to the
Eye-piece of the Microscope
as shown in Fig. 59 ; and its
action will be understood by
the accompanying diagram
(Fig. 60). The ray a b pro-
ceeding from the object, after
emerging from the eye-piece
of the Microscope passes
through the central perfora-
tion in the oblique mirror m
which is placed in front of
it, and so directly onwards
to the eye. On the other
hand, the ray a' b' proceeding
from the tracing-point, en-
ters the prism p, is reflected from its inclined surface to the
inclined surface of the mirror m, and is by it reflected to the eye
in such parallelism to the ray proceeding from the object, that
the two blend into one image.
Fig. 61.
The same effect is produced by
a contrivance which has been
devised by MM. Nachet for use
with vertical Microscopes. It
consists of a prism of a nearly
rhomboidal form (Fig. 61), which
is placed with one of its inclined
sides a c over the Eye-piece of
the Microscope ; to this side is
cemented an oblique segment e,
of a small glass cylinder, which
presents to the ray a b proceed-
ing directly upwards from the
object a surface at right angles
to it; so that this ray passes into
the small cylinder e, and out
from the side a b of the larger
prism, without sustaining any
refraction, and with very little
loss by reflexion from the in-
MICROMETEIC USE OF CAMERA LUCIDA. 129
elined surfaces at which they join. But the ray a! V which
comes from the tracing-point on entering the rhomboidal prism,
is reflected from its inclined side b d to its inclined side a c, and
thence it is again reflected to h in coincidence with the ray which
has directly proceeded from the object. — A prism of a different
shape, but constructed on the same principle, has been devised by
MM. JSTachet for use with a Microscope in the oblique position,
which is the one most comfortable to the delineator (see " Quart.
Journ. of Microsc. Science," Yol. viii. p. 158). — The Neutral Tint
Reflector, recommended by Dr. Beale, consists of a piece of neutral-
tint glass in a cap that is placed over the eye-piece, with which
it makes an angle of 45°. The arrangement of the Microscope is
the same as with the Camera Lucida. The eye looks through the
glass at a piece of drawing paper, or a ruler on the table, and re-
ceives a reflected image of the object.
82. It is so extremely useful to the Microscopist to be able to
take outlines with one or other of these instruments, that every one
would do well to practise the art. Although some persons at once
acquire the power of seeing the image and the tracing-point with
equal distinctness, the case is more frequently otherwise ; and
hence no one should allow himself to be baffled by the failure of
his first attempt. It will sometimes happen, especially when the
Prism is employed, that the want of power to see the pencil is due
to the faulty position of the Eye, too large a part of it being
over the prism itself. "When once a good position has been ob-
tained, the Eye should be held there as steadily as possible, until
the tracing shall have been completed. It is essential to keep in
view that the proportion between the size of the tracing and that
of the object is affected by the height of the eye above the paper ;
and hence that if the Microscope be placed upon a support of dif-
ferent thickness, or the Eye-piece be elevated or depressed by a
slight inclination given to the body, the scale will be altered. —
This it is, of course, peculiarly important to bear in mind, when a
series of tracings is being made of any set of objects which it is
intended to delineate on a uniform scale ; or when the Camera
Lucida (or any similar arrangement) is employed for the purpose
of Micrometry. All that is requisite to turn it to this account is
an accurately-divided Stage-micrometer, which, being placed in
the position of the object, enables the observer to see its lines pro-
jected upon the surface upon which he has drawn his outline; for
if the divisions be marked upon the paper, the average of several
be taken, and the paper be then divided by parallel lines at the
distance thus ascertained (the spaces being subdivided by inter-
mediate lines, if desirable), a very accurate scale is furnished, by
which the dimensions of any object drawn in outline under the
same power may be minutely determined. Thus if the divisions
of a Stage-micrometer, the real value of each of which is l-200th
of an inch, should be projected on the paper with such a magnify-
ing power as to be at the distance of an inch from one another, it
130 ACCESSOEY APPARATUS.
is obvious that an ordinary inch-scale applied to the measurement
of an outline, would give its dimensions in two-hundredths of an
inch, whilst each fifth of that scale would be the equivalent of
one-thousandth of an inch. When a sufficient magnifying power
is used, and the dimensions of the image are measured by the
' diagonal ' scale (which subdivides the inch into 1000 parts),
great accuracy may be obtained. It has been by the use of this
method, that Mr. Gulliver has made his admirable series of mea-
surements of the diameters of the Blood-corpuscles of different
animals.
83. Nose-piece. — It is continually desirable to be able to substi-
tute one Objective for another with as little expenditure of time
and trouble as possible; so as to be able to examine under a higher
magnifying power the details of an object of which a general view
has been obtained by means of a lower ; or to use the lower for
the purpose of finding & minute object (such as a particular Diatom
in the midst of a slide -full) which we wish to submit to high ampli-
fication. An arrangement for this purpose has been already noticed
in the description of Collins's " Harley Binocular" (Fig. 41) ; but
the one more commonly in use is the Nose-piece of Mr. C. Brooke,
which, being screwed into the object-end of the body of the Micro-
scope, carries two Objectives, either of which may be brought into
position by turning the arm on a pivot. In the original form of
this Nose-piece the arm is straight ; and its use is attended with
the inconvenience of often bring-
FlG- 62' ing down upon the Stage the Ob-
jective not in use, unless the re-
lative lengths of the two objectives
are specially adjusted to prevent
this. This inconvenience is still
more felt in triple and quadruple
nose-pieces. It is avoided, how-
ever, in the construction adopted
by Messrs. Powell and Lealand
(Fig. 62), and by MM. Nachet ;
Powell and Lealand's Modification ^ bend given to. the arm having
of Brooke's Nose-piece. the effect of carrying the Objective
not in use completely off the
Stage. — The working Microscopist will scarcely find any Ac-
cessory more practically useful to him than this simple piece of
apparatus.
84. Object-Marker.— All Microscopists occasionally, and some
continually, feel the need of a ready means of finding, upon a glass
slide, the particular object, or portion of an object, which they
desire to bring into view ; and various contrivances have been
suggested for the purpose. Where different magnifying powers
can be readily substituted one for another, as by the use of the
Erector (§ 69) or of the Nose-piece (§ 83), no special means are
required; since when the object has been found by a low power,
OBJECT-MARKER AND OBJECT-FINDER. 131
and brought into the centre of the field, it is rightly placed for
examination by any other Objective. Even this slight trouble,
however, may be saved by the adoption of more special methods ;
among the simplest of which is marking the position of the object
on the surface of the thin glass which covers it. The readiest mode
of doing this, when the object is large enough to be distinguished
by the naked eye or under the Simple Microscope, is to make a
small ring round it with a fine camel's-hair pencil dipped in Indian
ink ; but when the object is not thus visible, the slide must be
laid in position on the stage, the object 'found' in the Micro-
scope, the Condenser adjusted to give a bright and defined circle of
light, and then, the Microscope-body being withdrawn, the black
ring is to be marked around the illuminated spot. — The same end,
however, may be more precisely as well as more neatly accom-
plished, by attaching an object-marker to the Objective itself. That
of Mr. Tomes consists simply of an ivory cap, fitting over the l-4th
inch objective, having its extremity narrowed down (like that of
the objective itself) but perforated in the centre, so as to form a
minute ring; the object" having been ' found' and brought into the
centre of the field, the cap is placed upon the objective, the ring
is blackened with Indian-ink, and then, being carefully brought
by the focal adjustment into contact with the surface of the glass,
it stamps on this a minute circle enclosing the object.
85. Object-Finder. — The Mechanical Stage admits of a simple
addition, which very much facilitates the ' finding ' of objects
mounted in slides, that are so minute as not to be distinguishable
by the naked eye ; such, for example, as the particular forms
that present themselves in Diatomaceous deposits. This ' finder '
consists of two graduated Scales, one of them vertical, attached
to the fixed stage-plate, and the other horizontal, attached to an
arm carried by the intermediate plate ; the first of these scales
enables the observer to ' set ' the vertically- sliding plate to any
determinate position in relation to the fixed plate, while the
second gives him the like power of setting the horizontally-sliding
plate by the intermediate. In order to make use of these Scales,
it is of course necessary that the sliding and rotating platform on
which the object immediately rests, should be always brought into
one constant position upon the traversing plates beneath ; this is
accomplished by means of a pair of stops, against which it should
be brought to bear. So, again, this sliding-plate or object-platform
should itself be furnished with a ' stop ' for the glass slide to abut
against, so as to secure this being always laid in the same position.
These stops may be made removable, so as not to interfere with
the ordinary working of the stage. Now supposing an observer
to be examining a newly -mounted slide, containing any objects
which he is likely to wish to find on some future occasion; he
first lays the slide on the object-platform, with its lower edge
resting on the ledge, and its end abutting against the lateral
stop, and brings the object-platform itself into its fixed place
k2
132 ACCESSORY APPARATUS.
against the stops ; then, if, on giving motion to the slide by the
traversing action, he meet with any particular form worthy of note,
he reads-off its position upon the two scales, and records it in any
convenient mode. The scale may be divided to 50ths of an inch,
and each of these spaces may be again halved by the eye ; the
26
record may perhaps be best made thus, — Triceraiium favus ~
the npper number always referring to the upper scale, which is the
horizontal, and the lower to the vertical. Now whenever the
Microscopist may wish again to bring this object under examina-
tion, he has merely to lay the slide in the same position on the
platform, to bring the platform itself into its fixed place on the
traversing-plate below, and then to adjust the traversing-plates
themselves by their respective scales.*
86. Maltivood's Finder. — The ' finder' most commonly used, is that
invented by Mr. Maltwood, and first described in the " Transactions
of the Microscopical Society," Yol. vi. (1858), p. 59. This consists
of a glass slide 3 inches by 1|- inch, on which is photographed a
scale that occupies a square inch, and is divided by horizontal and
vertical lines into 2500 squares, each of which contains two num-
bers, one marking its ' latitude ' or place in the vertical series, and
the other its 'longitude' or place in the horizontal series. The slide,
when in use, should rest upon the ledge of the stage of the Micro-
scope, and be made to abut against a stop about 1^ inch from the
centre of the stage.— In order to use this ' finder,' the Object-slide
must be laid upon the Stage in such a manner as to rest upon its
ledge and to abut against the stop ; and when some particular
object, whose place it is desired to record, has been brought into
the field of view, the object-slide being removed and the Finder
laid down in its place, the numbers of the square then in the field
are to be read off and recorded. To find that object again at any
time, the Finder is to be laid in its place on the Stage, and the
stage moved so as to bring the recorded number into view ; and
the object-slide being then substituted for the Finder, the desired
object will present itself in the field. As care is taken in the pro-
duction of each ' Maltwood,' that the scale shall be at an exact dis-
tance from the bottom and left-hand end of the glass-slide, the
Microscopist may thus enable any other observer provided with a
similar Finder to bring into view any desired object, by informing
* This plan was suggested by Mr. Okeden in the " Quart. Microsc. Journal,"
Vol. iii. p. 166 ; and it appears to the Author that it might be adopted with so
little trouble or expense in every Microscope possessed of a mechanical stage,
that it would be very desirable for every such Microscope to be furnished with
these graduated scales. If the different Makers could agree upon some common
system of Graduation, in the same way as they have adopted the " Universal
Screw" for their Objectives, much trouble would be saved to Observers at a
distance from one another, who might wish to examine each other's objects ;
for the numerical reference attached to each object would then enable it to be
found by every observer whose Stage should be graduated upon the same
method.
DIAPHRAGM-PLATE. 133
him. of the numbers that mark its latitude and longitude. These
numbers may either be marked upon the object-slide itself, or re-
corded in a separate list.*
87. Diaphragm. — The Stage of every Microscope should be pro-
vided with some means of regulating the amount of light sent
upwards from the Mirror through transparent objects under exa-
mination. This is usually accomplished by means of a Diaphragm-
plate, perforated by apertures of different sizes, which is pivoted to
a removable fitting attached to the underside of the Stage (Fig. 36),
in such a manner that by rotating the plate, either of the apertures
can be brought into the optic axis of the instrument. This plate
should be always at least half an inch, below the object, since it is
otherwise comparatively inoperative. The largest of its apertures
should be made to carry a ground-glass (so fitted as to be remov-
able at pleasure), the use of which is to diffuse a soft and equable
light over the field when large Transparent objects (such as Sections
of Wood), are under examination ; between the smallest and the
largest aperture there should be an imperforated space, to serve as
a dark background for Opaque objects. The Diaphragm-plate
itself, the ' well ' of the Stage, in fact every part through which
light passes to the object from beneath, must be blackened, in order
to avoid the interference that would be occasioned by irregularly
reflected rays. The edge of the Diaphragm-plate should be notched
at certain intervals, and a spring-catch fitted so as to drop into
the notches, in order that each aperture may be brought into
its proper central position. — Another very simple and effective
arrangement for the same purpose, consists in the use of a single
Diaphragm having an aperture of about 3-16ths of an inch, which
is fixed in a tube that slides in a short tube fixed under the
aperture of the stage for carrying the Polariscope, &c. When
this diaphragm is pushed up so as to approach the Stage, it
cuts off only a small portion of the cone of rays reflected upwards
* Other " finders " have been suggested in the pages of the " Quart. Microsc.
Journal," by Mr. J. Tyrrell, Mr. E. G. Wright, Mr. T. E. Amyot, and Mr.
Bridgman, at pp. 234 and 302-304 of Vol. i. ; by Prof. Bailey, Mr. Amyot, and
Mr. Hodgson, at pp. 55, 151, 209, and 243 of Vol. iv. ; by Mr. Farrants, in
" Trans, of Microsc. Soc." Vol. v. p. 88 ; and by the Committee appointed for
the purpose, in the same volume, p. 95. Some of these have been superseded
by Maltwood's Finder, but as this cannot be conveniently used except with a
Mechanical Stage, those who do not possess that convenience must have re-
course to such of the above-mentioned plans as they may find most suitable to
their respective purposes. — Some of these methods only enable the Micro-
scopist to " find" his own object, whilst others enable him to indicate it to any
other observer. A very simple method of the former kind, applicable to Stages
fitted with side-springs for holding the slides (Figs. 34, 39), has been pointed
out to the Author by Mr. Moginie. If a small nick be filed in the inner edge
of each spring at about the middle of its length, it is easy, when an object has
been brought into position, to make two small ink dots upon the paper cover
of the slide, by a fine pen inserted into each nick ; and whenever the two dots
are brought again into their corresponding nicks, the object will be found in
the field.
134
ACCESSOEY APPAEATUS.
Fig. 63.
from the concave -mirror ; but when drawn downwards, it cnts off
more and more of the peripheral portion of that cone, and thus
gradually reduces the light. A small shutter for closing the aper-
ture, so as to give a black background for Opaque objects, is
generally supplied with a diaphragm of this kind. — So great an ad-
vantage is often derivable from a gradational reduction or augmen-
tation of the light, that the Microscopist who desires to avail him-
self of this will do well to provide himself with one of the forms
of Graduating Diaphragm which have been recently introduced.
That long ago invented by Dollond for Telescopic purposes is equally
applicable to the Microscope ; the
circumstance that its aperture is
square, instead of round, not consti-
tuting any practical objection to its
use. In another form, introduced by
Mr. Collins (Fig. 63), four shutters
are made, by acting on a lever-handle,
to move inwards simultaneously, so
as to narrow the aperture, the shape
of which always remains more nearly
circular than square. And in the
' Iris Diaphragm ' recently devised by
Mr. Brown,* the multiplication of
the number of shutters makes the
aperture practically circular. Either
Collins's Graduating Diaphragm, of these may be advantageously at-
tached to the Webster Condenser
(§ 89). Dr. Pigott obtains interesting and useful results by placing
an Iris Diaphragm over the objective, the aperture of which he
can thus modify at pleasure.
88. Achromatic Condensers. — In almost every case in which an
Objective of l-4th inch or any shorter focus is employed, its per-
formance is greatly improved by the interposition of an Achromatic
combination between the Mirror and the Object, in such a manner
that the rays reflected from the former shall be brought to a
focus in the spot to which the Objective is directed. A distinct
picture of the source of light is thus thrown on the object, from
which the rays emanate again as if it were self-luminous. The
Achromatic combination, which (at least in all First-class Micro-
scopes) is one specially adapted to the purpose, is furnished with a
Diaphragm plate (as first suggested by Mr. Gillett) immediately
behind its lenses ; and this is pierced with holes of such a form
and size, as to be adapted to cut off in various degrees, not merely
the peripheral, but also the central part, of the illuminating pencil.
The former of these purposes is of course accomplished by merely
* "Transactions of the Microscopical Society," Vol. xv. p. 74. — Another
form of Graduating Diaphragm, in which the reduction of the aperture is
effected by twisting a tube of Vulcanized Caoutchouc, is described by Mr. S. B.
Kincaid in the "Trans, of Microsc. Soc," Vol. xiv. p. 75.
ACHROMATIC CONDENSES.
135
E. and J. Beck's Achromatic
Condenser.
narrowing the aperture which limits the passage of the rays
through the central part of the lens ; the latter, on the other hand,
requires an aperture as large as that of the lens, having its central
part more or less completely occupied by a solid disk, which may
so nearly fill the circle as to
leave but a mere ring through Fig. 64.
which the light may pass. Such
apertures are shown in the
Diaphragm-plate in Fig. 64. —
The Condenser thus completed
is constructed on different plans
by the three principal Makers,
in accordance with the different
arrangements of their respective
stages. The thinness of the
Stage in Messrs. Smith and
Beck's Microscope allows the
diaphragm-plate to be made
upon the ordinary plan (Fig. 64),
since it can be brought suffi-
ciently near to the lenses of the
Condenser, without coming into too close contiguity with the
Stage ; and this is obviously the simpler arrangement.
Messrs. Powell and Lealand's condenser, in its last form, has an
angle of aperture of 170°, and a circular diaphragm-plate, con-
taining a numerous series of graduated apertures. The number
of stops being less than the number of apertures — the smaller ones
not requiring any — they are attached to an arm readily moved to
the right or left by touching a projecting pin; and by these
motions all the changes can be made with great facility. The
largest aperture of this condenser can only be utilised when the
object is mounted on thin glass.
Mr. Boss's latest form of Achromatic Condenser is represented
in Fig. 65. The combination of lenses has a focus of about 4-10ths
of an inch, and an angular aperture of about 110° ; and whilst this
aperture is found, when used with appropriate diaphragms, to give
rays of an obliquity sufficient for the resolution of the most
difficult tests, it is obvious that the focal length of this instrument
gives it an advantage over Condensers of shorter focus, the illu-
minating pencils of which cannot reach objects mounted on ordinary
slips of glass. The Diaphragm-plate, b, is furnished with a series
of eight apertures, which progressively bring down the angle of the
illuminating pencil from 110° to 20° ; whilst the Stop-plate, a, has
three circular stops for cutting-off the central rays in various
degrees, three marginal slots for limiting the passage of the illu-
minating rays to particular parts of the periphery, and a supple-
mentary aperture for the reception of any particular form of stop
or slot that the observer may wish to employ. The edges of each
plate are stamped with figures, which show what aperture is in
ACCESSOEY APPAKATUS.
use in the Diaphragm -plate, and what stop or slot in the Stop-
plate. It may be added that the outer lenses of this combination
Fig. 65.
Boss's Achromatic Condenser.
rw
are removable ; so that two or even one may be used alone, form-
ing a Condenser that is very suitable for use with Objectives of
medium power.
89. Webster Condenser. — Though the original idea of the
arrangement which has come into general use under this
designation, and which is at the same time comparatively inex-
pensive and applicable to a great variety of purposes, was given by
Mr. J. Webster (" Science Gossip," April 1, 1865), it has re-
ceived important modifications at the hands of the Opticians by
whom the instrument is manufactured ; and has, perhaps, not
even yet undergone its full development. In its present form the
arrangement of the lenses strongly resembles that used in the
Kellner Eye-piece (§ 27) ; the field-glass of the latter serving as a
Condenser to receive the cone of rays reflected upwards from the
mirror, and to make it converge upon a smaller Achromatic com-
bination, which consists of a double-convex lens of crown, with a
plano-convex lens of flint, the plane side of the latter being next
the object. These lenses are of large size and deep curvature ; so
that when their central part is stopped-out, the rays transmitted
WEBSTER-CONDENSEE.
137
from their peripheral portion meet at a wide angle of convergence,
and have the effect of those transmitted through the peripheral
portion of the ordinary Achromatic Condenser. When, on the other
hand, this combination is used with a diaphragm that allows only
the central rays to pass, these rays meet at a small angle ; and the
illumination thus given is very suitable for objects viewed with low
powers. Again, by stopping-out the central portion of the com-
bination, and removing the Condenser to a short distance beneath
the object, the effect of a Black ground illumination (§ 93) can be
very satisfactorily obtained with Objectives of moderate angular
aperture. Further, by stopping-out not only the central but also
a great part of the peripheral rays, so as only to allow the light to
enter from a small portion or portions of the margin, oblique illu-
mination (§ 90) can be most effectively obtained. All this can be
provided for by a Diaphragm-plate made to rotate at as short a
distance as possible beneath the condensing-lens ; but as the
number of apertures in this plate is necessarily Hmited, a greater
variety is obtained by the use of a Graduating Diaphragm (§ 87) for
the regulation of the central
aperture, and by making the
apertures in the rotating
plate subservient to the other
purposes already named, as
is done in the arrangement of
Mr. Highley (who employs
the Dollond Diaphragm) and
Mr. Collins (Fig. 66). — Still
greater variety can be ob-
tained by means of another
very simple arrangement
more recently introduced by
Mr. Collins ; the tube which
carries the lenses being fit-
ted with another tube which
slides within it ; and the
Fig. 66.
Webster's Condenser, fitted with Collins's
Graduating Diaphragm.
summit of this last being furnished with a socket into which
may be inserted a diaphragm of blackened card or of thin metal,
with an aperture or apertures of any shape or size that may be
desired. In this manner the Diaphragm may be carried up quite
close to the Condensing lens, which is a great advantage; and
when Oblique illumination is desired, the light may be transmitted
from any direction, by simply giving rotation to the tube carrying
a diaphragm with a marginal aperture. The "Webster Condenser
thus improved (which may also be used in combination with the
Polariscope) will be found one of the most universally-useful acces-
sories with which a Student's Microscope can be provided.
90. Oblique Illuminators. — It is frequently desirable to obtain a
means of illuminating Transparent objects with rays of more
obliquity than can be reflected to them from the Mirror, even when
138 ACCESSORY APPARATUS.
this is thrown as xmich as its mounting will permit out of the axis
of the Microscope ; or than can be transmitted by the ordinary
Achromatic Condenser, even when all bnt its marginal aperture is
stopped-out. Such oblique light may be used in two entirely
different modes. — The rays, although very far out of the axis of the
Microscope, may still not make too great an angle with, it to fall
beyond the aperture of the Objective ; and thus, entering its peri-
pheral portion after their passage through the object, they will
form the image in the ordinary way. The advantage of such
oblique illumination arises from its power of bringing-out markings
which cannot be seen when only direct rays are employed; and
when the rays come only from one side, so as to throw a strong
shadow, and either the Stage or the Illuminator is made to rotate so
that the light shall fall upon the object successively in every
azimuth, information may often be gained respecting the nature of
these markings, such as can be acquired in no other mode (§ 133). —
But the direction given to the rays may be so oblique that they
shall not enter the Object-glass at all ; in this case, they serve to
illuminate the Object itself, which shines by the light whose passage
it has interrupted ; and as the observer then receives no other light
than that which radiates from it, the object (provided it be of a
nature to stop enough, light) is seen bright upon a dark field. —
Each of these methods has its advantages for particular classes of
objects ; and it is advisable, in all doubtful cases, to have recourse
to every variety of oblique illumination that shall present the object
under a different aspect. Almost every Microscopist who has
especially devoted his attention to the more difficult lined or dotted
objects, has devised his own particular arrangement for Oblique
Illumination ; but those methods only can here be noticed which,
have acquired general approval.* As they have little in common
save their purpose, it seems scarcely possible to classify them
according to any other character than that afforded by the direction
which they give to the oblique rays ; some of them bringing these
to bear on the object from one side alone, and others from all
sides.
91. The Amid Prism, which causes the rays to be at once re-
flected by a plane surface and concentrated by lenticular surfaces,
so as to answer the purpose of Mirror and Condenser at the same
time, is much approved by many who have used it. Such a Prism
may be either mounted on a separate base, or attached to some
part of the Microscope-stand. The mounting adopted by Messrs.
E. and J. Beck, and shown in Fig. 67, is a very simple and con-
venient one ; this consists in attaching the frame of the prism to a
sliding bar, which works in dovetail grooves on the top of a cap
that may be set on the ' secondary body' beneath the stage ; the
slide serves to regulate the distance of the prism from the axis of
* Various other methods will be found described in the successive volumes
of the "Transactions of the Microscopical Society" and of the "Quarterly
Journal of Microscopical Science."
AMICI'S PEISM,— HEMISPHERICAL CONDENSER. 139
the microscope, and consequently the obliquity of the illumination ;
whilst its distance beneath the stage is adjusted by the rack-move-
ment of the cylindrical fitting.
In this manner, an illuminating Ftg. 67
pencil of almost any degree of
obliquity that is permitted by the
construction of the Stage may be
readily obtained ; but there is no
provision for the correction of its
aberrations. In order to use
this oblique illumination to the
greatest advantage, either the
Prism or the Object should be
made to rotate, thus causing the
oblique rays to fall upon the latter Ainici's Prism for Oblique
from every azimuth in succession, Illumination.
so as to bring out all its markings (§ 133).
92. For those who desire to obtain a very oblique illuminating
pencil, for the resolution of the most difficult lined Tests by means
of Objectives of large angular aperture, without having recourse
to more expensive arrangements, the Double Hemispherical Con-
dense!' of Mr. Eeade affords a very simple and convenient means.
This consists of a hemispherical lens of 1| or If inch diameter,
with its flat side next the object, surmounted by a smaller lens of
the same form, the flat side of which is covered with a Diaphragm
of thin brass or tin-foil, having an aperture or apertures close to
its margin. The single hemisphere originally used by Mr. Eeade
gave an angle of convergence of about 90° for its most oblique
rays ; which is about the same with that of the Webster Condenser
as at present constructed. By the addition of the second hemi-
sphere, however, the angle of convergence is augmented to 150° ;
and its power in ' bringing out' the lined tests is greatly augmented.
Such an arrangement, of course, involves a large amount of Chro-
matic dispersion ; but this is stated by Mr. Keade not to be a dis-
advantage in practice ; since with high powers the red, the yellow,
or the blue rays may be separately employed by altering the focus
of the condenser, so that the illumination becomes virtually mono-
chromatic. If the fineness of the lines under examination requires
that the Condenser should be closely approximated to the object,
the Diaphragms may be placed between the two hemispheres ; a slit
in the tube being provided for that purpose. The Diaphragms for
use with this or with the "Webster Condenser, when very oblique
illumination is required, may be cut out of thin brass or tin-foil,
and blackened with oxide of copper. The apertures should be
V-shaped, extending from the circumference to about a quarter of
an inch from the centre ; and it is often useful to have two such
apertures in the same diaphragm at angles of from 60° to 90° from
each other, so that two pencils of light may fall at the same time
in different directions upon two sets of lines. By an ingenious
140 ACCESSORY APPARATUS.
arrangement devised by Mr. Reade, a second adjustable diaphragm
may be made to shut off the inner portions of the V-shaped
apertures, leaving only such parts of their marginal portions as
may give the required obliquity to the illuminating rays.*
93. Black-Ground Illuminators. — Whenever the rays are directed
with such obliquity as not to be received into the Object-glass at
all, but are sufficiently retained by the Object to render it (so to
speak) self-luminous, we have what is known as the black-ground
illumination. For low powers whose angular aperture is small,
and for such objects as do not require any more special provision,
a sufficiently good ' black-ground' illumination may be obtained by
turning the concave Mirror as far as possible out of the axis of the
microscope, especially if it be so mounted as to be capable of a more
than ordinary degree of obliquity. In this manner it is often
possible, not merely to bring into view features of structure that
might not otherwise be distinguishable, but to see bodies of extreme
transparence (such, for instance, as very minute Animalcules) that
are not visible when the field is flooded (so to speak) by direct light ;
these presenting the beautiful spectacle of phosphorescent points
rapidly sailing through a dark ocean. Another very simple mode,
which answers sufficiently well for low powers and for the larger
objects which these are fitted to view, consists in substituting for
the ordinary Condenser a plano-convex lens of great convexity,
having on its plane side, which is the one turned towards the object,
a central stop to cut off the direct rays; for the rays passing
through the marginal portion of this Spot-Lens, being strongly re-
fracted by its high curvature, are made to converge upon the object
at an angle too wide for their entrance into an Objective of
moderate aperture, and thus the field is left dark ; whilst all the
light stopped by the object serves (as it were) to give it a luminosity
of its own. The same effect is gained by the use of the Webster
Condenser (§ 89) with a central stop placed immediately behind
the lower lens or upon the flat surface of the upper. Neither of the
foregoing plans, however, will answer well for Objectives of high
power, having such large Angles of Aperture that the light must
fall very obliquely to pass beyond them altogether. Thus if the
pencil formed by the Spot-Lens have an angle of 60°, its rays will
enter a l-4th Objective of 70°, and the field will not be darkened.
94. A greater degree of obliquity may be obtained by the Para-
bolic Illuminator^ (Fig. 68) now in general use ; which consists of
* See " Transactions of Microscopical Society," Vol. xv. p. 3. — Another Illu-
minator, giving a wide angular pencil, and specially devised by Mr. Wenham
for use with the Binocular Microscope, is described by him in il Quart. Journ.
of Microsc. Science," Vol. i. N.S. (1861), p. 111.
f A Parabolic Illuminator was first devised by Mr. Wenham, who, however,
employed a Silver speculum for the purpose. About the same time Mr.
Shadbolt devised an Annular Condenser of Glass for the same purpose (see
" Transact, of Microsc. Soc." Ser. 1, Vol. iii. pp. 85, 132). Both principles are
combined in the Glass Paraboloid.
PARABOLIC ILLUMINATOR. 141
a Paraboloid of Glass that reflects to its focus the rays which fall
upon its internal surface. A diagrammatic section of this instru-
ment, showing the course of the rays through it, is given in Fig. 69,
Fig. 68. Fig. 69.
Parabolic Illuminator.
the shaded portion representing the Paraboloid. The parallel rays
r r' r", entering its lower surface perpendicularly, pass on until they
meet its parabolic surface, on which they fall at such an angle as
to be totally reflected by it (§ 2), and are all directed towards its
focus f. The top of the Paraboloid being ground out into a
spherical curve of which f is the centre, the rays in emerging from
it undergo no refraction, since each falls perpendicularly upon the
part of the surface through which it passes. A stop placed at s
prevents any of the rays reflected upwards by the mirror from
passing to the object, which, being placed at f, is illuminated by
the rays reflected into it from all sides of the Paraboloid. Those
rays which pass through it diverge again at various angles ; and if
the least of these, g f h, be greater than the Angle of Aperture of
the Object-glass, none of them can enter it, so that the object is
seen only by the light issuing from itself, and is shown brightly
illuminated upon a black ground. The stop s is attached to a stem
of wire, which passes vertically through the Paraboloid and ter-
minates in a knob beneath, as shown in Fig. 68 ; and by means of
this it may be pushed upwards so as to cut off the less divergent
rays in their passage towards the object, by which means a black-
ground illumination may still be obtained with Objectives of
142 ACCESSORY APPARATUS.
an Angle of Aperture much wider than g f h. In using the
Paraboloid for delicate objects, the rays which are made to enter
it should be parallel, consequently the plane Mirror should always
be employed ; and when, instead of the parallel rays of Daylight,
we are obliged to use the diverging rays of a Lamp, these should be
rendered as parallel as possible, previously to their reflexion from the
mirror, by the interposition of the ' bull's-eye ' Condenser (Fig. 76)
so adjusted as to produce this effect. . There are many cases,
however, in which the stronger light of the concave Mirror is pre-
ferable. When it is desired that the light should fall on the object
from one side only, the circular opening at the bottom of the wide
tube (Fig. 68) that carries the Paraboloid may be fitted with a
diaphragm adapted to cover all but a certain portion of it ; and by
giving rotation to this diaphragm, rays of great obliquity may be
made to fall upon the object from every azimuth in succession. A
glass cone, with the apex downwards, and the base somewhat con-
vex, with a stop in the centre, is fitted by MM. Nachet to their
Microscopes for the same purpose ; and performs very effectively. —
Mr. Eeade's Double Hemispherical Condenser (§ 92) also may be
made to give a black-ground illumination with Objectives of wide
angles of aperture.
95. One of the great advantages of this kind of illumination
consists in this ; that, as the light radiates from each part of the
object as its proper source, instead of merely passing through it
from a more remote source, its different parts are seen much more
in their normal relations to one another, and it acquires far more
of the aspect of solidity. The rationale of this is easily made
apparent, by holding up a glass vessel with a figured surface
between one eye and a lamp or a window, so that it is seen by trans-
mitted light alone ; for the figures of its two surfaces are then
so blended together to the eye, that unless their form and dis-
tribution be previously known, it can scarcely be said with cer-
tainty which markings belong to either. If, on the other hand,
an opaque body be so placed behind the vessel that no rays are
transmitted directly through it, whilst it receives adequate illu-
mination from the circumambient light, its form is clearly dis-
cerned, and the two surfaces are distinguished without the least
difficulty.
96. Wenham's Reflex Illuminator for High Powers. — A very in-
genious and valuable illuminator for high powers has recently been
devised by Mr. Wenham and constructed by Messrs. Eoss. " It
is composed of a glass cylinder half an inch long and 4-10ths in
diameter, the lower convex surface of which is polished to a radius
of 4-10ths. The top is flat and polished. Starting from the
bottom edge, the cylinder is worked off to a polished face at an
angle of 64° ; close beneath the cylinder is set a plano-convex lens
of lj focus."* When parallel rays are thrown up through this
* " Monthly Microsc. Journ." June, 1872, p. 239.
WENHAM'S KEFLEX ILLUMIXATOE.
143
apparatus from the mirror, they impinge on the upper surface of a
glass slide at an angle of total reflexion; but if a suitable object ad-
heres to that surface, the light reaches it on an angle that admits
of its passage. The object is then seen brilliantly lit-up upon a
Wenham's Be flex Illuminator for High Powers.
o, glass cylinder, one side worked to angle of 64°, lower
surface convex, top flat ; 6, direction in which parallel rays
///-'would be reflected from flat top if there were no object
above ; c, slide, with object attached to its upper surface,
resting on top of a, with film of water intervening ; e, black
half-cylinder, with dot for centering; g, position of object
able to receive light; ft, point to which ///would converge
if continued through solid glass ; i i i, brass frame, lower
part fitting into sub-stage.
dark ground, and many hne markings that escape notice with
other methods become very distinct. It is advisable to rotate the
apparatus until the best position is attained. Some skill and
144 ACCESSORY APPARATUS.
practice are required to nse this apparatus to advantage, but it
will amply repay the trouble of mastering its difficulties. It is best
suited to thin flat objects ; with those that are thick and irregular
distortion is unavoidable. Although specially designed as a dark-
ground illuminator, good effects can with care be obtained for such
objects as difficult Diatoms, in balsam or dammar ; but the effect is
that of very oblique transparent illumination.
97. White-Cloud Illuminators. — It being universally admitted
that the light of a bright white cloud is the best of all kinds of
illumination for nearly every kind of Microscopic inquiry, various
attempts have been made to obtain such light from the direct rays
either of the Sun or of a Lamp, by what may be called an artificial
cloud. Some have replaced the plane mirror by a surface of
pounded glass or of carbonate of soda, or (more commonly) by a
disk of plaster-of- Paris, the latter being decidedly the preferable
method ; but a sufficiently bright light is not thus obtained, unless
a Condenser be employed to intensify the illumination of the
mirror. Such a Condenser may be most conveniently attached by
a jointed-arm to the frame which carries the disk, according to the
method of Messrs. Powell and Lealand, shown in Fig. 71 ; the
frame itself being made to fit upon
Fig. 71. the Mirror, and to turn with it in
every direction. Another very simple,
and for many purposes very efficient,
mode of obtaining a white-cloud illu-
mination (invented by Mr. Handf ord)
consists in coating the back of a
concave plate of glass, like that em-
ployed in the ordinary concave Mir-
ror, with white zinc paint, instead of
White-Cloud Illuminator. silvering it ; and then mounting this
in a frame, which may be fitted (like
the plaster-of -Paris disk just described) over the ordinary Mirror.
A concave surface of plaster-of-Paris, moreover, may easily be
obtained, by casting it when fluid upon the convex surface of
such a plate. When a concavity is thus given to the white sur-
face, its performance with low powers is much improved ; but
with high powers a special condensation of the light must be
adopted, and the arrangement above described seems the simplest
that could be devised. It is open, however, to certain objections,
which become apparent when very high powers are used and
difficult objects are under examination ; and to obtain the most
perfect white-cloud illumination possible, is the object of an appa-
ratus devised by Mr. Gillett. This consists of a small camphine
lamp, placed nearly in the focus of a Parabolic Speculum, which
reflects the rays either at once upon a disk of roughened Enamel,
or upon a second (hyperbolic) Speculum which reflects them
upon such a disk. A very pure and concentrated light is thus
obtained; and as the forms of the incident pencils are broken
POLARIZING APPAEATUS. 145
up by the roughened surface, that surface takes the place of a
lamp as the source* from which the rays primarily issue. The
advantage of this illumination is specially felt in the examina-
tion of objects of the most difficult class under the highest
powers. — Very pleasant white-ground effects may be obtained by
methods adopted by Mr. Slack. For large objects, viewed with
powers of 1| to 4 inches, he places under the stage a tube holding
a large disk (1| inch diameter) of ground glass, the ground surface
being protected by a plain glass cover over it. By this means
the peculiar tint of the freshly ground surface is permanently re-
tained. For 2-3rds and half -inch powers he employs a glass slide
carrying a disk or square of thin paper, saturated with spermaceti,
and protected from dirt by a thin glass cover that adheres to it.
This slide, disk downwards, is placed under the object.
98. Polarizing Apparatus. — In order to examine transparent
objects by Polarized Light, it is necessary to employ some means of
polarizing the rays before they pass through the object, and to
apply to them, in some part of their course between the object and
the eye, an analyzing medium. These two requirements may be
provided for in different modes. The Polarizer may be either a
bundle of plates of thin glass, used in place of the mirror, and
polarizing the rays by reflexion ; or it may be a ' single image' or
' Mcol ' prism of Iceland Spar, which is so constructed as to
transmit only one of the two rays into which a beam of ordinary
light is made to divaricate by passing through this substance ; or
it may be a plate of Tourmaline, or one of the artificial tourmalines
composed of the disulphate of iodine and quinine, known by
the designation of ' Herapathite ' after the name of their inventor.
Of these methods, the ' Nicol' prism is the one generally preferred,
the objection to the reflecting polarizer being that it cannot be
made to rotate ; the Tourmaline is undesirable, on account of the
colour which it imparts when sufficiently thick to produce an
Fig. 72.
Fitting of Polarizing Prism in Smith and Beck's Microscope.
effective polarization ; whilst the crystals of Herapathite are
seldom obtained perfect of sufficient size to afford a good illu-
mination, and when perfect are not always to be depended on
L
146 ACCESSORY APPARATUS.
for permanence. The Polarizing Prism is usually fitted into a
tube (Fig. 72, a a) with a large milled-head (c) at the bottom, by
which it is made to rotate in a collar (b) that is attached to the
microscope ; this collar may be fitted to the under side of the
Stage-plate, or, where a Secondary Stage is provided, it may be
attached to this : in the microscope of Messrs. Smith and Beck.
it screws into the lower part of a tube (Fig. 72, b) that slides
into the ' secondary body ' beneath the stage (Plate vn.). The
Analyzer, which may be either a ' Nicol ' prism, a Tourmaline, or
a crystal of Herapathite, is usually placed either in the interior of
the microscope, or between the eye-piece and the eye. If it be a
prism it is mounted in a tube, which may either be screwed into
the lower end of the body just above the Objective, or may be
fitted over the Eye-piece in place of its ordi-
nary cap (Fig. 73) : in the former situation it
has the advantage of not limiting the field, but
it stops a considerable proportion of the light ;
in the latter, it detracts much less from the
brightness of the image, but cuts off a good
deal of the margin of the field. In using the
Polarizing apparatus with the Binocular Mi-
croscope, the Analyzing prism must be placed
between the Wenham prism and the Objective ;
in a holder constructed so as to allow of being
rotated. By combining the Polarizing Appa-
ratus with the Achromatic Condenser, it may
fitting of Analyzing i j -,-, i • -i -i •,-, J
Prism upon the Eye- be ^ed with very high powers and with very
piece. oblique or even black-ground illumination.
And when low powers are employed with the
Webster Condenser or with a Spot-Lens, a very beautiful effect
may be produced in the case of many large semi-transparent
objects (ssa.cn as the horny polyparies of Zoophytes), by illu-
minating them on a black ground with Polarized rays reflected
upwards from the bundle of thin-glass plates which may be sub-
stituted for the mirror, and then viewing them through the
Analyzing prism in the usual manner *
99. For bringing out certain effects of Colour by the use of
Polarized Light (Chap, xx.), it is desirable to interpose a plate of
Selenite beneath the polarizer and the object ; and it is advan-
tageous that this should be made to revolve. A very convenient
mode of effecting this is to mount the Selenite plate in a revolving
collar, which fits into the upper end of the tube (Fig. 72, b) that
receives the Polarizing prism. In order to obtain the greatest
variety of coloration with different objects, films of Selenite of
different thicknesses should be employed ; and this may be accom-
* A Polarizer of Herapathite or Tourmaline may be used for this purpose
instead of the glass-plate polarizer, by mounting it in a cap, fitted above the
Condenser or Spot-Lens, at such a distance as to receive its converging
hollow pencil near its termination in the object.
POLARIZING- APPARATUS. — SELENITE-PLATE. 147
plished by substituting one for another in the revolving collar. A
still greater variety may be obtained by mounting three films,
which separately give three different colours, in collars revolving
in a frame resembling that in which hand-magnifiers are usually
mounted, so that they may be used singly or in double or triple
combinations ; as many as thirteen different tints may thus be
obtained. — When the construction of the Microscope does not
readily admit of the connexion of the Selenite plate with the
Polarizing prism, it is convenient to make use of a plate of brass
(Fig. 74) somewhat larger than the glass slides in which objects
are ordinarily mount-
ed, with a ledge near FlG- 74-
one edge for the slide
to rest against, and a
large circular aperture
into which a glass is
fitted, having a film of
selenite cemented to it ;
this ' Selenite Stage,'
or object-carrier, being
laid upon the Stage of Selenite Object-carrier,
the microscope, the
slide containing the object is placed upon it ; and, by an ingenious
modification contrived by Dr, Leeson, the ring into which the
Selenite plate is fitted being made moveable, one plate may be
substituted for another, whilst rotation may be given to the ring
hj means of a tangent-screw fitted into the brass-plate.* A very
excellent Selenite Stage more economical than other patterns, and
giving as great a variety of results, has been devised by Mr.
Ackland, A disk of selenite, cut so as to give hues from neutral
tint to mauve when the polarizing and analyzing prisms are
arranged to give a dark field, is made to revolve, by acting with
the finger on a small-toothed wheel. Above this may be placed
selenites cut to give retardation of f , f , and f. Each of these fit
into a circular groove, and rotate easily. By these means, and
the motion of the polarizing and analyzing prism, any object can
be excellently displayed.
100. Illuminators for Opaque Ohjects. — All objects through which
sufficient light cannot be transmitted to enable them to be viewed
in the modes already described, require to be illuminated by rays,
which, being thrown upon the surface under examination, shall be
* An improvement on the ordinary Selenite Object-carrier, enabling the
Selenite plates to be changed without disturbing the object, has been de-
scribed by Mr. James Smith in " Quart. Journ. of Microsc. Science," Vol. viii.
(1860), p. 203 ; and he has more recently added a very simple arrangement, by
which rotation may be given to the object, whilst the polarizing prism and
selenite remain stationary (see •' Transact, of Microsc. Soc," Ser. 2, Vol. xiv.
p. 101). — For an account of the nature and properties of Polarized Light, which
would be out of place in the present treatise, see the chapters on that subject
in Mr. Brooke's "Manual of Natura Philosophy."
l2
148 ACCESSOEY APPARATUS.
reflected from it into the microscope ; and this mode of viewing
them may often be advantageously adopted in regard to semi-
transparent or even transparent objects, for the sake of the diverse
aspects it affords. Among the various methods devised for this
purpose, the one most generally adopted consists in the use of a
Condensing Lens (Fig. 75), either attached to the Microscope, or
Fig. 75.
Ordinary Condensing Lens.
mounted upon a separate stand, by which the rays proceeding from
a lamp or from a bright sky are made to converge upon the object.
For the efficient illumination of large Opaque objects, such as In-
jected preparations, it is desirable to employ a Bull's Eye Condenser
(which is a plano-convex lens of short focus, two or three inches
in diameter), mounted upon a separate stand, in such a manner
as to allow of being placed in a great variety of positions. The
mounting shown in Fig. 76 is one of the best that can be adopted :
the frame which carries the lens is borne at the bottom upon a
swivel- joint, which allows it to be turned in any azimuth ; whilst
it may be inclined at any angle to the horizon, by the revolution
of the horizontal tube to which it is attached, around the other
horizontal tube which projects from the stem; by the sliding of one
of these tubes within the other, again, the horizontal arm may be
lengthened or shortened ; the lens may be secured in any position
(as its weight is apt to drag it down when it is inclined, unless the
BULL'S-EYE CONDENSER.
149
tubes be made to work, the one into the other, more stiffly than is
convenient) by means of a tightening collar milled at its edges ;
and finally the horizontal arm is attached to a sprung socket,
which slides up and down npon a vertical stem. The optical effect
of such a Lens differs according to the side of it turned towards
the light, and the condition of the rays which fall upon it. The
Fig. 76.
Bull's-Eye Condenser.
position of least Spherical Aberration is when its convex side is
turned towards parallel or towards the least diverging rays ; con-
sequently, when used by Daylight, its plane side should be turned
towards the object ; and the same position should be given to it
when it is used for procuring converging rays from a Lamp, the
lamp being placed four or five times farther off on one side than
the object is on the other. But it may also be employed for the
150
ACCESSOKY APPARATUS*
Fig. 77.
purpose of reducing the diverging rajs of the Lamp to parallelism,
for use either with the Parabolic illuminator (§ 94), or with the
Side Beflector to be presently described ; and the plane side is
then to be turned towards the Lamp, which must be placed at
such a distance from the Condenser, that the rays which have
passed through the latter shall form a luminous circle equal to it
in size, at whatever distance from the lens the screen may be held.
For viewing minute objects under high powers, the smaller Con-
densing Lens may be used to obtain a further concentration of
the rays already brought into convergence by the ' Bull's Eye*
(§ 136).
101. The Illumination of Opaque objects may be effected by
■reflexion as well as by refraction ; and the most convenient as well
as most efficient instru-
ment yet devised for
this purpose is the
Parabolic Speculum of
Mr. K. Beck (Fig. 77),
which is attached to a
spring-clip that fits
upon the Objectives (2
inch, 1| inch, 1 inch,
2-ords inch), to which
it is especially suited,
and is slid up or down
or turned round its
axis, when the object
has been brought into
focus, until the most
suitable illumination
has been obtained. The
ordinary rays of dif-
Beck's Parabolic Speculum.
fused Daylight, which may be considered as falling in a parallel
direction on the Speculum turned towards the window to receive
them, are reflected upon a small object in the focus of the Spe-
culum, so as to illuminate it sufficiently brightly for most pur-
poses ; but a much stronger light may be concentrated on it,
when the Speculum receives its rays from a Lamp placed near
the opposite side of the stage, a Bull's Eye being interposed to
give parallelism to the rays. — There is a valuable addition to this
apparatus, not shown in the figure, which consists of an arm
carrying a plane mirror at an angle of 45°, so that a movement of
the finger brings it over the object, and substitutes its action for
that of the parabola. The result is, that light is thrown verti-
cally upon the object, and brings out the surface-markings of
minerals, &c, in an admirable way. — For the sake of Micro-
scopists who may desire to use this admirable instrument with
Objectives to which it has not been specially fitted, Mr.
Crouch has contrived an Adapter, by which it may be used with
PARABOLIC SPECULUM.— LIEBERKUHN.
151
Crouch's Adapter for .Para-
bolic Speculum.
any objective of suitable focus. This consists of a collar (Fig. 78, a)
which is interposed between the lower end of the body of the
Microscope and the Objective ; on this
collar is fitted the ring b, which turns
easily round it, and carries the hori-
zontal arm c c, jointed at each end ;
from this hangs vertically the stem d,
which can be lengthened or shortened
at pleasure ; and to the lower end of
this the Speculum f is attached by the
ball-and-socket joint e. This arrange-
ment may be used not only with the
Objectives already named, but also
with those of one-half or 4-10ths inch
focus, if these do not approach the
object so nearly as to interfere with
the reflexion of the illuminating rays
from the Speculum.
102. Lieberkuhn. — A mode of Illu-
minating Opaque objects by a small
concave Speculum reflecting directly
down upon them the light reflected up
to it from the Mirror, was formerly
much in use, but is now compara-
tively seldom employed. This concave Speculum, termed a ' Lie-
berkiihn,' from the celebrated Microscopist who invented it, is
made to fit upon the end of the Objective, having a perforation
in the centre for the passage of the rays from the object to the
lens ; and in order that it may receive its light from the Mirror
beneath (Fig. 79, a), the object must be so mounted as only to
stop-out the central portion of the rays that are reflected upwards.
The curvature of the Speculum is so adapted to the focus of
the Object-glass, that, when the latter is duly adjusted, the rays
reflected up to it from the mirror shall be made to converge
strongly upon the part of the object that is in focus ; a sepa-
rate Speculum is consequently required for every Object-glass.
The disadvantages of this mode of illumination are chiefly these : —
first, that by sending the light down upon the object almost per-
pendicularly, there is scarcely any shadow, so that the inequalities
of its surface and any minute markings which it may present
are but faintly or not at all seen ; second, that the size of the
object must be limited by that of the Speculum, so as to allow
the rays to pass to its marginal portion ; and third, that a special
mode of mounting is required, to allow the light to be reflected from
the mirror around the margin of the object. The first objection may
be in some degree removed by turning the Mirror considerably out
of the axis, so as to reflect its light obliquely upon the Lieberkuhn,
which will then send it down obliquely upon the object (Fig. 79, b) ;
or by covering one side of the Lieberkuhn by a diaphragm, which
152 ACCESSORY APPARATUS.
should be made capable of rotation, so that light may be reflected
from the uncovered portion in every azimuth : the illumination,
however, will in neither case be so good as that which is afforded,
Fig. 79.
with powers up to 2-ords inch, by the Parabolic Speculum just
described. The mounting of Opaque objects in wooden slides
(Fig. 98), which affords in many cases the most convenient means
of preserving them, completely prevents the employment of the
Lieberkuhn in the examination of them ; and they must be set for
this purpose either upon disks which afford them no protection, or
in cells (Fig. 106) with a blackened background. The cases
wherein the Lieberkuhn is most useful, are those in which it is
desired to examine small Opaque objects, such as can be held in
the Stage-Forceps (§ 105), or mounted on small disks (§ 106), or laid
upon a slip of glass, with Objectives of half-inch focus or less ;
since a stronger light can be thus concentrated upon them, than
can be easily obtained by side-illumination. In every such case, a
black background must be provided, of such a size as to fill the
field, so that no light shall come to the eye direct from the Mirror,
and yet not large enough to create any unnecessary obstruction to
the passage of the rays from the mirror to the speculum. With
each Lieberkuhn is commonly provided a blackened stop of appro-
priate size, having a well-like cavity, and mounted upon a pin
which fits into a support connected with the under side of the
stage ; but though this ' dark well' serves to throw out a few
objects with peculiar force, yet, for all ordinary purposes, a spot
of black paper or black varnish will answer the required purpose
BECK'S VERTICAL ILLUMINATOR.
153
very effectually, this spot being either made on the under side of
the cell which contains the object, or upon a separate slip of glass
laid upon the stage beneath this.
103. Vertical Illumination for High Powers. — Various attempts
have been made by Mr. Wenham and others to view Opaque objects
under powers too high for the advantageous use of the Lieberkiihn,
by employing the Objective itself as the illuminator, light being
transmitted into it downwards from above. By Professor Smith,
of Kenyon College, U.S., a pencil of light admitted from a lateral
aperture above the Objective, is reflected downwards upon the object
through its lenses, by means of a small silver speculum placed on
one side of its axis and cutting off a portion of its aperture. By
Messrs. Powell and Lealand a piece of plane glass is placed at an
angle of 45° across a tube placed like an Adapter between the Objec-
tive and the body of the Microscope ; and whilst a pencil of light,
entering at the side a|:>erture and striving against this inclined
surface, is reflected by it downwards through the Objective on to
the object, the rays proceeding upwards from the object pass up-
wards (with some loss by reflexion) through the plane glass into
the body of the Microscope. For this fixed plate of glass, Mr. K.
Beck substituted a disk of thin glass attached to a milled-head
(Fig. 80, b), by the rotation of which its angle may be exactly
adjusted ; and this is introduced by a slot (shown at e, Fig. 80, a)
Fig. 80.
Beck's Vertical Illuminator.
into the interior of an Adapter that is interposed between the
Objective (e, d) and the nose (c) of the Microscope. The light which
154 ACCESSORY APPARATUS.
enters at the lateral aperture (a, a) falling upon the oblique surface
of the disk (c, b), is reflected downwards, and is concentrated by
the lenses of the Objective upon the object beneath. There is this
advantage in the method of Mr. Beck over that of Messrs. Powell
and Lealand, that not only does the former give a power of adjust-
ment which it is very important the Reflector should possess, but
also that the natural surface of the thin-glass disk reflects a much
larger proportion of the luminous rays impinging upon it, than does
any artificially polished plane. On the other hand, Messrs. Powell
and Lealand's arrangement is provided with a diaphragm, having
a series of apertures, which are very useful in diminishing the
false light to which this method is liable. — In using this Illumi-
nator, the Lamp should be placed at a distance of about 8 inches
from the aperture ; and when the proper adjustments have been
made, the image of the flame should be seen upon the object. The
Illumination of the entire field, or the direction of the light more
or less to either side of it, can easily be managed by the interposi-
tion of a small Condensing Lens placed at about the distance of its
own focus from the lamp ; and slight alterations in its position will
produce the effect of the insertion of Diaphragms into the side
aperture. The Objects viewed by this mode of illumination are best
uncovered ; since, if they are covered with thin glass, so large a por-
tion of the light sent down upon them is reflected from the cover
(especially when Objectives of large angle of aperture are employed)
that very little is seen of the objects beneath, unless their reflective
power is very high. It is specially applicable to Diatoms, Poly-
cystina, minute Foraminifera, and the Scales of Lepidopterous and
other Insects, viewed under Objectives of from 4-10ths to l-5th
of an inch; and it often makes objects present appearances that
would not in the least be suspected from their ordinary aspect,
when viewed as Transparent objects mounted in Canada Balsam.
104. Stephenson's Safety Stage. — In examining objects with those
higher powers which focus extremely close to the covering glass, the
Fig 81 slightest inadvertence is likely to lead to a frac-
ture of the glass, and perhaps to the destruction
of a valuable slide. This is a serious matter with
Moller's Diatom Type Slide, or Robert's Test
Lines, or with many others that are expensive or
perhaps impossible to replace. To remove this
source of danger, Mr. Stephenson contrived the
" Safety Stage," shown in Fig. 81. The frame on
which the slide carrying the object rests, is hinged
at its upper part, and kept in its true position
by slight springs, which give way directly the
slide is pressed by the objective. It is found
that springs firm enough to insure the steadiness
required for high powers, may yet be sufficiently
flexible to give way before very thin glass is en-
dangered, and a glance at the stage shows if it is made to deviate
STAGE-FOECEPS. — DISK-HOLDEE. 155
,from the normal position in which its npper and lower edges are
parallel.
Section 2. Apparatus for the Presentation of Objects.
105. Stage-Forceps and Vice. — I*or bringing under the Object-
glass in different positions such small Opaque objects as can be
conveniently held in a pair of forceps, the Stage-Forceps (Fig. 82)
supplied with most
Microscopes afford a FlG- 82-
ready means. These
are mounted by means
of a joint upon a pin,
which fits into a hole
eixher in the corner of
the Stage itself or in
the Object platform ; Stage-Forceps.
the object is inserted
by pressing the pin that projects from one of the blades, whereby
it is separated from the other; and the blades close again by
their own elasticity, so as to retain the object when the pressure
is withdrawn. By sliding the wire stem which bears the Forceps
through its socket, and by moving that socket vertically upon its
joint, and the joint horizontally upon the pin, the object may
be brought into the field precisely in the position required ; and
it may be turned round and round, so that all sides of it may be
examined, by simply giving a twisting movement to the wire stem.
The other extremity of the stem often bears a small brass box
filled with cork, and perforated with holes in its side ; this affords
a secure hold to common pins, to the heads of which small objects
can be attached by gum, or to which disks of card, &c, may be
attached, whereon objects are mounted for being viewed with the
Lieberkuhn (§ 102). This method of mounting was formerly much
in vogue, but has been less employed of late, since the Lieberkuhn
has fallen into comparative disuse.
The Stage Vice, as made by Mr. Ross for Mr. Slack, was con-
trived for the purpose of holding small hard bodies, such as mine-
rals, apt to be jerked out by the angular motion of the blades of
the forceps, or very delicate substances that will not bear rough
compression. In this apparatus the blades meet horizontally, and
their movements can be regulated to a nicety with a fine screw. The
Stage Yice fits into a plate, as is the case with Beck's disk-holder,
Fig. 83.
106. For the examination of Objects which cannot be conveniently
held in the Stage-forceps, but which can be temporarily or perma-
nently attached to Disks, no means is comparable to the Dish-
Holder of Mr. R. Beck (Fig. 83) in regard to the facility it affords
for presenting them in every variety of position. The Object being
attached by gum (having a small quantity of glycerine mixed with
156
ACCESSOEY APPAEATUS.
Beck's Disk-Holder.
it), or by gold- size, to the surface of a small blackened metallic
Disk, this is fitted by a short stem projecting from its under sur-
face into a cylindrical holder ; and the holder, carrying the disk,
„ can be made to rotate
around a vertical axis
by turning themilled-
head on the right,
which acts on it by
means of a small
chain that works
through the hori-
zontal tubular stem ;
whilst it can be made
to incline to one side
or to the other, until its plane becomes vertical, by turning the
whole movement on the horizontal axis of its cylindrical socket.*
The supporting plate being perforated by a large aperture, the
object may be illuminated by the Lieberkiihn rf desired. The
Disks are inserted into the Holder, or are removed from it, by
a pair of Forceps constructed for the purpose; and they may
be safely put away by inserting their stems into a plate perfo-
rated with holes. Several such plates, with intervening guards to
prevent them from coming into too close apposition, may be
packed into a small box. To the value of this little piece of
apparatus the Author can bear the strongest testimony from his
own experience, having found his study of the Foraminifera
greatly facilitated by it. — A less costly substitute, however, which
answers sufficiently well for general purposes, is found in the
Object-Holder of Mr. Morris (Fig. 84), which consists of a support-
Fig. 84.
Morris's Object-Holder.
ing plate that carries a ball-and-socket joint in its centre, into the
ball of which can be fitted by a tapering stem either a holder for
small cardboard disks, or a larger holder suitable for carrying an
* A small pair of Forceps adapted to take up minute objects may be fitted
into the cylindrical Holder, in place of a disk, as proposed by Capt. Hutton
(see "Quart. Journ. of Microsc. Science," N.S. Vol. vi. p. 61).
GLASS STAGE-PLATE.— GROWING-SLIDE. 157
ordinary slide. By the free play of the ball-and-socket joint in
different directions, the object may either be made to rotate, or may
be so tilted as to be viewed obliquely or almost laterally. This in-
strument can, of course, be used only by side -illumination, and in
order to turn it to the best account, the objects to be viewed by it
must be mounted on special disks ; but it has an advantage over
the preceding in being applicable also to objects mounted in ordi-
nary slides.
107. Glass Stage-Plate. — Every Microscope should be furnished
with a piece of Plate-Glass, about 4 in. by 1^ in., to one margin of
which a narrow strip of glass is cemented, so as to form a ledge.
This is extremely useful, both for laying objects upon (the ledge
preventing them from sliding down when the Microscope is in-
clined), and for preserving the Stage from injury by the spilling of
sea-water or other saline or corrosive liquids, when such are in use.
Such a plate not only serves for the examination of transparent,
but also of opaque objects ; the dark background being furnished
by the Diaphragm-plate, and the Condensing -lens being so placed
as to throw a side-light upon them. — A small addition may be
conveniently made to the glass stage-plate, which adapts it for use
as a Groiving-Slide. A circular aperture of about the diameter
of a test-tube is made near one end of the plate (the length of
which, for this purpose, had better be not less than 5 inches), and
in this is to be fitted a little cup, formed of the end of a test-tube,
about three-quarters of an inch deep, in such a manner that its
rim shall project a little above the surface of the plate. The cup
may be closed by an ordinary cork, or (to avoid danger of splitting
it) by a disk of glass cemented to a ring of cork which shall
embrace the exterior of the tube ; but a small aperture must be
left by grinding a notch in the rim of the cup, sufficient to admit
the passage of two or three threads of lamp-cotton. The manner
in which the ' growing -slide ' is used is this : — Supposing we wish
to follow the changes undergone by some minute Alga or Infu-
sorium, which we have just detected in a drop of liquid under
examination upon an ordinary slip of glass (and covered with thin
glass), — we transfer this slip to the ' growing-slide,' fill the cup
with distilled water mixed with a small proportion of the water in
which the organism was found, and then so arrange the threads
(previously moistened with distilled water) that they shall pass
from the cup to the edge of the liquid in which the object is con-
tained. Thus, as the water evaporates from beneath the thin
glass, the threads will afford a continuous supply ; and the threads
will not become dry until the whole of the liquid has been ab-
sorbed by them and has been dissipated by evaporation. Fresh
supplies should of course be introduced into the cup from time to
time, as may be needed, so as to prevent any loss of liquid from
beneath the thin glass ; and in this manner the most important
requisite for the continued growth of aquatic organisms, — a con-
stant supply of liquid, without an exclusion of air, — may be
158 ACCESSOEY APPAEATUS.
secured.* — Prof. Smith's Growing- Slide (made by Mr. Baker) is
composed of two plates of glass slightly separated by four glass
slips so as to form a large flat cell. It is filled through an aperture
left at one corner, and is perforated by a small hole, near which
the object whose growth is to be watched is placed, covered with
thin glass. The water-supply in this growing-slide lasts several
days. Its chief disadvantage arises from the growth of vegetable
matter inside, which it is not
Fig. 85. easy to remove. — Dr. Maddox's
Gh'oiving- Slide will be under-
stood from the annexed sketch.
The shaded parts are pieces
of tinfoil fastened with shell-
lac glue to a glass slide. The
minute fungi or spores to be
grown are placed on a glass
cover, large enough to cover the
tinfoil, with a droplet of the
fluid required. This, after ex-
amination to see that no extra-
neous matter is introduced, is placed over the tinfoil, and the edges
fastened with wax softened with oil, leaving free the spaces x x
for entrance of air. — Growing-slides of this description could be
made cheaply with thin glass instead of tinfoil.
Dr. Maddox has found the following fluid sufficiently hygrometric
to keep the spores moist, and to be adapted to fungoid growth : — ■
Dextrine 2 grains.
Phosphate of Soda and Ammonia 2 „
Saturated Solution of Acetate Potash . . .12 drops.
Grape Sugar , 16 grains.
Freshly Distilled Water 1 oz.
The water is to be boiled in large test-tube or beaker for
15 minutes, and covered whilst boiling and cooling ; when settled,
it should be poured into perfectly clean 2 -drachm stoppered bottles
and kept for use. Sometimes other cultivating media are added.f
108. Live Boxes and Cells. — The live box consists of a short piece
of wide brass tube, fixed perpendicularly at one end into a flat
plate of brass (Fig. 86), which is itself perforated by an aperture
equal in diameter to that of the tube, and having its opposite ex-
tremity closed by a disk of glass (b, b) ; over this fits a cover,
formed of a piece of tube just large enough to slide rather stiffly
upon that which forms the box, closed at the top by another disk
of glass (b a). The cover being taken off, a drop of the liquid to
* For descriptions of other forms of Growing Slide, see " Transact, of
Microsc. Soc." Vol. xiv. p. 34, and " Quai't. Journ. of Microsc. Science," N.S.
Vol. vii. p. 11.
f See paper on Cultivation of Microscopic Fungi, in ." Monthly Microsc.
Journ." June, 1870, p. 14.
LIVE-BOX OR ANIMALCULE-CAQE.
159
be examined, or any thin object which can be most advantageously
looked at in fluid, is placed upon the lower plate ; the cover is then
slipped over it, and is
pressed down until the
drop of liquid be spread
out, or the object be flat-
tened, to the degree most
convenient for observation.
If the glass disk which
forms the lid be cemented
or burnished into the brass
ring which carries it, a small
hole should be left for the
escape of air or superfluous
fluid ; and this hole may be
closed up with a morsel of
wax, if it be desired to pre-
vent the included fluid from
evaporating. But as it is
desirable that this glass
should be thin enough to
allow a 1-ith inch Objec- Live Box or Animalcule Cage, as seen in per-
tive to be employed for 'the spective at a, and in section at b.
examination of Animalcu-
les, &c, and as such thin glass is extremely apt to be broken, it is
a much better plan to furnish the brass cover with a screw-cap,
which holds the glass disk with sufficient firmness, but permits it
to be readily replaced when broken ; and as the looseness of this
fitting gives ample space for the escape of air or fluid around the
margin of the disk, no special aperture is needed. It is always
desirable, if possible, to prevent the liquid from spreading to the
edge of the disk ; since any objects it mav contain are very apt in
such a case to be lost under the opaque ring of the cover ; this is
to be avoided by limiting the quantity of liquid introduced, by
laying it upon the centre of the lower plate, and by pressing down
the cover with great caution, so as to flatten the drop equally on all
sides, stopping short when it is spreading too close to the margin.
With a little practice, this object may_ in general be successfully
attained ; but if so much superfluous liquid should have been in-
troduced that it has flooded the circumference of the enclosed space
and exuded around the edge of the disk, it is better to wipe the
whole perfectly dry and then to introduce a fresh drop, taking more
care to limit its quantity and to restrain it within convenient
bounds. If the box be well constructed, and the glass disks be
flat, they will come into such close contact that objects of extreme
thinness may be compressed between them ; hence not only may
such small animals as Water-fleas (Ento7iiostraca) be restrained
from the active movements which preclude any careful observation
of their structure, — and this without any permanent injury being
160
ACCESSOEY APPAEATUS.
inflicted upon them, — but much smaller creatures, such as Wheel-
animalcules (Rotifera), or Bryozoa, may be flattened out, so as to
display their internal organization more clearly, and even the larger
Infusoria may be treated in like manner. — The Live Box is less
used than in former times, as it is not adapted for illumination with
the achromatic condenser or the parabolic illuminator, on account
of its standing up above the stage.
109. Infusoria, minute Algas, &c, can be well seen by placing a
drop of the water containing them on an ordinary slide, and laying
a thin piece of covering glass on the top. Objects of somewhat
greater thickness can be shown in shallow cells made by placing
a loop or ring of fine cotton-thread upon an ordinary slide, to keep
the covering-glass at a small distance from it. The object to be
examined with a drop of water is placed on the slide, and the cover-
ing-glass gently pressed down till it touches the ring. For deeper
cells, glass rings cemented with shell-lac glue to ordinary slides
answer excellently. When the cells are filled, glass covers adhere
by capillary attraction, so that they will remain in place when the
Microscope is inclined, provided the superfluous fluid be removed by
the Syringe (§ 115) or by blotting-paper. Mr. Carter (at Baker's)
has contrived ingenious cells by fixing rotating glass covers to
hollow glass slides : the only disadvantage of this plan arises from
the facility with which the glasses may be broken. Small cell-
slides with their covers are, however, particularly convenient for im-
prisoning minute insects.
110. Zoophyte Trough. — For the examination of living Aquatic
objects too large to be conveniently received into the Animalcule
cage, the Zoophyte trough,
contrived by Mr. Lister,
may be employed with great
advantage. This consists of
a trough of the shape re-
presented in Fig. 87, formed
of plates and slips of plate-
glass, cemented together by
marine glue ; of a loose ver-
tical plate of glass, just so
much smaller than the front
or back of the inside of the
trough as to be able to move
freely between its sides ;
and of a horizontal slip of
glass, whose length equals
that of the inside-bottom
Fig. 87
Zoophyte Trough.
of the trough, but whose breadth is inferior by the thickness of
the plate just mentioned. The trough being filled with water
(fresh or salt, as the case may be), the horizontal slip is laid at
the bottom, and the vertical plate is placed in contact with the
front of the trough, its lower margin being received into the space
ZOOPHYTE-TROUGH. — COMPEESSOPJUM. 161
left at the front edge of the horizontal slip which serves to hold it
there, acting as a kind of hinge ; a small ivory wedge is then in-
serted between the front-glass of the trough and the upper part of
the vertical plate, which it serves to press backwards ; but this
pressure is kept in check by a little spring of bent whalebone,
which is placed between the vertical plate and the lack-glass of
the trough. By moving the ivory wedge up or down, the amount
of space left between the upper part of the vertical plate and the
front-glass of the trough can be precisely regulated ; and as their
lower margins are always in close apposition, it is evident that the
one will incline to the other with a constant diminution of the
distance between them from above downwards. Hence a Zoophvte,
or any similar body, dropped into this space, will descend until it
rests against the two surfaces of the glass, and will remain there
in a situation extremely convenient for observation ; and the regu-
lating-wedge, by increasing or diminishing the space, serves to
determine the level to which the object shall fall. — It is convenient
for the working Microscopist to be furnished with several simple
Water-troughs of different sizes ; and he may easily construct for
himself thin ones suitable for observing delicate Zoophytes, or for
growing Ghara or Nitella, in the following manner. A piece of
plate-glass of thickness equal to the water-space which it is desired
to give, is cut to the size 'suitable for the trough, and strips are cut
from three of its edges ; these strips are cemented with marine
glue, in their original relative positions, on a glass plate, so as to
form the bottom and ends of the trough j ( ; and a thin-glass
cover being cemented on them, the trough is complete ; or, what is
usually more handy, the thin glass may be simply laid in its place
after a little water has been placed in the trough, to the sides of
which it will adhere by capillary attraction. Small troughs of
this kind may be conveniently made from ordinary Glass Slides
cut into halves ; the three strips being cut from one-half, and the
other half, if thin enough, serving as the cover.
111. Compressorium. — The purpose of this instrument is to apply
a gradual pressure to objects whose structure can only be made
out when they are thinned by extension. For such as will bear
tolerably rough treatment, a well-constructed Aquatic Box may
be made to answer the purpose of a compressor ; but there is a
very large class whose organization is so delicate as to be con-
fused or altogether destroyed by the slightest excess of pressure ;
and for the examination of such, an instrument in which the
degree of compression can be regulated with precision is almost
indispensable. The Compressorium represented in Fig. 88 was
originally devised by Schiek of Berlin, whilst its details were
modified by M. de Quatrefages, who constantly employed it in his
elaborate and most successful researches on the organization of the
Marine Worms. It consists of a plate of brass between 3 and
4 inches long, and from 1^ to 1| inches broad, having a central
aperture of from | to f of an inch. This central aperture is covered
M
162 ACCESSOEY APPARATUS.
on its upper side by a disk of thin glass, which may be cemented
to the brass plate by Canada balsam ; and the under side of it is
Fig. 88.
Compressorium.
bevelled away, so that the thickness of the edge shall not interfere
with the approach of the objective to its margin, when that side is
made the uppermost. Near one extremity of the plate is a strong
vertical pin, that gives support to a horizontal bar which turns on
it as on a swivel ; through the end of this bar that projects beyond
the plate, there passes a screw with a milled-head ; and at the
other end is jointed a second bar, against one end of which the
screw bears, whilst the other carries a frame holding a second disk
of thin glass. This frame is a small circular plate of brass,
having an aperture equal in size to that of the large plate ; to its
under side, which is flat, a disk of thin glass is cemented by
Canada balsam, while its upper side is bevelled off as it approaches
the opening, for the purpose just now specified ; and by beino>
swung between pivots in a semicircle of brass, which is itself
pivoted to the moveable arm, it is made capable of a limited move-
ment in any direction. The upper disk, with the apparatus which
supports it, having been completely turned aside around the
swivel- joint, the object to be compressed is laid upon the lower
disk ; the upper disk is then turned back so as to lie precisely over
it, and by the action of the milled-head screw is gradually
approximated to the lower, to which the pivot-movements of its
frame allow it to take up a parallel position, whatever may be the
inclination of the bar. — As it is frequently of great importance to
be able to look at either side of the object under compression, the
principal plate is provided with two pins at the extremity farthest
from the milled-head, which, being exactly equal in length to the
swivel-pin, afford with it a support to the instrument, when it is
so turned that the side represented as undermost in the figure
shall be uppermost ; and it is in order that high powers may be
used in this case as in the other, that the disk which then covers
the object is made of thin glass, instead of being (as in the original
form of the instrument) a piece of thick glass plate. Either°disk
may be replaced with extreme facility, if broken, by simply warm-
ing the part of the instrument to which it is attached, so as to
loosen the cement that holds it. Some observers prefer a modifica-
COMPEESSOKIUM.
163
Fig. 89.
tion of this instrument, in which the brass plate is made to carry
an ordinary Glass Slide, on which the object may be prepared
nnder the Dissecting Microscope before being subjected to com-
pression. By transferring it to the Compressorium on the slide
on which it has been dissected, we avoid Disturbing the object, but
sacrifice the advantage of being able to look at it through thin
glass from the under side.
112. The chief defect in the preceding apparatus consists in the
absence of any provision for securing the parallelism of the ap-
proximated surfaces. Such
a provision is made in Ross's
Improved Compressorium,
shown in Fig. 89 ; in which
the upper plate d is attached
to a slide that works between
grooves in the vertical piece c,
so that when raised or lowered
by the milled-head, it always
maintains its parallelism to
the lower plate a. The thin
glass carried by the upper
plate d (which can be turned
aside on a swivel joint," as
shown in the lower figure)
is a square that slides into
grooves on its under side, so
as to be easily replaced if
broken. The glass to which
it is opposed is a circular disk
lodged in a shallow socket in
plate b, which is received into
a part of the lower plate a
that is sunk below the rest.
The plate b carrying the
lower glass can be drawn out
Ross's Improved Corapressorium.
(as shown in the lower figure) and laid upon the Dissecting Micro-
scope, and can then be replaced in the Compressorium after the
object has been prepared for compression.
113. Beck's Reversible Compressoriums. — The most convenient
Compressoriums for general use are those made by Messrs. Beck,
shown in Figs. 90 — 94. In both, the upper and lower glasses are
fixed, upon a plan devised by Mr. Slack, by means of flat-headed
screws, two to each glass. The heads of these screws fit into
holes of the opposite frame, and thus permit the close approxi-
mation of the two glass surfaces. In Figs 90, 91 {the Parallel
Plate Compressor), the degree of pressure and approximation is
regulated by the screw b, wl^ch works out of centre in a conical
hole of the lower frame ; so that the further it is introduced, the
closer the two frames, with their glasses, are approximated. This
164
ACCESSORY APPAEATUS.
pattern works equally well whichever side is uppermost. Figs.
92, 33 show the plan npon which the glasses are fixed; and
Fig. 90.
Fig. 91.
«s>Ofg;
"' :;'.'' 'I':"": ".!':' ■ ■ ■"" '■■■■:■ ,;il 'I'l,
Figs. 93, 94 illustrate the Reversible Cell Compressor. The upper
frame a screws on to the lower one, giving any degree of pressure
Fig. 92.
Fig. 93.
required. "When screwed together they form a cell fitting into the
plate b, which rests on the stage; c is a milled-head, by means of
which this cell is attached to b, from which it can be instantly
detached and replaced in a reverse position. In both these Com-
pressoriums it is easy to vary the thickness of the glass within
convenient limits. Fig. 90 is perhaps the handiest when slight
pressure is required. Fig. 94 allows a stronger pressure without
disturbing the parallelism of the glasses. The observer should be
provided with a stock of glass slips, as shown in Figs. 92-3, some
of very thin, and others of moderately stout covering glass. In
sea- side and many other investigations, thin glasses are very liable
to fracture from the presence of sharp sand particles ; and the
power of immediately replacing them without the employment of
cement is a great convenience.
114. Dipping-tubes. — In every operation in which small quantities
of liquid, or small objects contained in liquid, have to be dealt
DIPPING-TUBES. —GLASS SYRINGE.
165
Fig. 95.
ABC
with, by the Microscopist, he will find it a very great convenience
to be provided with a set of Tubes of the forms represented in
Fig. 95, but of somewhat larger dimensions. These
were formerly designated as ' fishing tubes ;' the
purpose for which they were originally devised
having been the fishing-out of Water-fleas, aquatic
Insect Larvae, the larger Animalcules, or other
living objects distinguishable either by the unaided
eye or by the assistance of a magnifying-glass,
from the vessels that may contain them. But
they are equally applicable, of course, to the selec-
tion of minute Plants ; and they may be turned
to many other no less useful purposes, some of
which will be specified hereafter. When it is de-
sired to secure an object which can be seen either
with the eye alone or with a magnifying-glass,
one of these tubes is passed down into the liquid,
its upper orifice having been previously closed by
the forefinger, until its lower orifice is imme-
diately above the object ; the finger being then
removed, the liquid suddenly rises into the tube,
probably carrying the object up with it ; and if
this is seen to be the case, by putting the finger
again on the top of the tube, its contents remain
in it when the tube is lifted out, and may be
deposited on a slip of glass or on the lower disk
of the Aquatic Box, or, if too copious for either
receptacle, may be discharged into a large glass
cell (Fig. 117). In thus fishing for any but the
minutest objects, it will be generally found con-
venient to employ the open-mouthed tube c ; and
when its contents have been discharged, if they
include but a single object of the desired kind,
this may be taken up by one of the finer tubes, a, b,
or, if more convenient, the whole superfluous fluid
may be sucked up by the mouth, and the object
left with no more than is suitable ; or, if there be
many of the objects in the fluid first selected,
these may be taken up from it, one by one, by
either of the finer tubes.
115. Glass Syringe. — In dealing with minute Aquatic objects,
great advantage will be found in the use of a small Glass Syringe
of the pattern represented in Fig. 96, and of about double the
dimensions. When this is firmly held between the fore and middle
fingers, and the thumb is inserted into the ring at the summit of
the piston-rod, such complete command is gained over the piston
that its motion may be regulated with the greatest nicety ; and
thus minute quantities of fluid may be removed or added, or any
minute object may be selected (by the aid of the simple Microscope,
Dipping-tubes.
166 ACCESSORY APPARATUS.
if necessary) from amongst a number in the same drop, and trans-
ferred to a separate slip. A set of such Syringes, with points
Fig. 96.
Glass Syringe.
drawn to different degrees of fineness, and bent to different cur-
vatures, will be found to be among the most useful ' tools' that the
working Microscopist can have at his command, as they are
capable of a great number of applications, several of which will
be particularized hereafter.
116. Forceps. — Another instrument so indispensable to the
Microscopist as to be commonly considered an appendage to the
Microscope, is the Forceps for taking up minute objects ; many
forms of this have been devised, of which one of the most con-
venient is represented in Fig. 97 of something less than the actual
Fig. 97.
Forceps.
size. As the forceps, in marine researches, have continually to be
plunged into sea-water, it is better that they should be made of
brass or of German silver, than of steel, since the latter rusts far
more readily ; and as they are not intended (like Dissecting -forceps)
to take a firm grasp of the object, but merely to hold it, they may
be made very light, and their spring-portion slender. As it is
essential, however, to their utility, that their points should meet
accurately, it is well that one of the blades should be furnished
with a guide-pin passing through a hole in the other.
The foregoing constitute, it is believed, all the most important
pieces of Apparatus which can be considered in the light of Acces-
sories to the Microscope. Those which have been contrived to
afford facilities for the prejmration and mounting of Objects, will
be described in a future chapter (Chap. v.). It may be thought
that some notice ought to be taken of the Frog -Plate and Fish-
Pan, with the former of which many Microscopes are supplied,
whilst the latter has scarcely jTet gone altogether out of use.
But the Author, having been accustomed to gain all the ad-
ACCESSOEY APPARATUS. 167
vantages of these by methods far more simple, whilst at least
equally efficacious, does not consider them as presenting any
advantages which render it desirable to expend time or space in
giving a detailed account of them ; and he will explain the methods
alluded to under the appropriate head (Chap, xvm., Circulation of
the Blood).
CHAPTEE IV.
MANAGEMENT OE THE MICROSCOPE.
117. Table. — The Table on which the Microscope is placed when
in use, should be one whose size enables it also to receive the
various appurtenances which the observer finds it convenient to
have within his reach, and whose steadiness is such as to allow of
his arms being rested upon it without any yielding ; it should,
moreover, be so framed, as to be as free as possible from sny ten-
dency to transmit the vibrations of the building or floor ivhereon
it stands. The working Microscopist will find it a matter of great
convenience to have a Table specially set apart for his use, famished
with drawers, in which are contained the various Accessories he
may require for the preparation and mounting of objects. If he
should desire to carry about with him all the apparatus he may
require for the prosecution of his investigations in different locali-
ties, and for the mounting of his preparations on the sp*t, he will
find it very convenient to provide himself with a small Cabinet,
fitted with drawers, in which every requisite can be securely packed,
and of such a height that, when laid upon an ordinaiy table, it
may bring up the Quekett Dissecting Microscope (Fig. 32) placed
upon it to the position most convenient for use * — If the Mcroscope
be one which is not very readily taken out from and put back into its
case, it is very convenient to cover it with a large bell-glass ; which
may be so suspended from the ceiling, by a cord carrying a counter-
poise at its other end, as to be raised or lowered with the least
possible trouble, and to be entirely out of the way when the Mcro-
scope is in use. Similar but smaller bell-glasses (wine-gllsses
whose stems have been broken answer very well) are also useM for
the protection of objects which are in course of being examined or
prepared, and which it is desirable to seclude from dust. — Foj the
purpose of Demonstration in the Lecture Eoom, a small traveling
* The dimensions of the Cabinet which the Author has had construct^ for
himself (its size being so adapted to that of the box of his Crouch's Bintular
that the two are received into the same travelling-case) are 14 incheslong
7 inches broad, and A\ inches high. In the middle there are five shllow
drawers, 5 inches broad, containing dissecting apparatus, large flat cells
covers, syringes, &c. ; on one side are two drawers, each 3^ inches broa
upper one, containing slides, cells, &c, rather more than one inch deep ijside,
the lower, for larger pieces of apparatus, 2 inches deep ; on the other sic
single drawer of the same breadth and 2>\ inches deep, for bottles contesting
solutions, cements, &c.
DAYLIGHT AXD LAMPLIGHT. 169
Platform may be constructed to run easily upon rollers, and to
carry the Microscope and Lamp securely clamped down upon it, so
as to be passed from one observer to another. For Demonstration
to a small party sitting round a circular table, it is convenient to
employ a A- shaped platform, the vertical angle of which is pivoted
to a weight placed in the centre of the table, whilst the angles at
the base are supported upon castors, so that the platform may run
round to each observer in succession. Or the table itself, if not too
large, may be made to rotate (like a dumb-waiter) upon its central
pillar, as made by Messrs. Beck.
118. Light. — Whatever may be the purposes to which the Micro-
scope is applied, it is a matter of the first importance to secure a
pure and adequate Illumination. For the examination of the
greater proportion of objects, good daylight is to be preferred to
any other kind of light; but good lamvplight is preferable to bad
daylight. When Daylight is employed, the Microscope should be
placed near a window, whose aspect should be (as nearly as may be
convenient) opposite to the side on which the sun is shining ; for
the light of the sun reflected from a bright cloud is that which the
experienced Microscopist will almost always prefer, the rays pro-
ceeding from a cloudless blue sky being by no means so well fitted
for his purpose, and the 'dull lurid reflection of a dark cloud being
the worst of all. The direct rays of the Sun are far too powerful
to be used with advantage, unless its intensity be moderated,
either by reflection from a plaster-of -Paris or some other ' white-
cloud' mirror (§ 97), or by passage through some medium which
stops a greater or less proportion of its rays. This may be done
by placing coloured glasses over the eye-pieces, as recommended by
Mr. Wenham ; or by placing the ' moderator' specially contrived
by Mr. Rainey for lamp or gaslight illumination (§ 119) between
the window and the mirror. Direct sunlight, is, however, occasionally
used by some observers to work out intricate markings or fine colour :
it may sometimes be of advantage for these purposes, but without
great care would be a fertile source of error. The young Micro-
scopist is earnestly recommended to make as much use of daylight
as possible ; not only because, in a large number of cases, the view
of the object which it affords is more satisfactory than that which
can be obtained by any kind of lamplight, but also because it is
much less trying to the eyes. So great, indeed, is the difference
between the two in this respect, that there are many who find
themselves unable to carry on their observations for any length of
time by Lamplight, although they experience neither fatigue nor
strain from many hours' continuous work by Daylight.
119. Lamps. — When recourse is had to Artificial light, it is
essential, not only that it should be of good quality, but" that the
arrangement for furnishing it should be suitable to the special
wants of the Microscopist. The most useful light for ordinary use
is that furnished by the steady and constant flame of a Lamp, fed
either with Oil, Camphine, Parafnne (of its best varieties), or Gas ;
170
MANAGEMENT OF THE MICROSCOPE.
Fig. 98.
it should be capable of adjustment to any height above the table ;
and a moveable shade should be provided, by which the light may
be prevented from coming direct to the observer's eyes, or from
diffusing itself too widely through the room. These requisites are
supplied by the Lamp commonly known as the ' University' or
6 reading' lamp, which has a circular foot with a vertical stem, on
which the oil-reservoir (carrying with it the burner) and the shade
can be fixed at any convenient height. French and German lamps,
on the same general construction, but having the reservoir contrived
on the ' bird-fountain' principle, are also to be obtained, being
largely imported for the use of watchmakers ; these have the
advantage of burning out all their oil, which is not the case with
the ordinary ' reading' lamp, as it does not burn well except when
full, or nearly so ; and they are now made with shades, well suited
to the wants of the Microscopist.* The Paraffine or Belmontine
lamps, which have come into such general use, have many advan-
tages for the Microscopist ; and are probably, on the whole, when
constructed with express reference to his
requirements, the most convenient lamps
he can employ. The Author can strongly
recommend, from his own experience of
its use, the form known as the Bockett
Lamp (Fig. 98), manufactured by Mr.
Collins. This is attached by a transverse
arm to a tubular slide, which moves up
and down upon the stem that rises from
the foot, and can be fixed by a milled-
head ; and this slide also carries the Con-
denser, which is thus raised or lowered
with the lamp itself, far more conveniently
than when mounted on a separate foot.
The flat wick may be so turned as to
present to the mirror or condenser either
its whole breadth, or only its edge, or any
intermediate aspect ; the light in the se-
cond case being much increased in inten-
sity, but restricted to a smaller surface. —
To every one who has a supply of Gas at
command, the use of it for his Microscope-
lamp (by means of a flexible tube) strongly
recommends itself, on account of its ex-
treme convenience, and its freedom from any kind of trouble. The
lamp should be constructed on the general plan already described,
the burner being made to slide up and down on a stem rising per-
pendicularly from a foot, which also carries a shade ; and the burner
should be one which affords a bright and steady cylindrical flame,
either ' Leslie's' or the ' cone' burner being probably the best. Even
Bockett Lamp.
* A very excellent Lamp of this kind is sold by Mr. Pillischer.
MICEOSCOPE-LAMPS. 171
the best light supplied by a Gas-lamp, however, is inferior in quality
to that of a good Oil-lamp ; and is more injurious and unpleasant
to the eye. Hence the interposition of some kind of artificial
medium adapted to keep back the yellow rays, whose predominance
in the lamp-flame is the chief source of its injurious action, is espe-
cially required when Gaslight is used. This may be partly effected
by the simple expedient of using a chimney of bluish glass, known
as 'Leblond's ;' but, in addition, it is advantageous to cause the
light to pass through a screen of bluish -black or neutral -tint glass ;
and it will then be nearly purified as to quality, though much
reduced in intensity.* Mr. Rainey, who has paid great attention
to the best means of obtaining a good illumination by artificial
light, recommends, as the best moderator, one piece of dark-blue
glass free from any tint of red, another of very pale-blue with a
slight shade of green, and two of thick white plate-glass, all
cemented together with Canada balsam ; this, as already stated, may
be used with Sunlight, as well as with Lamplight.
Mr. Fiddian's Lamp is preferred by some microscopists. It is
supplied with a copper chimney lined with plaster-of-Paris. The
light escapes through a circular aperture made to receive a plain
or coloured disk of glass, or a bull's-eye condenser. This lamp
is fed with paraffine or photogen. It gives an excellent light ; and
is so mounted that it will burn well when it is slanted considerably
out of the perpendicular, and thus brought parallel to the stage
mirror, arranged at a convenient angle. It has the advantage of
not diffusing any general illumination, which is a matter of im-
portance in examining very delicate objects. Its chief disadvantage
is that the plaster-of-Paris wants occasional renewing, and if used
after it is much cracked will suddenly tumble off. Fresh plaster-
of-Paris can, however, be applied in a few minutes by pouring
it in mixed with water to the consistency of cream.
Messrs. Home and Thornthwaite, Collins, How, Baker, and
others, supply lamps with white porcelain cylindrical shades over
the ordinary white glass chimney. These shades have a hole on
one side through which a very white light passes.
The Bockett and Fiddian lamps are made with an upright stem
at one edge of _ the circular foot, and consequently are steady in
only one direction, that in which the lamp stands over the centre
of the foot. Other patterns have the stem rising from the centre
of a foot sufficiently heavy, or spreading in three directions, so
that the lamp is safe whichever way it is turned.
Mr. Moginnie and others have devised useful portable lamps for
travelling. _ They can be carried safely in the pocket.
120. Position of the Light. — When the Microscope is used by
Daylight, it will usually be found most convenient to place it in
such a manner that the light shall be at the left hand of the
* A Gas-lamp provided with these and other appurtenances for regulating
the illumination, and also with a water-bath and mounting-plate, has been
devised by Mr. S. Highley.
172 MANAGEMENT OF THE MICROSCOPE.
observer. It is most important that no light should enter his eye,
save that which comes to it through the Microscope ; and the
access of direct light can scarcely be avoided, when he sits with
his face to the light. Of the two sides, it is more convenient to
have the light on the left ; first, because it is not interfered with
by the right hand, when this is employed in giving the requisite
direction to the mirror, or in adjusting the illuminating apparatus ;
and secondly, because, as most persons in using a Monocular
Microscope employ the right eye rather than the left, the projection
of the nose serves to cut off those lateral rays which, when the
light comes from the right side, glance between the eye and the
eye-piece. The side-shades fitted by Mr. Collins to the eye-pieces
of his Harley Binocular (Fig. 41) may be advantageously employed
with every instrument of that class. — "When Artificial light is em-
ployed, the same general precaution should be taken. The Lamp
should always be placed on the left side, unless the use of the
mirror be dispensed with, or some special reason exist for placing
it otherwise. If the object under examination be transparent,
the lamp should be placed at a distance from the eye about mid-
way between that of the stage and that of the mirror ; if on the
other hand, the object be opaque, it should be at a distance about
midway behind the eye and the stage ; so that its light may fall,
in the one case upon the Mirror, in the other case upon the Stage,
at an angle of about 45° with the axis of the Microscope. The
passage of direct rays from the flame to the eye should be guarded
against by the interposition of the lamp-shade ; and no more light
should be diffused through the apartment, than is absolutely neces-
sary for other purposes. If observations of a very delicate nature
are being made, it is desirable, alike by Daylight and by Lamp-
light, to exclude all lateral raj^s from the eye as completely as
possible ; and this may be readily accomplished by means of a
shade made like the upper part of a mask, and lined with black
cloth or velvet, which should be fixed on the ocular end of the
Microscope.
121. Care of the Eyes. — Although most Microscopists who
habitually work with the Monocular Microscope acquire a habit of
employing only one eye (generally the right), yet it will be decidedly
advantageous to the beginner that he should learn to use either eye
indifferently ; since by employing and resting each alternately, he
may work much longer, without incurring unpleasant or injurious
fatigue, than when he always employs the same. — Whether or not
he do this, he will find it of great importance to acquire the habit
of keeping open the unemployed eye. This, to such as are unaccus-
tomed to it, seems at first very embarrassing, on account of the
interference with the microscopic image which is occasioned by the
picture of surrounding objects formed upon the retina of the second
eye ; but the habit of restricting the attention to that impression
only which is received through the microscopic eye, may generally
be soon acquired ; and when it has once been formed, all difficulty
CAEE OF EYES AND OF MICROSCOPE. 173
ceases. Those who find it unusually difficult to acquire this habit,
may do well to learu it in the first instance with the assistance of
the shade just described; the employment of which will permit
the second eye to be kept open without any confusion. — So much
advantage, however, is derived from the use of the Binocular
Microscope, even with objects not requiring its stereoscopic effect,
that the Author would strongly recommend its use to every observer
who wishes to take advantage of the best means of avoiding injury
to his sight. — There can be no doubt that the habitual use of the
Microscope, for many hours together, especially by lamplight, and
with high magnifying powers, has a great tendency to injure the
sight. Every Microscopist who thus occupies himself, therefore,
will do well, as he values his eyes, not merely to adopt the various
precautionary measures already specified, but rigorously to keep to
the simple rule of not continuing to observe any longer than he can
do so without fatigue*
122. Care of the Microscope. — Before the Microscope is brought
into use, the cleanliness and dryness of its glasses ought to be
ascertained. If dust or moisture should have settled on the Mirror,
this can be readily wiped off. If any spots should show them-
selves on the field of view when it is illuminated by the mirror,
these are probably due to particles adherent to one of the lenses of
the Eye -piece : and this may be determined by turning the eye-
piece round, which will cause the spots also to rotate, if their source
lies in it. It may very probably be sufficient to wipe the upper
surface of the eye-glass (by removing its cap), and the lower surface
of the field-glass ; but if, after this has been done, the spots should
still present themselves, it will be necessary to unscrew the lenses
from their sockets, and to wipe their inner surfaces ; taking care
to screw them firmly into their places again, and not to confuse the
lenses of different eye-pieces. Sometimes the eye-glass is obscured
by dust of extreme fineness, which may be carried off by a
smart puff of breath ; the vapour which then remains upon the
surface being readily dissipated by rapidly moving the glass back-
wards and forwards a few times through the air. And it is always
desirable to try this plan in the first instance ; since, however soft
the substance with which the glasses are wiped, their polish is
impaired in the end by the too frequent repetition of the process.
The best material for wiping glass is a piece of soft wash-leather,
* The Author attributes to his rigorous observance of the above rule his
entire freedom from any injurious affection of his visual organs, notwith-
standing that of the whole amount of Microscopic study which he has prose-
cuted for thirty-five years past, a large proportion has been necessarily earned
on by Artificial light, most of his daylight hours being occupied in other ways.
He has found the length of time during which he can ' microscopize ' without
the sense of fatigue, to vary greatly at different periods ; half -an -hour's work
being sometimes sufficient to induce it, whilst on other occasions none has
been left by three or four hours' almost continuous use of the instrument, —
his power of visual endurance being usually in relation to the vigour of his
general system.
174 MANAGEMENT OF THE MICKOSCOPE.
from which the dust it generally contains has been well beaten
ont. — If the Object-glasses be carefully handled, and kept in their
boxes when not in use, they will not be likely to reqnire cleansing.
One of the chief dangers, however, to which they are liable in the
hands of an inexperienced Microscopist, arises from the neglect of
precaution in using them with fluids ; which, when allowed to
come in contact with the surface of the outer glass, should be
wiped off as soon as possible. In screwing and unscrewing them,
great care should be taken to keep the glasses at a distance from
the surface of the hands ; since they are liable not only to be
soiled by actual contact, but to be dimmed by the vaporous exha-
lation from skin which they do not touch. This dimness will be
best dissipated by moving the glass quickly through the air. It
will sometimes be found, on holding an Object-glass to the light,
that particles either of ordinary dust, or more often of the black
coating of the interior of the Microscope, have settled upon the
surface of its back-lens ; these are best removed by a clean and dry
camel's-hair pencil. If any cloudiness or dust should still present
itself in an object-glass, after its front and back surfaces have been
carefully cleansed, it should be sent to the maker (if it be of
English manufacture) to be taken to pieces, as the amateur will
seldom succeed in doing this without injury to the work ; the
foreign combinations, however, being usually put together in a
simpler manner, may be readily unscrewed, cleansed, and screwed
together again. ~Not unfrequently an objective is rendered dim by
the cracking of the cement by which the lenses are united, or by
the insinuation of moisture between them ; this last defect occa-
sionally arises from a fault in the quality of the glass, which is
technically said to ' sweat.' In neither of these cases has the
Microscopist any resource, save in an Optician experienced in this
kind of work, since his own attempts to remedy the defect are
pretty sure to be attended with more injury than benefit.
123. General Arrangement of the Microscope for Use. — The
inclined position of the instrument, already so frequently referred
to, is that in which observation by it may be so much more advan-
tageously carried on than it can be in any other, that this should
always be had recourse to unless particular circumstances render
it unsuitable. The precise inclination that may prove to be most
convenient, will depend upon the ' build' of the Microscope, upon
the height of the Observer's seat as compared with that of the table
on which the instrument rests, and lastly, upon the sitting height
of the individual ; and it must be determined in each case by his
own experience of what suits him best, — that which he finds most
comfortable being that in which he will be able not only to work
the longest, but to see most distinctly. — The selection of the
Object-glasses and Eye-pieces to be employed must be entirely
determined by the character of the object. Large objects presenting
no minute structural features should always be examined in the
first instance by the lov:est powers, whereby a general view of their
GENERAL ARRANGEMENT FOR USE. 175
nature is obtained ; and since, with lenses of comparatively long
focus and small angle of aperture, the precision of the focal
adjustment is not of so much consequence as it is with the higher
powers, not only those parts can be seen which are exactly in
focus, but those also can be tolerably well distinguished which
are not precisely in that plane, but are a little nearer or more
remote. When the general aspect of an object has been sufficiently
examined through low powers, its details may be scrutinized under
a higher amplification ; and this will be required in the first
instance, if the object be so minute that little or nothing can be
made out respecting it save when a very enlarged image is formed.
The power needed in each particular case can only be learned by
experience ; that which is most suitable for the several classes of
objects hereafter to be described will be specified under each head.
In the general examination of the larger class of objects, the
range of power that is afforded by the Erector in combination with
the Draw-tube (§§ 68, 69) will often be found useful ; whilst for
the ready exchange of a low power for a higher one, great con-
venience is afforded by the ]N"ose-piece (§ 83).
124. When the Microscopist wishes to augment his magnifying
power, he has a choice between the employment of an Objective of
shorter focus, and the use of a deeper Eye-piece. If he possess a
complete series of Objectives, he will frequently find it best to sub-
stitute one of these for another without changing the Eye-piece for
a deeper one ; but if his ' powers' be separated by wide intervals,
he will be able to break the abruptness of the increase in amplifi-
cation which they produce, by using each Objective first with the
shallower and then with the deeper Eye-piece. Thus if a Micro-
scope be provided only with two Objectives, of 1 inch and l-4th
inch focus respectively, and with two Eye-pieces, one nearly double
the power of the other, such a range as the following may be
obtained, — 60, 90, 240, 360 diameters; or, with two Objectives
of somewhat shorter focus, and with deeper Eye-pieces (as in some
French and German instruments), — 88, 176, 350, 700 diameters.
In the examination of large Opaque objects having uneven sur-
faces, it is generally preferable to increase the power by the Eye-
piece rather than by the Objective ; thus a more satisfactory view
of such objects may usually be obtained with a 3-inch or 2-inch
Objective and the b Eye-piece, than with a l|-inch or 1-inch
Objective and the a Eye-piece. The reason of this is, that in
virtue of their smaller Angle of Aperture, the Objectives first named
have a much greater amount of ' penetrating power' or ' focal
depth' than the latter (§ 145, 1.) ; and that in the case just specified
this quality is of the first importance. The use of the Draw-
tube (§ 68) enables the Microscopist still further to vary the mag-
. nifying power of his instrument, and thus to obtain almost any
exact number of diameters he may desire, within the limits to
which he is restricted by the focal length of his Objectives. The
advantage to be derived, however, either from ' deep Eye-piecing '
176 MANAGEMENT OF THE MICROSCOPE.
or from the use of the Draw-tube, will mainly depend upon the
quality of the Object-glass. For, if it be imperfectly corrected, its
errors are so much exaggerated, that more is lost in definition than
is gained in amplification ; whilst, if its aperture be small, the loss
of light is an equally serious drawback. On the other hand, an
Objective of perfect correction and adequate angle of aperture
will sustain this treatment with so little impairment in the perfec-
tion of its image, that a magnifying power may be obtained by its
use, such as, with an inferior instrument, can only be derived from
an Objective of much shorter focus combined with a shallow Eye-
piece. In making any such comparisons, it must be constantly
borne in mind that the real question is, ivliat can be seen ? It is
always desirable for the purposes of research to employ the lowest
power with which the details of structure can be clearly made out ;
since, the lower the power, the less is the liability to error from
false appearances, and the better can the mutual relations of the
different parts of the object be appreciated. Hence, in testing the
optical quality of a Microscope, the first question should be, not
what is its greatest magnifying power, but, what is the least mag-
nifying power under which it will show objects of a given degree
of difficulty.
125. In making the Focal Adjustment, when low powers are
used, it will scarcely be necessary to employ any but the coarse
adjustment, or ' quick motion ;' provided that the rack be well
cut, the pinion work in it smoothly and easily, without either
'spring,' "loss of time,' or 'twist,' and the milled-head be large
enough to give the requisite leverage. All these are requisites
which should be found in every well-constructed instrument ; and
its possession of them should be tested, like its freedom from
vibration, by the use of high powers, since a really good coarse
adjustment should enable the observer to ' focus' an Objective of
l-8th inch with precision. What is meant by ' spring' is the
alteration which may often be observed to take place on the with-
drawal of the hand ; the object which has been brought precisely
into focus, and which so remains as long as the milled-head is
between the fingers, becoming indistinct when the milled-head is
let go. The source of this fault may lie either in the rack-move-
ment itself, or in the general framing of the instrument, which is
so weak as to allow of displacement by the mere weight or pressure
of the hand : should the latter be the case, the ' spring' may be in
great degree prevented by carefully abstaining from bearing on the
milled-head, which should be simply rotated between the fingers.
By ' loss of time ' is meant the want of sufficient readiness in the
action of the pinion upon the rack, so that the milled-head may be
moved a little in either direction without affecting the body ;
thus occasioning a great diminution in the sensitiveness of the
adjustment. This fault may sometimes be detected in Microscopes
of the best original construction, which have gradually worked loose
owing to the constancy with which they have been in employ-
FOCAL ADJUSTMENT. 177
ment ; and it may often be corrected by tightening the screws that
bring the pinion to bear against the rack. And by ' twist' it is
intended to express that apparent movement of the object across
the field, which results from a real displacement of the axis of the
body to one side or the other, owing to a want of correct fitting
in the working parts, As this last fault depends entirely on bad
original workmanship*, there is no remedy for it ; but it is one
which most seriously interferes with the convenient use of the in-
strument, however excellent may be its optical performance. In
the use of the coarse adjustment with an Objective of short focus,
extreme care is necessary to avoid bringing it down upon the
object, to the injury of one or both ; for although the spring with
which the tube for the reception of the object-glass is furnished,
whenever the Fine Adjustment is immediately applied to this,
takes off the violence of the crushing action, yet such an action,
even when thus moderated, can scarcely fail to damage or disturb
the object, and may do great mischief to the lenses. Where no
such spring tube is furnished, the fine adjustment being otherwise
provided for, or being not supplied at all, still greater care is of
course required. — It is here, perhaps, well to notice, for the guidance
of the young Microscopist, that the actual distance between the
Object-glass and the object, when a distinct image is formed, is
always considerably less than the nominal focal length of the
object-glass : thus, the distance of the 1 inch or 2-3rds inch object-
glass may be little more than half an inch : that of the 4-10ths inch
may be but little more than one-tenth of an inch ; that of a l-4th
or a l-5th inch may scarcely exceed one-twentieth ; that of a l-8th
inch may not be one-fortieth ; and that of a l-12th or a l-16th inch
may be so close as not to admit the intervention of a piece of glass
more than one two-hundredth of an inch thick. One more precau-
tion it may be well to specify ; namely, that either in changing one
object for another, or in substituting one Objective for another —
save when powers of such focal length are employed as to remove
all likelihood of injury — the Body should have its distance from the
Stage increased by the ' coarse adjustment.' This precaution is
absolutely necessary when Objectives of short focus are in use, to
avoid injury either to the lenses or to the object ; and when it is
habitually practised with regard to these, it becomes so much like
an ' acquired instinct,' as to be almost invariably practised in other
cases.
126. In obtaining an exact Focal Adjustment with Object-glasses
of less than half-an-inch focus, it will be generally found con-
venient to employ the fine adjustment or 'slow motion;' and as
recourse will frequently be had to its assistance for other purposes
also, it is very important that it should be well constructed and m
good working order. The points to be particularly looked to in
testing it, are for the most part the same with those already noticed
in relation to the coarse movement. It should work smoothly and
equably, producing that graduated alteration of the distance of
178 MANAGEMENT OF THE MICEOSCOPE.
the Object-glass from the object which, it is its special duty to
effect, without any jerking or irregularity. It should be so sen-
sitive, that any movement of the milled-head should at once make
its action apparent by an alteration in the distinctness of the
image, when high powers are employed, without any ' loss of time.' *
And its action should not give rise to any twisting or displacing
movement of the image, which ought not to be in the least degree
disturbed by any number of rotations of the milled-head, still less
by a rotation through only a few degrees. One great use of the
Fine adjustment consists in bringing into view different strata
of the object, and this in such a gradual manner that their con-
nexion with one another shall be made apparent. Whether an
Opaque or a Transparent object be under examination, only that
part which is exactly in focus can be perfectly discerned under any
power ; and when high powers of large angular aperture are
employed, this is the only part that can be seen at all. A minute
alteration of the focus often causes so different a set of appearances
to be presented, that, if this alteration be made abruptly, their
relation to the preceding can scarcely be even guessed at ; and the
gradual transition from the one to the other, which the Fine
adjustment alone affords, is therefore necessary to the correct in-
terpretation of either. To take a very simple case : — The trans-
parent body of a certain . animal being traversed by vessels lying
in different planes, one set of these vessels is brought into view by
one adjustment, another set by ' focussing' to a different plane ;
and the connexion of the two sets of vessels, which may be the
point of most importance in the whole anatomy of the animal,
may be entirely overlooked for want of a Fine adjustment, the
graduated action of which shall enable one to be traced con-
tinuously into the other. What is true even of low and medium
powers, is of course true to a still greater degree of high powers ;
for although the ' quick motion' may enable the observer to bring-
any stratum of the object into accurate focus, it is impossible
for him by its means to secure that transitional 'focussing'
which is often much more instructive than an exact adjustment
at any one point. A clearer idea of the nature of a doubtful
structure is, in fact, often derived from what is caught sight of in
the act of changing the focus, than by the most attentive study
and comparison of the different views obtained by any number of
separate ' focussings.' The experienced Microscopist, therefore,
whilst examining an object of almost any description, constantly
keeps his finger upon the milled-head of the ' slow motion,' and
watches the effect produced by its revolution upon every feature
* It will sometimes happen that the 'slow motion' will seem not to act,
merely because it has been so habitually worked in one direction rather than
the other, that its screw has been turned too far. In that case, nothing more
is required for its restoration to good working order, than turning the screw in
the other direction, until it shall have reached about the middle of its range of
action.
ADJUSTMENT OF OBJECT-GLASS.
179
which, he distinguishes ; never leaving off until he be satisfied that
he has scrutinized not only the entire surface, but the entire thick-
ness of the object. It will often happen, that where different
structural features present themselves on different planes, it will
be difficult or even impossible to determine with the Monocular
Microscope which of them is the nearer and which the more
remote (§ 95), unless it be ascertained by the use of the ' slow
motion,' when they are successively brought into focus, whether
the Object-glass has been moved toivards or away from the object.*
Even this, however, will not always succeed in certain of the most
difficult cases, in which the difference of level is so slight as to be
almost inappreciable ; as, for instance, in the case of the markings
on the siliceous loricod of the Diatomaceas (§ 141).
127. "When Objectives of short focus and of wide Angular Aper-
ture are being employed, something more is necessary than exact
focal adjustment ; this being the Adjustment of the Object-glass
itself, which is required to neutralize the disturbing effect of the
glass cover upon the course of the rays proceeding from the object
(§ 17). For this adjustment, it will be recollected, a power of
altering the distance between the front pair and the remainder of
the combination is required; and this power is obtained in the
following manner : — The front pair of lenses is fixed into a tube
Fig. 99.
Section of an Adjusting Object-Glass.
(Fig. 99, a), which slides over an interior tube (b) by which the
other two pairs are held ; and it is drawn up or down by means of
* It is in objects of this kind that the great advantage of the Stereoscopic
Binocular arrangement makes itself most felt (§§ 28-37).
N 2
180 MANAGEMENT OF THE MICKOSCOPE.
a collar (c), which works in a furrow cut in the inner tube, and
upon a screw-thread cut in the outer, so that its revolution in the
plane to which it is fixed by the one tube gives a vertical move-
ment to the other. In one part of the outer tube an oblong slit is
made, as seen at d, into which projects a small tongue screwed on
the inner tube ; at the side of the former two horizontal lines are
engraved, one pointing to the word ' uncovered,' the other to the
word ' covered ;' whilst the latter is crossed by a horizontal mark,
which is brought to coincide with either of the two lines by the
rotation of the screw-collar, whereby the outer tube is moved up or
down. When the mark has been made to point to the line ' un-
covered,' it indicates that the distance of the lenses of the object-
glass is such as to make it suitable for viewing an object without
any interference from thin glass ; when, on the other hand, the
mark has been brought by the revolution of the screw-collar into
coincidence with the line ' covered,' it indicates that the front lens
has been brought into such proximity with the other two, as to
produce a ' positive aberration ' in the Objective, fitted to neutralize
the ' negative aberration ' produced by the interposition of a glass
cover of a certain thickness. It is evident, however, that unless the
particular thickness of glass for which this degree of alteration is
suited be always employed for this purpose, the correction cannot
be exact ; and means must be taken for adapting it to every grade
of thickness which may be likely to present itself in the glass
covers. Unless this correction be made with the greatest pre-
cision, the enlargement of the Angle of Aperture, to which our
Opticians have of late applied themselves with such remarkable
success, becomes worse than useless ; being a source of diminished
instead of increased distinctness in the details of the object, which
are far better seen with an Objective of greatly inferior aperture,
possessing no special adjustment for the thickness of the glass.
The following general rule is given by Mr. Wenham for securing
the most efficient performance of an Object-glass with any ordinary
object : — " Select any dark speck or opaque portion of the object,
and bring the outline into perfect focus ; then lay the finger on
the milled-head of the fine motion, and move it briskly backwards
and forwards in both directions from the first position. Observe
the expansion of the dark outline of the object, both when within
and when without the focus. If the greater expansion, or coma,
is when the object is ivithout the focus, or farthest from the Objec-
tive, the lenses must be placed farther asunder, or towards the
mark 'uncovered.' If the greater coma is when the object is within
the focus, or nearest to the Objective, the lenses must be brought
closer together, or towards the mark ' covered.' When the object-
glass is in proper adjustment, the expansion of the outline is
exactly the same both within and without the focus." A different
indication, however, is afforded by such ' test-objects' as present
(like the Podura-scale and the DiatomaceaBj a set of distinct dots
or other markings. For " if the dots have a tendency to run into
ADJUSTMENT OF OBJECT-GLASS. 1S1
lines when the object is placed ii'Wioiit the focns, the glasses nmst
he bronght closer together ; on the contrary, if the lines appear
when the object is within the focal point, the object nmst be far-
ther separated."* When the Angle of Aperture is very wide, the
difference in the aspect of any severe Test under different adjust-
ments becomes at once evident ; markings which are very distinct
when the correction has been exactly made, disappearing almost
instantaneously when the screw-collar is turned a little way
round.f
128. Although the most perfect Correction required for each par-
ticular object (which depends not merely upon the thickness of
its glass cover, but upon that of the fluid or balsam in which it
may be mounted) can only be found by experimental trial ; yet for
all ordinary purposes, the following simple method, first devised
by Mr. Powell, will suffice. The object-glass, adjusted to 'un-
covered,' is to be ' focussed' to the object; the screw-collar is next to
be turned until the surface of the glass cover comes into focus, as
may be perceived by the spots or strise by which it may be marked ;
the object is then to be again brought into focus by the ' slow
motion.' The edge of the screw-collar being now usually graduated,
the particular adjustment which any object may have been found
to require, and of which "a record has been kept, may be made
again without any difficulty. By Messrs. Smith and Beck, however,
who first introduced this Graduation, a further use is made of it.
By experiments such as those described in the last paragraph, the
correct adjustment is first found for any particular object, and the
number of divisions observed through which the screw-collar must
be moved in order to bring it back to 0°, the position suitable for
an uncovered object. The thickness of the glass cover must then
be measured by means of the ' slow motion ;' this is done by bring-
ing into exact focus, first the object itself, and then the surface of
the glass cover, and by observing the number of divisions through
which the milled-head (which is itself graduated) has passed in
making this change. A definite ratio between that thickness of
glass and the correction required in that particular Objective is
thus established; and this serves as the guide to the requisite
correction for any other thickness, which has been determined in
like manner by the ' slow motion.' Thus supposing a particular
thickness of glass to be measured by 12 divisions of the milled-
* See "Quart. Journ. of Microsc. Science," Vol. ii. (1854), p. 138.
t Mr. Wenhani remarks (loc. cit.), not without justice, upon the difficulty of
making this adjustment even in the Objectives of our best Opticians ; and he
states that he has himself succeeded much better by making the outer tube
the fixture, and by making the tube that carries the other pairs slide within
this ; the motion being given by the action of an inclined slit in the revolving
collar, upon a pin that passes through a longitudinal slit in the outer tube to
be attached to the inner. The whole range'of adjustment is thus performed
within a third part of a revolution, with scarcely any friction, and with such
an immediate transition from good to bad definition, that the best point is
made readily apparent.
182 MANAGEMENT OF THE MICEOSCOPE,
head of the ' slow motion,' and the most perfect performance of
the Object-glass to be obtained by moving the screw-collar through
8 divisions, then a thickness of glass measured by 9 divisions of
the milled-head would require the screw-collar to be adjusted to
6 divisions in order to obtain the best effect. The ratio between the
two sets of divisions is by no means the same for different combi-
nations ; and it ought to be determined for each Objective by its
maker, who will generally be the best judge of the best ' points '
of his lenses ; but when this ratio has been once ascertained, the
adjustment for any thickness of glass with which the object may-
happen to be covered is readily made by the Microscopist him-
self. Although this method appears somewhat more complex
than that of Mr. Powell, yet it is more perfect; and when the
ratio between the two sets of divisions has been once determined,
the adjustment does not really involve more trouble. — Another
use is made of this adjustment by Messrs. Smith and Beck,
namely, to correct the performance of the Objectives which is
disturbed by the increase of distance between the Objective and
the Eye-piece that is occasioned by the use of the Draw-tube
(§ 68). Accordingly, they mark a scale of inches on the Draw-
tube (which is useful for many other purposes), and direct that for
every inch the body is lengthened, the screw-collar of the Objective
shall be moved through a certain number of divisions.
129. Arrangement for Transparent Objects. — If the Object be
already ' mounted ' in a Slide, nothing more is necessary, in order
to bring it into the right position for viewing it, than to lay the
slide upon the Object-platform of the Stage, and to support it in
such a position (by means of the sliding ledge or other contrivance)
that the part to be viewed is, as nearly as can be guessed, in the
centre of the aperture of the stage, and therefore in a line with the
axis of the body. If the object be not ' mounted,' and be of such
a kind that it is best seen dry, it may be simply laid upon the
glass Stage-plate (§ 107), the ledge of which will prevent it from
slipping off when the Microscope is inclined, and a plate of thin
glass may be laid over it for its protection, if its delicacy should
seem to render this desirable. If, again, it be disposed to curl up,
so that a slight pressure is needed to flatten or extend it, recourse
may be had to the use of the Aquatic Box (§ 108) or of the Com-
pressorium (§ 111), without the introduction of any liquid between
the surfaces of glass. In a very large proportion of cases, how-
ever, either the objects to be examined are already floating in
fluid, or it is preferable to examine them in fluid on account of
the greater distinctness with which they may be seen. If such
objects be minute, and the quantity of liquid be small, the drop is
simply to be laid on a slip of glass, and covered with a plate of
thin glass ; if the object or the quantity of liquid be larger, it will
be better to place it in a concave slide or cell ; whilst, if the object
have dimensions which render even this inconvenient, the Zoophyte
Trough (§ 110) will afford the best medium for its examination. In
ARRANGEMENT EOR TRANSPARENT OBJECTS. 183
tlie case of living animals, whose movements it is desired to limit
(so as to keep them within the field of view) withont restrain-
ing them by compression, the Author has fonnd the following
plan extremely convenient. The drop of water taken up with
the animal by the Dipping-tube being allowed to fall into a concave
slide (Fig. 117), the whole of the superfluous water may be removed
by the Syringe (§ 115), only just as much being left as will keep
the animal alive. If the animal be very minute, it is convenient
to effect this withdrawal by placing the slide on the stage of the
Dissecting Microscope (§ 41), and by working the Syringe under
the magnifier ; and it will be found, after a little practice, that the
complete command which the operator has over the movements of
the piston, as well as over the place of the point of the syringe,
enables him to remove every drop of superfluous water without
drawing the animal into the syringe. When, on the other hand,
it is desired to isolate a particular animal from a number of others,
the syringe may be conveniently used, after the same fashion, to
draw it up and transfer it to another slide ; care being, of course,
taken that the syringe so employed has a sufficient aperture to
receive it freely. If it be wished to have recourse to compression,
for the expansion or flattening of the object, this may be made
upon the ordinary slide, by pressing down the thin-glass cover with
a pointed stick ; and this method, which allows the pressure to be
applied where it may chance to be most required, will generally be
found preferable for delicate portions of tissue which are easily
spread out, and which, in fact, require little other compression than
is afforded by the weight of the glass cover, and by the capillary
attraction which draws it into proximity with the slide beneath.
A firmer and more enduring pressure may be exerted by the
dexterous management of a well-constructed Aquatic Box ; and
this method is peculiarly valuable for confining the movements of
minute animals, so as to keep them at rest under the field of the
microscope, without killing them. It is where a firm but graduated
pressure is required, for the flattening- out of the bodies of thin
semi-transparent animals, without the necessity of removing them
from the field of the Microscope, that the Compressorium (§ 111)
is most useful.
130. In whatever way the Object is submitted to examination,
it must be first brought approximately into position, and supported
there, just as if it were in a mounted Slide. The precise mode of
effecting this will differ, according to the particular plan of the
instrument employed; thus, in some it is only the ledge itself
that slides along the stage ; in others it is a carriage of some kind,
whereon the object-slide rests ; in others, again, it is the entire
platform itself that moves upon a fixed plane beneath. Having
guided his object, as nearly as he can do by the unassisted eye,
into its proper place, the Microscopist then brings his light
(whether natural or artificial) to bear upon it, by turning the
Mirror in such a direction as to reflect upon its under surface the
184
MANAGEMENT OF THE MICROSCOPE,
rays which are received by itself from the sky or the lamp, Tlie
concave Mirror is that which should always he first employed, the
plane being reserved for special purposes ; and it should bring the
rays to convergence in or near the plane in which the object lies
(Fig. 100). The distance at which it should be ordinarily set
Fig. 100,
Arrangement of Microscope for Transparent Objects,
beneath the Stage, is that at which it brings parallel rays to a f ocns ;
bnt this distance shonld be capable of elongation, by the length-
ening of the stem to which the Mirror is attached ; since the rays
diverging from a lamp at a short distance are not so soon brought
to a focus. The correct focal adjustment of the Mirror may be
judged of by its formation of images of window-bars, chimneys,
&c, upon any semi-transparent medium placed in the plane of the
object. It is only, however, when small objects are being viewed
under high magnifying powers, that such a concentration of the
light reflected by the Mirror is either necessary or desirable ; for,
with large objects seen under low powers, the field would not in
this mode be equally illuminated. The diffusion of the light over
a larger area may be secured, either by shifting the Mirror so much
above or so much below its previous position, that the pencil will
fall upon the object whilst still converging or after it has met and
diverged ; or, on the other hand, by the interposition of a plate of
Ground-glass in the course of the converging pencil ; this last
method, which is peculiarly appropriate to lamp-light, being very
easily had recourse to, if the diaphragm-plate, as formerly recom-
mended (§ 87), have had its larger aperture filled with such a
diffusive medium. The eye being now applied to the Eye-piece,
ILLUMINATION OF TEANSPAEENT OBJECTS. 185
and the body being ' focnssed,' the object is to be brought into the
exact position required by the nse of the traversing movement, if
the stage be provided with it ; if not, by the nse of the two hands,
one moving the object-slide from side to side, the other pnshing the
ledge, fork, or holder that carries it, either forwards or backwards
as may be reqnired. It is always to be remembered, in making
snch adjustments by the direct use of the hands, that, owing to the
inverting action of the Microscope, the motion to be given to the
object, whether lateral or vertical, must be precisely opposed to
that which its image seems to require, save when Erectors (§§ 69,
70,) are employed. When the object has been thus brought fully
into view, the Mirror may require a more accurate adjustment.
What should be aimed at is the diffusion of a clear and equable
light over the entire field ; and the observer should not be satisfied
until he has attained this object. If the field should be darker on
one side than on the other, the Mirror should be slightly turned in
such a direction as to throw more light upon that side ; perhaps in
so doing, the light may be withdrawn from some part previously
illuminated ; and it may thus be found that the pencil is not large
enough to light up the entire field. This may be owing to one of
three causes : either the cone of rays may be received by the object
too near to its focal apex, the remedy for which lies in an altera-
tion in the distance of the Mirror from the stage ; or, from the
very oblique position of the Mirror, the cone is too much narrowed
across one of its diameters, and the remedy must be sought in a
change in the position either of the Microscope or of the Lamp, so
that the face of the Mirror may not be turned so much away from
the axis of vision ; or, again, from the centre of the Mirror being
out of the optical axis of the instrument, the illuminating cone is
projected obliquely, an error which can be rectified without the
least difficulty. If the cone of rays should come to a focus in the
object, the field is not unlikely to be crossed (in the day-time) by
the images of window-bars or chimneys, or (at night) the form of
the lamp-flame may be distinguished upon it ; the former must be
got rid of by a slight change in the inclination of the Mirror ; and
if the latter cannot be dissipated in the same way, the lamp should
be brought a little nearer.
131. The equable illumination of the entire field having been
thus obtained, the quantity of light to be admitted should be regu-
lated by the Diaphragm-plate (§ 87). This must depend very
much upon the nature of the object, and upon the intensity of the
light. _ Generally speaking, the more transparent the object, the
less light does it need for its most perfect display ; and its most
delicate markings are frequently only made visible, when the major
part of the cone of rays has been cut off. Thus the movement of
the cilia— those minute vibratile filaments with which almost
every Animal is provided in some part of its organism, and which
many of the humbler Plants also possess — can only be discerned
in many instances when the light is admitted through the smallest
186 MANAGEMENT OF THE MICROSCOPE.
aperture. On the other hand, the less transparent objects usually
require the stronger illumination which is afforded by a wider cone
of rays ; and there are some (such as semi-transparent sections of
Fossil Teeth) which, even when viewed with low powers, are better
seen with the intenser light afforded by the Achromatic Condenser.
— In every case in which the object presents any considerable ob-
struction to the passage of the rays through it, great care should
be taken to protect it entirely from incident light; since this
extremely weakens the effect of that which is received into the
Microscope by transmission. It is by daylight that this inter-
ference is most likely to occur ; since, if the precautions already
given (§ 120) respecting the use of lamp-light be observed, no great
amount of light can fall upon the upper surface of the object.
The observer will be warned that such an effect is being produced,
by perceiving that there is a want not only of brightness but of
clearness in the image, the field being veiled, as it were, by a kind
of thin vapour ; and he may at once satisfy himself of the cause
by interposing his hand between the stage and the source of light,
when the immediate increase of brilliancy and of distinctness will
reveal to him the occasion of the previous deficiency in both.
ISTothing more is necessary for its permanent avoidance, than the
interposition of an opaque screen (blackened on the side towards
the stage) between the window and the object ; care being of course
taken that the screen does not interfere with the passage of light
to the Mirror. Such a screen may be easily shaped and adapted
either to be carried by the stage itself, or by the stand for the
condenser ; but it is seldom employed by Micro scopists, as it inter-
feres with access to the left side of the stage ; and the interposi-
tion of the hand, so often as it may be needed, is more frequently
had recourse to in preference, as the more convenient expedient.
The young Microscopist who maybe examining transparent objects
by daylight, is recommended never to omit ascertaining whether
the view which he may obtain of them is in any degree thus marred
by incident light..
132. Although the illumination afforded by the Mirror alone is
quite adequate for a very large proportion of the purposes for which
the Microscope may be profitably employed (nothing else having
been used by many of those who have made most valuable contri-
butions to Science by means of this instrument), yet, when high
magnifying powers are employed, and sometimes even when but a
very moderate amplification is needed, great advantage is gained
from the use of a Condenser. The form which has been described
under the name of the Webster Condenser (§ 89) answers so well
for most purposes, and may in addition be so easily converted into
a Black-Ground Illuminator, that the working Microscopist will
find it convenient to keep it always in place ; substituting an
Achromatic Condenser of greater power (§ 88) only when specially
needed. Special care is needed in the use of this last, both as to
the coincidence of its optic axis with that of the Microscope
ILLUMINATION OF TRANSPARENT OBJECTS. 187
itself, and as to its focal distance from the object. The centering
may be most readily accomplished by so adjusting the distance of
the Condenser from the Stage (by the rack-and-pinion action, or the
sliding movement, with which it is always provided), that a sharp
circle of light shall be thrown on any semi-transparent medium
laid npon it ; then, on this being viewed through the Microscope
with an Objective of sufficiently low power to take in the whole of
it, if this circle be not found to be concentric with the field of
view, the axis of the Condenser must be altered by means of the
milled-head tangent- screws with which it is provided. The focal
adjustment, on the other hand, must be made under the Objective
which is to be employed in the examination of the object, by
turning the Mirror in such a manner as to throw upon the visual
image of the object (previously brought into the focus of the Micro-
scope) an image of a chimney or a window-bar, if daylight be
employed, or of the top, bottom, or edge of the lamp-flame, if
lamp -light be in use ; the focus of the condenser should then be so
adjusted as to render the view of this as distinct as possible ; and
the direction of the Mirror should then be sufficiently changed to
displace the image, and to substitute for it the clearest light that
can be obtained. It will generally be found, however, that although
such an exact focussing gives the most perfect results by Daylight,
yet that by Lamp-light the best illumination is obtained when the
Condenser is removed to a somewhat greater distance from the
object, than that at which it gives a distinct image of the lamp. In
every case, indeed, in which it is desired to ascertain the effect of
variety in the method of illumination, the effects of alterations in
the distance of the condenser from the object should be tried ; as
it will often happen that delicate markings become visible when
the condenser is a little out of focus, which cannot be distinguished
when it is precisely in focus. The regulation of the amount of
light transmitted through the object is often of the very first
importance ; and no means of accomplishing this is so convenient
as a Graduating Diaphragm (§ 87). For some objects of great
transparence, the White-Cloud illumination (§ 97) may be had
recourse to with advantage. For the most difficult class of objects,
however, when viewed by lamp-light under the highest powers, it
is better to dispense with the Mirror altogether, placing the lamp
in the axis of the Micro scope, so that its light shall fall directly on
the Condenser.
133. There are many Transparent Objects, however, whose pecu-
liar features can only be distinctly made out when they are viewed
by light transmitted through them obliquely instead of axially ; and
this is especially the case with such as have their surfaces marked
by very delicate and closely-approximated furrows, the direction of
the oblique rays being then a matter of primary importance. Thus
suppose that an object be marked by longitudinal strias too deli-
cate to be seen by ordinary direct light ; the oblique light most
fitted to bring them into view will be that proceeding in either of
138 MANAGEMENT OF THE MICEOSCOPE.
the directions c or d ; that which falls upon it in the directions A
and b tending to obscure the striae rather than to disclose them.
But, moreover, if the striae should be due to furrows or promi-
nences which have one side inclined and the other side abrupt,
they will not be brought into view indifferently
by light from c or d, but will be shown best
by that which . makes the strongest shadow :
hence if there be a projecting ridge, with an
abrupt side looking towards c, it will be best
seen by light from d ; whilst if there be a
furrow with a steep bank on the side of c, it
will be by light from that side thai it will be
best displayed. But it is not at all unfrequent for the longitudinal
striae to be crossed by others ; and these transverse striae will usu-
ally be best seen by the light that is least favourable for the
longitudinal ; so that, in order to bring them into distinct view,
either the illuminating pencil or the object must be moved a
quarter round. The simplest mode of obtaining this end is to
make the Mirror capable of being turned into such a position as
to reflect light into the object from one side and at a very oblique
angle ; and to give the Stage a rotatory movement, so that the
object may be presented to that light under every aspect. But
where sufficient obliquity cannot be given to the Mirror, nearly
the same effect may be produced by placing the Lamp in the
desired direction, and interposing an ordinary Condensing lens
between it and the object.
134. For objects of the greatest difficulty, however, it is better
to have recourse to the Accessories which are specially provided to
furnish oblique illumination in the most effectual manner. Thus
by using the Webster Condenser (§ 89) or an Achromatic Con-
denser of large angular aperture (§ 96) with a central stop, rays
of great obliquity are admitted from every azimuth at once ; and
there are some objects which are best seen in this manner. Either
of these condensers, again, may be used, like Mr. Reade's Hemi-
spherical Condenser (§ 92), with diaphragms that allow light to
pass only from some particular portion or portions of their peri-
phery ; thus illuminating the object from the exact direction or
directions best adapted to develop its markings. In the best
Achromatic Condensers there are stops with radial slots : a single
slot admitting light from one azimuth only, two slots at right
angles to each other, and two at an obtuse angle, all susceptible
of having the obliquity of their illumination varied by the dia-
meters of the apertures employed in combination with them. The
single slot stop is particularly useful in combination with a rota-
tory stage. A stop with two peripheral slots shows some lined
objects advantageously. — With fine Objectives from l-4th upwards,
using deep Eye-pieces when necessary, all but the most difficult
Diatoms and similar objects can be shown by a small pencil of
central light ; and as a general rule the chances of error will be
ILLUMINATION OF TEANSPAEENT OBJECTS.
189
Fig. 101.
diminished by employing the smallest obliquity that will answer
the purpose, and by receiving light from one or two known direc-
tions rather than from a multiplicity of azimuths. If the Stage
of the Microscope should not be capable of rotation in the optic
axis of the instrument, the required variety of direction may be
given by rotating the eccentric Diaphragm. In first-class Micro-
scopes, the sub-stage carrying the Illuminating apparatus can be
rotated by a rack-and-pinion move-
ment. Yery oblique illumination
in one direction only may also be
conveniently obtained by the use
of the Amici Prism (§ 91), which
combines the action of Mirror and
Condenser, and which may be ren-
dered still more effective by being
made achromatic ; and. where it is
desired to bring out simultaneously
two sets of lines crossing each other
transversely or obliquely, two such
prisms may be employed at once,
so fixed as to throw the light of
two separate lamps in the most
advantageous directions. A good
example of the variety of appear-
ances which the same object may ex-
hibit when illuminated in different
modes and with slight changes of
focussing, is shown in Fig. 101,
which represents portions of a valve
of Pleurosigma formosum as seen
under a power of 1300 diameters ;
the markings shown at a, b, and c
are brought out by oblique light in-
different directions, which, how-
ever, when carefully used, does not
produce these erroneous aspects ;
whilst at d is shown the effect of
central illumination with the Achro-
matic Condenser.
135. There are many kinds of
Transparent Objects — especially
such as either consist of thin plates,
disks, or spicules of Siliceous or
Calcareous matter, or contain such
bodies, — which are peculiarly well
seen under the Black-ground illu-
mination (§§ 93, 94) ; for not only does the brilliant luminosity
which they then present, in contrast with the dark ground behind
them, show their forms to extraordinary advantage ; but this
!■■■ BB»«" I L"«3
Valve of Pleurosigma formosum,
with portions A, B, c, D, showing
diverse effects of Illumination.
190 MANAGEMENT OF THE MICROSCOPE.
mode of illumination imparts to them an appearance of solidity
which they do not exhibit by ordinary transmitted light (§ 95) ;
and it also frequently brings ont surface-markings which are
not otherwise distinguishable. Hence, when any object is under
examination that can be supposed to be a good subject for this
method, the trial of it should never be omitted. For the low
powers, the use of the Spot-lens or the Webster Condenser with
the central stop, will be found sufEciently satisfactory; for the
higher, the Paraboloid should be employed. — Similar general re-
marks may be made respecting the examination of objects by
Polarized light. Some of the most striking effects of this kind of
illumination are produced upon bodies whose particles have a
crystalline aggregation ; and hence it may often be employed with
great advantage to bring such bodies into view, when they would
not otherwise be distinguished ; thus, for example, the Baphides of
Plants are much more clearly made out by its means, in the midst
of the vegetable tissues, than they can be by any other. But the
peculiar effects of Polarized light are also exerted upon a great
number of other Organized substances, both animal and vegetable ;
and it often reveals differences in the arrangement or in the relative
density of their component particles, the existence of which would
not otherwise have been suspected : hence the Microscopist will do
well to have recourse to it, whenever he may have the least
suspicion that its use can give him an additional power of
discrimination.
136. Arrangement for Opaque Ohjects. — There are many objects
of the most interesting character, the opacity of which entirely
forbids the transmission of light through them, and of which, there-
fore, the surfaces only can be viewed by means of the incident rays
which they reflect. These are, for the most part, objects of com-
paratively large dimensions, for which a low magnifying power
suffices ; and it is specially important, in the examination of such
objects, not to use a lens of shorter focus than is absolutely necessary
for discerning the details of the structure ; since, the longer the
focus of the Objective employed, the less is the indistinctness pro-
duced by inequalities of the surface, and the larger, too, may be
its aperture, so as to admit a greater quantity of light, to the great
improvement of the brightness of the image. Objectives of long
focus are especially required in Microscopes that are to be used for
Educational purposes ; since it is most important that the young
should be trained in a knowledge of the wonders and beauties of
the familiar objects around them, and of these an endless variety
may be found by srich as will take the trouble to search for them,
which can thus be viewed with great facility* The mode of bringing
Opaque objects under view will differ according to their ' mounting^'
and to the manner in which it is desired to illuminate them. If
* The makers of Educational Microscopes supply at a small cost single
(triplet) combinations of 3 inches, 2 inches, 1| inch, or 1-inch focus, which
are quite adequate for ordinary requirements.
ARRANGEMENT FOE OPAQUE OBJECTS.
191
the object be mounted in a ' slide' of glass or wood, upon a large
Opaque surface, the slide must be laid on the stage in the usual
manner, and the object brought as nearly as possible into position
by the eye alone (§ 129). If it be not so mounted, it may be simply
laid upon the glass Stage-plate, resting against its ledge ; and the
Diaphragm-plate must then be so turned as to afford it a black
background, light being thrown upon it by a Condensing Lens or
Bull's-eye placed as in Fig. 102, or (still better) by Beck's Parabolic
Fig. 102.
Arrangement of Microscope for Opaque Objects.
Speculum, which gives a far better illumination by diffused daylight
than can be obtained by any other means yet devised, and which is
equally well adapted to lamp-light, when used in combination with
the Bull's-eye (§ 100). Direct sunlight cannot be employed without
the production of an injurious glare, and the risk of burning the
object; but the sunlight reflected from a bright cloud is the best
light possible. When a Condensing Lens is used, it should always
be placed at right angles to the direction of the illuminating rays,
and at a distance from the object which will be determined by the
size of the surface to be illuminated and by the kind of light re-
quired. If the magnifying power employed be high, and the field
of view be consequently limited, it will be desirable so to adjust
the lens as to bring the cone of rays to a point upon the part of
the object under examination ; and this adjustment can only be
rightly made whilst the object is kept in view under the Microscope,
the Condenser being moved in various modes until that position has
been found for it in which it gives the best light. If, on the other
192 MANAGEMENT OF THE MICROSCOPE.
hand, the power be low, and it be desired to spread the light equably
over a large field, the Condenser shonld be placed either within or
beyond its focal distance ; and here, too, the best position will be
ascertained by trial. It will often be desirable also to vary both
the obliquity of the light and the direction in which it falls npon
the object ; the aspect of which is greatly affected by the manner
in which the shadows are projected npon its surface, and in which
the lights are reflected from the various points of it. Many objects,
indeed, which are distinguished by their striking appearance when
the light falls upon them on one side, are entirely destitute both of
brilliancy of colour and of sharpness of outline when illuminated
from the opposite side. Hence it is always desirable to try the
effect of changing the position of the object ; which, if it be
' mounted,' may be first shifted by merely reversing the place of
the two ends of the slide, and then, if this, be not satisfactory,
may be more completely as well as more gradually altered, by
making the object-platform itself to revolve. With regard to the
obliquity of the illuminating rays, it is well to remark, that if the
object be 'mounted' under a glass cover, and the incident rays fall
at too great an angle with the perpendicular, a large proportion of
them will be reflected, and the brilliancy of the object will be greatly
impaired; and hence when Opaque objects are being examined
under high powers with a very oblique illuminating pencil, they
should always be uncovered.
137. The same general arrangement must be made when Arti-
ficial light is used for the illumination of Opaque objects ; the Lamp
being placed in such a position in regard to the Stage that its rays
may fall in the direction indicated in Fig. 102, and these rays being
collected and concentrated by the Condenser, as already directed.
Since the rays proceeding from a lamp within a short distance are
already diverging, they will not be brought by the Condenser to
such speedy convergence as are the parallel rays of daylight ; and
it must, therefore, be further removed from the object to produce
the same effect. By modifying the distance of the Condenser from
the lamp and from the object respectively, the cone of rays may be
brought nearly to a focus, or it may be spread almost equably over
a large surface, as may be desired. And the same effect may be
produced by shifting the position of the Condenser, when Beck's
Parabolic Speculum is employed in combination with it. No more
effective illumination can be desired for objects viewed under the
low powers to which the Parabolic Speculum is adapted, than that
which is afforded by this combination ; the Bockett Lamp (Fig. 98)
supplying a most convenient means of using it, as the Author can
testify from a very large experience. In the illumination of Opaque
objects, Artificial light has the advantage over ordinary daylight of
beino- more easily concentrated to the precise degree, and of being-
more readily made to fall in the precise direction that may be found
most advantageous. Moreover, the contrast of light and shadow
will be more strongly marked when no light falls upon the object
ILLUMINATION OF OPAQUE OBJECTS. 193
except that proceeding from the lamp used for its illumination,
than it can be when the shadows are partially lightened by the
rays which fall upon the object from every quarter, as must be the
case if it be viewed by daylight. If a more concentrated light be
required, the small Condensing Lens may be used in combination
with the Bull's-eye, being so placed as to receive the cone pro-
jected by it, and to bring its rays to a more exact convergence. In
this manner very minute bodies may be viewed as Opaque objects
■under high magnifying powers, provided that the brasswork of the
extremities of the Objectives be so bevelled-off as to allow the
illuminating cone to have access to the object. As none but a very
oblique illumination, however, can be thus obtained, the view of
the object will be by no means complete, unless it be supplemented
by that which may be obtained by means of the Vertical Illumi-
nator (§ 103), which supplies for high powers the kind of illumi-
nation that is given by the Lieberkuhn for the lower.
138. There are many Opaque objects which it is desirable to
view from all sides, in order that their features may be completely
made out. For such as can be conveniently attached to small
disks, Beck's Disk -holder (§ 106) affords by far the most convenient
and effective mode of presenting them in every variety of aspect.
Many small objects, such as the Capsules of Mosses, may be
grasped in the Stage-Forceps ; and by a little care in manipulation
every part may be brought into view successively. In either of
these cases the Lieberkuhn can be employed with powers that are
too high for the Parabolic Speculum ; and light of considerable
obliquity may be obtained by its means, either by turning the
Mirror out of the axis, or by covering the greater part of the re-
flecting surface of the Lieberkuhn by means of a cap, or by a com-
bination of both methods. Whenever the Lieberkuhn is employed,
care must be taken that the direct light from the Mirror be entirely
stopped out by the interposition of a ' dark well ' or of a black
disk, of such a size as to fill the field given by the particular
Objective employed, but not to pass much beyond it. Opaque
objects that are permanently mounted either upon cardboard disks
or in the slides specially provided for them, may be presented to
the Microscope in a considerable variety of directions by means
of Morris's Object-holder (Fig. 84), which, however, can only be
employed with side-illumination. If it be desired to make the
most advantageous use of this instrument, objects mounted in
slides should be so placed that the parts to be brought into view
by its tilting movement may look towards the long edges of the
slide ; since it is obvious that a much greater inclination may be
given to it in either of these directions, than in the direction of
either of its extremities.
139. Errors of Interpretation. — The correctness of the con-
clusions which the Microscopist will draw regarding the nature of
any object, from the visual appearances which it presents to him
when examined in the various modes now specified, will necessarily
o
194 MANAGEMENT OF THE MICEOSCOPE.
depend in a great degree upon his previous experience in Microscopic
observation, and npon his knowledge of the class of bodies to which
the particular specimen may belong. Not only are observations of
any kind liable to certain fallacies, arising out of the previous
notions which the observer may entertain in regard to the consti-
tution of the objects or the nature of the actions to which his
attention is directed, but even the most practised observer is apt
to take no note of such phenomena as his mind is not prepared to
appreciate. Thus, for example, it cannot be doubted that many
Physiologists must have seen those appearance in thin slices of
Cartilage which are now interpreted as denoting its cellular orga-
nization, without in the least degree suspecting their real import,
which Schwann was the first to deduce from the study of the
development of that tissue. It was not known before his time
" what cells mean" in animal organization ; and the visual appear-
ances, which now suggest the idea of them to the mind of even the
tyro in the study of Histology, passed almost entirely unnoticed
by keen-sighted and intelligent Microscopists previously to 1839.
Errors and imperfections of this kind can only be corrected, it is
obvious, by general advance in scientific knowledge ; but the his-
tory of them affords a useful warning against hasty conclusions
drawn from a too cursory examination. If the history of almost
any scientific investigation were fully made known, it would gene-
rally appear that the stability and completeness of the conclusions
finally arrived-at had only been attained after many modifications,
or even entire alterations, of doctrine. And it is, therefore, of such
gieat importance to the correctness of our conclusions as to be
almost essential that they should not be finally formed and an-
nounced until they have been tested in every conceivable mode.
It is due to Science that it should be burdened with as few false
facts and false doctrines as possible. It is due to other truth-
seekers that they should not be misled, to the great waste of their
time and pains, by our errors. And it is due to ourselves that we
should not commit our reputation to the chance of impairment by
the premature formation and publication of conclusions, which
may be at once reversed by other observers better informed than
ourselves, or may be proved to be fallacious at some future time,
perhaps even by our own more extended and careful researches.
The suspension of the judgment, whenever there seems room for
doubt, is a lesson inculcated by all those Philosophers who have
gained the highest repute for practical wisdom ; and it is one
which the Microscopist cannot too soon learn, or too constantly
practise.
140. Besides these general warnings, however, certain special
cautions should be given to the young Microscopist, with regard to
errors into which he is liable to be led by the misinterpretation of
appearances peculiar to objects thus viewed, even when the very
best instruments are employed. Thus, the sharpness of the outline
of any transparent object is impaired by a change in the course of
DIFFRACTION.— DIFFRACTING SPECTRUM. 195
the rays that merely pass by its edges, which is termed Inflection or
Diffraction. If any Opaque object be held in the conrse of a cone
of rays diverging from a focus, the shadow which it will form upon
a screen held to receive it will not possess a well-defined edge, but
will have as its boundary a shaded band, gradually increasing in
brightness from the part of the screen on which the shadow is most
intense, to that on which the illumination is most complete. If the
light be homogeneous in its quality, the shaded band will possess
no colours of its own ; but if the light be decomposable, like the
ordinary solar beam, the band will exhibit prismatic fringes.* It is
obvious that such a diffraction must exist in the rays transmitted
through the substance, as well as along the edges, of transparent
objects ; and that it must interfere with the perfect distinctness,
not merely of their outlines, but of their images, the various
markings of which are shadows of portions that afford obstacles,
more or less complete, to the perfectly free transmission of the
rays. There are many objects of great delicacy, in which the
' diffraction-band ' is liable to be mistaken for the indication of an
actual substance ; on the other hand, the presence of an actual
substance of extreme transparence may sometimes be doubted or
denied, through its being erroneously attributed to the ' diffraction-
band.' No rules can be given for the avoidance of such errors,
since they can only be escaped by the discriminative power which
education and habit confer. The practised Microscopist, indeed,
almost instinctively makes the requisite allowance for diffraction ;
and seldom finds himself embarrassed by it in the interpretation of
the visual appearances which he obtains through a good instru-
ment.—Besides this unavoidable result of the inflection of the rays
of light, there is a peculiar phenomenon attendant upon oblique
illumination at certain angles in one direction, which consists in
the production of a double image, or a kind of overlying shadow,
sometimes presenting markings equally distinct with those of the
object itself. This image, which is not unlike the secondary image
formed by reflection from the outer surface of a silvered-glass
Mirror, has been called the ' diffracting spectrum ;' but its origin
does not really lie in the diffraction of the luminous rays, since on
the one hand it cannot be explained according to the laws of dif-
fraction, and on the other it may be traced to an entirely different
cause. An object thus illuminated is seen by two different sets of
rays ; those, namely, of transmitted light, which pass through it
oliquely from the source of the illumination to the opposite side
of the object-glass ; and those of the radiated light, which, being
intercepted by the object, are given off from it again in all direc-
_* This phenomenon is explained, on the Undulatory Theory of light, by the
disturbance which takes place in the onward propagation of waves, when
subsidiary centres of undulation are developed by the impact of the principal
undulations on obstacles in their course ; the Chromatic Dispersion being due
to the inequality in the lengths of the undulations proper to the severally-
coloured rays.
o2
196
MANAGEMENT OF THE MICEOSCOPE.
tions. (The latter alone are the rays whereby the images are
formed in any kind of ' Black -Ground ' illumination (§§ 93, 94).
Hence two different images will be formed when the illuminating
pencil is very oblique and the angular aperture of the object-glass
is wide, one of them by the light transmitted to one extreme of its
aperture, the other by the light radiated to its general surface ;
and one or the other of these images may be stopped-out, by cover-
ing that portion of the lens which receives, or that which does not
receive, the transmitted pencil. This ' diffracting spectrum ' may
be produced at pleasure, in an object illuminated by direct light
and seen with an Objective of large angular aperture, by holding
a needle or a horsehair before its front lens.
141. Errors of interpretation arising from the imperfection of
the Focal adjustment are not at all uncommon amongst young
Microscopists. With lenses of high power, and especially with
those of large angular aperture, it very seldom happens that all
the parts of an object, however minute and flat it may be, can be
in focus together ; and hence when the focal adjustment is exactly
made for one part, everything that is not in exact focus is not only
more or less indistinct, but is often wrongly represented. The
indistinctness of outline will sometimes present the appearance of
a pellucid border, which, like the diffraction-band, may be mistaken
for actual substance. But the most common error is that which is
produced by the reversal of the lights and shadows resulting from
the refractive powers of the object itself : thus, the bi-concavity of
the blood-disks of Human (and other Mammalian) Blood occasions
their centres to appear dark when in the focus of the Microscope,
through the dispersion of the rays which it occasions ; but when
they are brought a little within the focus by a slight approximation
of the object-glass, the centres appear brighter than the peripheral
parts of the disks. An opposite reversal presents itself in the
case of the markings of certain Diatomacece. False appearances
may be obtained by view-
B Fig. 103. ing a Diatom formed of
rows of spherical beads out
of focus, such as Pleuro-
sigma angulatum. a is the
aspect a little inside the
focus (Fig. 103, a) ; and yet,
when the surface is slightly
beyond the focus, the hex-
agonal areolas are dark, and
the intervening partitions
light (Fig. 103, b)^ The best
way of ascertaining when
hexagonal appearances of
Diatoms or analogous bodies are real, and when they are spurious, is
to view fractured specimens. The lines of fracture will pass through
the weakest parts. In P. angulatum the fractures occur between
False hexagonal areolation of Pleurosigma
angulatum, as seen in a Photograph magni-
fied to 15,000 diameters.
EEEOES OF INTERPRETATION, 197
the bead rows, and single terminal beads will be seen at the tips of
sharp angles. Goscinodiscus oculus Iridis can be shown according
to focussing and illumination, either as composed of large beads,
or as a structure with hexagonal depressions. The reality of these
depressions is evidenced by the lines of fracture passing through
them. The experienced Microscopist will find in the optical effects
produced by variations of Focal adjustment the most certain indi-
cations in regard to the nature of such inequalities of surface as
are too minute to be made apparent by the use of the Stereoscopic
Binocular. For, as Welcker has pointed out,* superficial elevations
must necessarily appear brightest when the distance between the
Objective and the Object is increased, whilst depressions must
appear brightest when that distance is diminished. — The student
should be warned against supposing that, in all cases, the most
positive and striking appearance is the truest ; for this is often
not the case. Mr. Slack's optical illusion, or silica-crack slide,
illustrates an error of this description. A drop of water holding
colloid silica in solution is allowed to evaporate on a glass slide,
and, when quite dry, covered with thin glass to keep it clean.
The silica deposited in this way is curiously cracked, and the
finest of these cracks can be made to present a very positive
and deceptive appearance of being raised bodies like glass threads.
It is also easy to obtain diffraction lines at their edges, giving an
appearance of duplicity to that which is really single. — The silica
films on these slides exhibit exquisite fragments of Newton's
rings when viewed as opaque objects with |th or -|th, and illumi-
nated on Professor Smith's plan.
142. A very important and very frequent source of error, which
sometimes operates even on experienced Microscopists, lies in the
refractive influence exerted by certain peculiarities in the internal
structure of objects upon the rays of light transmitted through
them ; this influence being of a nature to give rise to appearances
in the image, which suggest to the observer an idea of their cause
that may be altogether different from the reality. Of this fal-
lacy we have ' pregnant instance' in the misinterpretation of the
nature of the lacunae and canaliculi of Bone, which were long
supposed to be solid corpuscles with radiating filaments of peculiar
opacity, instead of being, as is now universally admitted, minute
chambers with diverging passages excavated in the solid osseous
substance. For, just as the convexity of its surfaces will cause
a transparent cylinder to show a bright axial band,f so will the
concavity of the internal surfaces of the cavities or tubes hollowed
out in the midst, of highly -refracting substances occasion a di-
vergence of the rays passing through them, and consequently
* See "Quart. Journ. of Microsc. Science," Vol. vii. (1859), p. 240, and
Vol. viii. (1860), p. 52.
| This was the appearance which gave rise to the erroneous notion that
long prevailed amongst Microscopic observers, and still lingers in the Public
mind, of the tubular structure of the Human Hair.
198 MANAGEMENT OF THE MICROSCOPE.
render tnem so dark that they are easily mistaken for opaque solids.
That snch is the case with the so-called ' bone corpuscles,' is shown
by the effects of the infiltration of Canada balsam through the
osseous substance ; for when this fills up the excavations, being
nearly of the same refractive power with the bone itself, it
obliterates them altogether. So, again, if a person who is unaccus-
tomed to the use of the Microscope should chance to have his
attention directed to a preparation mounted in liquid or in
balsam that might chance to contain Air -bubbles, he will be almost
certain to be so much more strongly impressed by the ajypear-
ances of these than by that of the object, that his first remark will
be upon the number of strange -looking black rings which he sees,
and his first inquiry will be in regard to their meaning.
143. Although no experienced Microscopist could now be led
astray by such obvious fallacies as those alluded to, it is necessary
to notice them, as warnings to those who have still to go through
the same education. The best method of learning to appreciate
the class of appearances in question, is the comparison of the
aspect of globules of Oil in water, with that of globules of "Water
in oil, or of bubbles of Air in water or Canada balsam. This com-
parison may be very readily made by shaking up some oil with
water to which a little gum has been added, so as to form an
emulsion ; or by simply placing a drop of oil of turpentine and a
drop of water together on a slip of glass, laying a thin-glass cover
upon them, and then moving the cover several times backwards
and forwards upon the slide* ISTow when such a mixture is
examined with a sufficiently high magnifying power, all the
globules present nearly the same appearance, namely, dark
margins with bright centres ; but when the test of alteration of
the focus is applied to them, the difference is at once revealed ;
for whilst the globules of Oil surrounded by water become darker
as the object-glass is depressed, and lighter as it is raised, those of
Water surrounded by oil become 'more luminous as the object-glass
is depressed, and darker as it is raised. The reason of this lies in
the fact that the high refracting power of the Oil causes each of its
globules to act like a double-convex lens of very short focus ; and
as this will bring the rays which pass through it into convergence
above the globule {i.e., between the globule and the Objective), its
brightest image is given when the object-glass is removed some-
what further from it than the exact focal distance of the object.
On the other hand, the globule of Water in oil, or the minute
bubble of air in water or balsam, acts, in virtue of its inferior re-
fractive power, like a double-concave lens ; and ■ as the rays of this
diverge from a virtual focus beloiv the globule (i.e., between the
globule and the Mirror), the spot of greatest luminosity will be
* If this latter mode be adopted, it is preferable, as suggested by the Authors
of the " Micrographic Dictionary" (Introduction, p. xxxii.), to colour the oil of
turpentine with alkanet, or some similar substance, for its more ready dis-
tinction.
MOLECULAR MOVEMENT. 199
found by causing the object-glass to approach within the proper
focns. — Now in the ' protoplasm' of the cells of the lower Plants,
and in the ' sarcode ' of the lower Animals, oil-particles and
vacuoles (or void spaces) are often interspersed ; and these at first
sight present so very striking a resemblance, that the inexperienced
observer may well be pardoned for mistaking the ' vacuoles ' for
larger globules of a material more refractive than the gelatinous
substance around them. But the difference in the effects of
alterations of focus on the two sets of appearances at once serves
to make evident the difference of their causes ; and this, moreover,
is made obvious by the effect of oblique light, which will cause
the strongest shadow to exhibit itself on opposite sides in the two
cases respectively.
144. Among the sources of fallacy by which the young Micro -
scopist is liable to be misled, one of the most curious is the
Molecular Movement which is exhibited by the particles of nearly
all bodies that are sufficiently finely divided, when suspended in
water or other fluids. This movement was first observed in the
fine granular particles which exist in great abundance in the con-
tents of the Pollen-grains of plants (sometimes termed the fovilla),
and which are set free by crushing them ; and it was imagined
that they indicated the possession of some special vital endowment
by these particles, analogous to that of the Spermatozoa of
animals. In the year 1827, however, it was announced by Dr.
Kobert Brown that numerous other substances, Organic and
Inorganic, when reduced to a state of equally minute division,
exhibit a like movement, so that it cannot be regarded as indi-
cative of any endowment peculiar to the fovilla-granules ; and sub-
sequent researches have shown that there is no known excep-
tion to the rule, that such motion takes place in the particles of all
substances, though some require to be more finely divided than
others before they will exhibit it. Nothing is better adapted to
show it than a minute portion of Gamboge, Indigo, or Carmine,
rubbed up with water ; for the particles of these substances which
are not dissolved, but only suspended, are of sufficiently large size
to be easily distinguished with a magnifying power of 250 diameters,
and are seen to be in perpetual locomotion. Their movement is
chiefly of an oscillatory kind ; but they also rotate backwards and
forwards upon their axis, and they gradually change their places
in the field of view. It may be observed that the movement of
the smallest particles is the most energetic, and that the largest
are quite motionless, whilst those of intermediate size move
with comparative inertness. The movement is not due (as some
have imagined) to evaporation of the liquid ; for it continues,
without the least abatement of energy, in a drop of aqueous fluid
that is completely surrounded by oil, and is therefore cut off from
all possibility of evaporation : and it has been known to continue
for many years in a small quantity of fluid enclosed between two
glasses in an air-tight case. It is, however, greatly accelerated, and
200 MANAGEMENT OF THE MICEOSCOPE.
rendered more energetic, by Heat ; and this seems to show that
it is due, either directly to some calorical changes continually
taking place in the fluid, or to some obscnre chemical action
between the solid particles and the fluid, which is indirectly pro-
moted by heat. It is cnrions that the closer the conformity
between the specific gravity of the solid particles and that of the
liquid, the less minute need be that reduction in their size which
is a necessary condition of their movement ; and it is from this
that the substances just named are so favourable for the exhibition
of it. On the other hand, the particles of Metals, which are from
seven to twelve times as heavy as water, require to be reduced to
a minuteness many times greater than that of the particles of
carmine or gamboge, before they become subject to this curious
action. In any case in which the motions of very minute par-
ticles, of whatever kind, are in question, it is necessary to make
allowance for this 'molecular movement ;' and the young Micro-
scopist will therefore do well to familiarize himself with its ordi-
nary characters, by the careful observation of it in such cases
as those just named, and in any others in which he may meet
with it.
145. Comparative Values of Object- Glasses ; Test-Objects. — In
estimating the comparative values of different Object-glasses,
regard must always be had to the purpose for which each is de-
signed ; since it is impossible to construct a combination which
shall be equally serviceable for every requirement. It is commonly
assumed that an Objective which will show certain Test-objects
must be very superior for everything else to a glass which will not
show these ; but this is known to every practical Microscopist to be
a great mistake, — the qualities which enable it to resolve some of the
more difficult ' tests' not being by any means identical with those
which make it most useful in all the ordinary purposes of Scientific
investigation. Four distinct attributes have to be specially con-
sidered in judging of the character of an Object-glass, viz. — (1) its
defining poiver, or power of giving a clear and distinct image of all
well-marked features of an object, especially of its boundaries ;
(2) its penetrating power, or focal depth, by which the observer is
enabled to look into the structure of objects ; (3) its resolving povoer,
by which it enables closely-approximated markings to be dis-
tinguished ; and (4) the flatness of the field which it gives.
I. The ' Defining power' of an Objective mainly depends upon
the completeness of its corrections, both for Spherical and for
Chromatic aberration (§§ 9-15) ; and it is an attribute essential to
the satisfactory performance of any Objective, whatever be ita
other qualities. Good definition may be more easily obtained with
lenses of small or moderate than with lenses of large angular
aperture ; and in the aim to extend the aperture, the perfection of
the definition is not unfrequently impaired. An experienced
Microscopist will judge of_ the defining power of a lens by the
quality of the image which it gives of almost any object with which
PENETRATING POWER OF OBJECT-GLASSES. 201
lie may be familiar ; but there are certain ' tests,' to be presently
described, which are particularly appropriate for the determination
of it. Amy imperfection in Defining power is exaggerated, as
already pointed ont (§§ 25, 124), by the nse of deep Eye-pieces ; so
that, in determining the value of an Objective, it is by no means
sufficient to estimate its performance under a low Eye-piece, — an
image which appears tolerably clear when moderately magnified,
being often found exceedingly deficient in sharpness when more
highly amplified. The use of the Draw-Tube (§ 68) affords an
additional means of testing the Defining power ; but this cannot
be fairly had recourse to, unless an alteration be made in the
adjustment for the thickness of the glass that covers the object
(§ 127), in proportion to the nearer approximation of the object to
the Objective which the lengthening of the body involves.
ii. The penetrating power or Focal Depth of an Object-glass
(good definition being of course presupposed) mainly depends upon
the degree of distinctness with which parts of the object that are
a little out of focus can be discerned ; and this will be found to vary
greatly in different Objectives, being, within certain limits, in an
inverse proportion to the extent of the Angle of Aperture* (§ 10), as
can be easily proved on Optical principles. f Hence an Objective
of comparatively limited angular aperture may enable the observer
to gain a view of the ivhole of an object, the several parts of whose
structure lie at different distances from it, sufficiently good to
afford an adequate idea of the relation of those parts to each other ;
whilst if the same object be looked at with an Objective of very
wide angle of aperture, which only enables what is precisely in
focus to be seen at all, each part can only be separately discerned,
and the mutual relations of the whole cannot be brought into
* As the young Microscopist may be perplexed by the fact that an Objective
having a large actual aperture may have but a small angular aperture, and that
the lenses of largest angular aperture may be those of the smallest actual
aperture, it may be well to recall his attention to Fig. 10 ; from which he will
see that the Angle of aperture a b c depends on the actual aperture of the
Objective, and the distance of the object (when in focus) from its front lens,
conjointly. Hence two Objectives may have the same actual aperture, and yet
one may have a much larger angular aperture than the other, because the focal
distance of the object is less. On the other hand two lenses may have the
same angular aperture, yet the actual aperture of one shall be much greater
than that of the other, the focal distance of the object being greater. And
thus, as a general rule, Objectives of low power or long focal distance have
the largest actual apertures ; whilst those of high power or short focus have
the largest angular apertures. If the focal distance be constant, the angular
aperture will increase or diminish with the actual aperture ; whilst, if the
actual aperture be constant, the angular aperture will increase with the short-
ening of the focal distance, and will decrease with its elongation.
t Thus the Portrait-lens of a Photographic Camera having a large angle of
aperture, is quite unsuitable for Landscape purposes: and the greater the
range of distances it is desired to obtain in a photographic picture (as, for
example, in taking the interior of a long Sculpture Gallery, or a Landscape
with near fore-ground and remote back-ground), the more must the aperture of
the lens be reduced by ' stops.'
202 MANAGEMENT OF THE MICROSCOPE.
view. The want of this Focal Depth is a serious drawback in the
performance of many Objectives which are distinguished by the
possession of other admirable qualities. The possession of a high
measure of it is so essential, in the Author's opinion, to the satis-
factory performance of those Objectives which are to be employed
for the general purposes of Scientific investigation, that he cannot
consider its deficiency to be compensated by the possession of any
degree of the Eesolving power, whose use is comparatively limited.
The value of Penetrating power is especially felt when the
Binocular arrangement is employed ; since the assistance which it
is able to give in the estimation of the solid forms of objects is
in great degree neutralized by the employment of Objectives of
such wide angular aperture as not to show any part of the object
distinctly save what is precisely in focus ; whilst, in addition,
those forms are untruly represented through the exaggeration of
projection occasioned by the too great dissimilarity of the pictures
received through the two halves of the Objective (§ 36). And the
Author has found that all who have made much use of this instru-
ment are now come to an agreement as to the superior value of
Objectives of a moderate, or even a comparatively small, Angle of
Aperture for ordinary working purposes ; the special utility of the
very wide apertures being limited to particular classes of objects.
in. The ' Eesolving power,' by which very minute markings —
whether lines, striae, or dots — are discerned and clearly separated
from each other, may be said to stand in close relation to the ex-
tent of its Angle of Aperture,* that is, to the obliquity of the rays
which it can receive from the several points of the surface of the
object. This is not so much the case where the markiugs depend
upon the interposition of opaque and semi-opaque particles in the
midst of a transparent substance, so that the lights and shadows
* Of the various modes which have been proposed for measuring the Angle
of Aperture of Microscopic Object-glasses, the following is one of the simplest
and most convenient: — The Microscope is to be placed perpendicularly on a
table covered with dark cloth, and is to be used after the manner of a diminish-
ing Telescope, the ordinary Eye-piece being removed, and a common pocket or
watchmaker's hand-glass of two or three inches focal length being held at such
a distance from the Objective as to give a distinct image of objects lying on the
surface of the table. A strip of white cardboard or paper is then to be laid on
either side of the centre of the field of view, and to be gradually moved out-
wards until its edge is just vanishing ; then if lines be drawn from the centre
of the front glass of the Objective to the inner edges of these strips, the angle
included between them will be that of the aperture of the Object-glass; and it
may be either measured by an ordinary graduated scale or protractor, so held
that its straight edge shall be parallel to the table, whilst the central point of
that edge shall coincide with the centre of the front lens of the Objective ; or
it may be calculated by dividing half the horizontal distance between the card-
board edges by the vertical distance of the Objective from the table, and finding
in a table of Natural Tangents the angle corresponding to the product, which
when doubled, will be the Angle of Aperture. This is the true available angle
for the formation of distinct images ; and will be found in many cases con-
siderably less than the angle of admission of diffused light.
RESOLVING POWER OF OBJECT-GLASSES. 203
of the image represent the absolute degrees of greater or less
transparence in its several parts ; as it is where, the whole sub-
stance being equally transparent, the markings are due to the
refracting influence which inequalities of the surface exert upon
the course of the rays that pass through it. It may be readily
perceived, on a little reflection, that the information given
about such inequalities by rays of light transmitted axially
through the object, must be very inferior to that which can be
gained from rays of light transmitted obliquely ; and thus it hap-
pens that, as already explained (§§ 133, 134), many such markings
are seen by Oblique illumination, which could not be seen under
the same Object-glass by light transmitted more nearly in the
axis of the Microscope. When an object, however, is seen by
transmitted light, no degree of obliquity in the illuminating rays
can be useful, which exceeds that at which the Object-glass can
receive them ; but the illumination of objects which are seen by
radiated light (§ 95) depends upon these very rays ; and thus it is
that the ' black-ground' illumination by the Paraboloid or by any
other effective contrivance (§§ 93, 94) will often bring surface-
markings into view, which cannot be seen by transmitted light. An
Object-glass of very wide aperture, however, will receive, even with
axial illumination, so many rays of great obliquity, that the same
kind of effect will be produced as by oblique illumination with an
Objective of smaller aperture ; but when oblique illumination is
used with the former, a greater resolving power is obtained than
the latter can afford. In comparing the Resolving power of dif-
ferent Object-glasses, it is obviously essential to a correct judgment
that the illumination should be the same ; for it will often happen
that an observer who knows the ' points' of his own instrument
will ' bring-out' tests which another does not resolve with Object-
glasses of much greater capability, simply for want of proper
management. Moreover, it must be borne in mind that great
Resolving power may exist, even though the definition may be far
from exact ; since the former depends more upon Angle of Aper-
ture than upon the perfection of the corrections : and yet there
cannot be the slightest question that, of two Objectives of the
same focal length, one perfectly corrected up to a moderate angle
of aperture, the other with a wider aperture but less perfectly
corrected, the former will be the one most suitable to the general
purposes of the Microscopist.
iv. The ' Flatness of the field' afforded by the Object-glass is
a condition of great importance to the advantageous use of the
Microscope, since the real extent of the field of view practically
depends upon it. Many Objectives are so constructed that, even
with a perfectly flat object, the foci of the central and of the peri-
pheral parts of the field are so different, that when the adjustment
is made for one, the other is entirely indistinct. Hence, when the
central portion is being looked at, no more information is gained
respecting the peripheral than if it had been altogether stopped
204 MANAGEMENT OF THE MICEOSCOPE.
out. With a really good Object-glass, not only should the image
be distinct even to the margin of the field, but the marginal por-
tion should be as free from Chromatic fringes as the central
portion. In many Microscopes of inferior construction, the imper-
fection of the Objectives in this respect is masked by the contraction
of the aperture of the diaphragm in the Eye-piece (§ 26), which
limits the dimensions of the field ; and the performance of one
Objective within this limit may scarcely be distinguishable from
that of another, although, if the two were compared under an Eye-
piece of larger aperture, their difference of excellence would be
at once made apparent by the perfect correctness of one to the
margin of the field, and by the entire failure of the other in every
part save its centre. In estimating the relative merits of two
lenses, therefore, as regards this condition, the comparison should
of course be made under the same Eye-piece.
v. It may be safely affirmed that the most perfect Object-glass
is that which combines all the preceding attributes in the highest
degree in which they are compatible one with another. But, as
has just been shown, two of the most important — namely, Pene-
trating power and Resolving power — stand in such opposite rela-
tions to the Angular Aperture, that the highest degree of which
each is in itself capable can only be attained by some sacrifice of
the other ; and, therefore, of two Objectives which are respectively
characterized by the predominance of these opposite qualities, one
or the other will be preferred by the Microscopist, according to the
particular class of researches which he may be carrying on ; just
as a man who is about to purchase a horse will be guided in his
choice by the kind of work for which he destines the animal.
Hence it shows, in the Author's estimation, just as limited an
appreciation of the practical applications of the instrument, to
estimate the merits of an Object-glass by its capability of showing
certain lined or dotted Tests, without any reference to its pene-
trating or defining power, as it would be to estimate the merits of
a Horse merely by the number of seconds within which he could
run a mile, or by the number of pounds he could draw ; without
any reference, in the first case, either to the weight he could carry
or the length of time during which he could maintain his speed,
and in the second case, either to the rate of his draught or his
power of continuing the exertion. The greatest capacity for speed
alone, the power of sustaining it not being required, and burthen
being reduced almost to nothing, is that which is sought in the
Racer ; the greatest power of steady draught, the rate of move-
ment being of comparatively little importance, is that which is
most valued in the Cart-horse ; but for the ordinary Carriage-horse
or Roadster, the highest merit lies in such a combination of speed
and power with endurance, as cannot co-exist with the greatest
perfection in either of the two first. — The Author feels it the more
important that he should express himself clearly and strongly on
this subject, as there is a great tendency at present both among
QUALITIES OF OBJECTIVES :— TEST-OBJECTS. 205
amateur Microscopists and among Opticians, to look at the attain-
ment of that Resolving power which is given by Angular aperture
as the one thing needful ; those other attributes which are of far
more importance in almost every kind of Scientific investigation,
being comparatively little thought of. It is neither the only nor
yet the chief work of the Microscope (as some appear to suppose)
to resolve the markings of the siliceous valves of the Diatomacece ;
in fact the interest which attaches to observations of this class
per se is of an extremely limited range. If one-tenth of the
attention which these objects have received, had been devoted to
the careful study of the Life-history of the tribe of Plants which
furnishes them, it cannot be doubted that great benefit would have
accrued to Physiological Science* And the more carefully we look
into the history of those contributions to our knowledge which
have done most to establish the value of the Microscope as an in-
strument of scientific research, the more clear does it become that
for almost every purpose except the resolution of the Diatom-
tests, Objectives of moderate Angular Aperture are to be decidedly
preferred.
146. Test-Objects. — It is usual to judge of the optical perfection
of a Microscope by its capacity for exhibiting certain objects, which
are regarded as Tests of the merits of its Object-glasses ; these tests
being of various degrees of difficulty, and that being accounted the
best instrument which shows the most difficult of such tests. Now
it must be borne in mind that only two out of the four qualities
which have been just enumerated — namely, Defining power and
Resolving power — can be estimated by any of the tests usually
relied on ; and the greater number of them, being objects whose
surface is marked by lines, strias, or dots, are tests of Resolving
power, and thus of Angular Aperture only. Hence, as already
shown, an Objective may show some very difficult test-objects, and
yet may be very unfit for ordinary use. Moreover, these Test-objects
are only suitable to Object-glasses of very short focus and high
magnifying power ; whereas the greater part of the real work of
the Microscope is done with Objectives of low and medium power ;
and the enlargement of the Angular Aperture, which enables even
these to resolve (under deep Eye-pieces) many objects which were
formerly considered adequate tests for higher powers, is for ordi-
nary purposes rather injurious than beneficial. In estimating the
value of an Object-glass, it should always be considered for what
purpose it is intended ; and its merits should be judged of accord-
ing to the degree in which it fulfils that purpose. We shall
therefore consider what are the objects proper to the several
'powers' of Object-glasses — low, medium, and high; and what are
the objects by its mode of exhibiting which it may be fairly
judged.
* The discovery of the conjugation of the Diatomacese (§ 240) by Mr.
Thwaites was made by means of an instrument certainly not superior to the
" Society of Arts Educational Microscope."
206 MANAGEMENT OF THE MICROSCOPE.
I. By Object-glasses of low power we may understand any whose
focal length is greater than half-an-inch. The 'powers' usually
made in this country are known as 3 inch,* 2 inch, 1^ inch, 1 inch,
and 2-3rds inch focus ; and they give a range of amplification of
from 13 to 60 diameters with the A eye-piece, and of from 20 to
90 diameters with the B eye-piece. These are the Objectives
most used in the examination of opaque objects, and of
transparent objects of large size and of comparatively coarse
texture ; and the qualities most desirable in them are a suffi-
ciently large Aperture to give a bright image, combined with
such accurate Definition as to give a clear image, with Focal Depth
sufficient to prevent any moderate inequalities of surface from
seriously interfering with the distinctness of the entire picture,
and with perfect flatness of the image when the object itself is flat.
For the 3 inch, 2 inch, or 1-| inch Objectives,f no ground of judg-
ment is better than the manner in which it shows such an injected
preparation as the interior of a Frog's Lung (Fig. 430) or a portion
of the villous coat of the Monkey's Intestine (Fig. 424) ; for the
aperture ought to be sufficient to give a bright image of such
objects by ordinary daylight, without the use of any illuminator;
the border of every vessel should be clearly defined, without any
thickness or blackness of edge ; every part of such an object that
comes within the field should be capable of being made out when
the focal adjustment is adapted for any other part; whilst, by
making that adjustment a medium one, the whole should be seen
without any marked indistinctness. If the aperture be too small,
the image will be dark : but if it be too large, details are brought into
view (such as the separateness of the particles of the vermilion
injection) which it is of no advantage to see ; whilst, through the
sacrifice of penetration, those parts of the object which are brought
exactly into focus being seen with over-minuteness, the remainder
are enveloped in a thick fog through which even their general con-
tour can scarcely be seen to loom : whilst if the corrections be imper-
fectly made, no line or edge will be seen with perfect sharpness.
For Defining power, the Author has found the Pollen-grains of the
Hollyhock or any other flower of the Mallow kind (Fig. 248, a)
viewed as an opaque object, a very good test ; the minute spines
with which they are beset being but dimly seen with any save a
good Object-glass of these long foci, and being really- well exhibited
only by adding such power to the Eye-piece as will exaggerate any
want of definition on the part of an inferior lens. For Flatness of
field no test is better than a section of Wood (Fig. 228), or a large
* Mr. T. Boss introduced a 4-inch, useful for large objects requiring much
penetration, such as living groups of Polyzoa, &c. ; it is now made by several
other Opticians. A 5-inch is also made for ' Tank-microscopes.'
f These are ordinarily composed of two pairs of lenses only, as the correc-
tions can be adequately made by this combination for an Angular aperture of
20°, which is the largest that is found practically useful for the 1^-inch. (See
p. 190, note.)
QUALITIES OF OBJECTIVES :— TEST OBJECTS. 207
Ech\inus-spine (Fig. 315), Tinder an Eye-piece that will give a field
of the diameter of from 9 to 12 inches. The general performance
of Object-glasses of 1-inch and 2-3rds inch focns may be partly
judged-of by the manner in which they show such injections as
those of the Gill of the Eel (Fig. 429), or of the Bird's Lung
(Fig. 431), which require a higher magnifying power for their reso-
lution than those previously named ; still better, perhaps, by the
mode in which they exhibit a portion of the wing of some Lepi-
dopterous Insect having well-marked scales. The same qualities
should here be looked-for, as in the case of the lowest powers ; and
a want of either of them is to be distinguished in a similar manner.
The increase of Angular Aperture which these Objectives may
advantageously receive up to 30°, should render them capable of
resolving all the easier ' test' scales of Lepidoptera, such as those
of the Morpho menelaus (Fig. 360), in which, with the B eye-
piece, they should show the transverse as well as the longitudinal
markings. The Proboscis of the common Fly (Fig. 373)* is one of
the best transparent objects for enabling a practised eye to estimate
the general performance of Object-glasses of these powers ; since it
is only under a really good lens that all the details of its structure
can be well shown ; so that an Objective which shows this well may
be trusted to for any other object of its kind. For Flatness of field
sections of small Echinus-spines (Plate II., fig. 1) are very good
tests. The exactness of the corrections in lenses of these foci may
be judged of by the examination of objects which are almost sure
to exhibit Colour if the correction be otherwise than perfect. This
is the case, for example, with the glcmdulcB of Coniferous wood
(Fig. 223), the centres of which ought to be clearly defined under
such objectives, and ought to be quite free from colour ; and also
with the tracliece of Insects (Fig. 377), the spires of which ought
to be distinctly separated from each other without any appearance
of intervening chromatic fringes.
n. We may consider as Object-glasses of medium, power the
Half -inch, 4-10ths inch, l-4th inch, and l-5th inch ; the magnifying
power of which ranges from about 90 to 250 diameters under the A
eye-piece, and from about 150 to 400 diameters with the B eye-piece.
The first three can only be advantageously employed in the examina-
tion of such small opaque objects as Diatoms, Polycystina, portions
of small feathers, capsules of the lesser Mosses, Hairs, &c. The l-4th
for these purposes should not exceed 80° Aperture. Larger-angled
l-4ths and l-5ths are only fit for opaque objects of unusual minute-
ness, shown by Professor Smith's or some analogous iEumination
(§ 103). The great value of these powers lies in the information they
enable us to obtain regarding the details of organized structures and
of living actions, by the examination of properly-prepared trans-
parent objects by transmitted light; and it is to them that the
* This object should be mounted in Glycerine-jelly ; for when mounted in
Balsam, the parts are usually flattened out and squeezed together, bo that their
real forms and relative positions cannot be seen.
208 MANAGEMENT OF THE MICROSCOPE.
remarks already made respecting Angular Aperture (§ 145, v.) espe-
cially apply ; since it is here that the greatest difference exists between
the ordinary requirements of the Scientific investigator, and the
special needs of those who devote themselves to the particular classes
of objects for which the greatest Besolving power is required. A
moderate amount of such power is essential to the value of every Ob-
jective within the above-named range of foci : thus, even a good Half-
inch should enable the markings of the larger scales of the Polyom-
matus argus (' azure-blue ' Butterfly) to be well distinguished — these
being of the same kind with those of the Menelaus, but more delicate
— and should clearly separate the dots of the small or ' battledoor'
scales (Fig. 362) of the same insect, which, if unresolved, are seen
as coarse longitudinal lines ; a good 4-10ths inch should resolve the
larger scales of the Podura (Plate II., fig. 2) without difficulty ; and
a good l-4th or l-5th-inch should bring out the markings on the
smaller scales of the Podura, and should resolve the markings on
the Pleurosigma angulatum into longitudinal and oblique lines.*
Even the Half -inch or the 4-10ths inch may be made with angles
of aperture sufficiently wide to resolve the objects named as fair
tests for the powers above them ; but for the reasons already stated,
the Author thinks it most undesirable that they should be thus
forced up to the work altogether unsuited to their powers, by a
sacrifice of those very qualities which constitute their special value
in the study of the objects whereon they can be most appropriately
and effectively employed. And he is decidedly of opinion that an
angular aperture of 50° is as great as should be given to a Half-
inch, 60° to a 4-10ths inch, and 90° to a l-4th inch, that are des-
tined for the ordinary purposes of scientific investigation ; whilst
his own experience would lead him to prefer an angle of 40° for
the Half- inch (§ 36), and of 75° for the l-4th inch, provided the
corrections are perfect.f Objectives of these apertures should
show the easier tests first enumerated with perfect Definition, a
fair amount of Penetrating power, and complete Flatness of field.
ISTo single object is so useful as the Podura-scale for the purpose of
testing these qualities in a l-4th inch or l-5th inch Objective ; and
it may be safely said that a lens which brings out its markings
satisfactorily will suit the requirements of the ordinary working
Microscopist, although it may not resolve difficult Diatoms. In
every case the Objective should be tried with the B and 0 as well
as with the A eye-piece ; and the effect of this substitution will be
a fair test of its merits. Where markings are undistinguishable
under a certain Objective, merely because of their minuteness or
their too close approximation, they may be enlarged or separated
by a deeper Eye-piece, provided that the Objective be well cor-
rected. But if, in such a case, the image be darkened or blurred,
* When the valves are small, or the markings delicate, the B or 0 eye-pieces
must be used.
f Several Opticians now make Objectives of these limited apertures, of
excellent quality, and very moderate price.
QUALITIES OF OB JECTIVES. — TEST OBJECTS. 209
so as to be rather deteriorated tlian improved, it may be concluded
that the Objective is of inferior quality, having either an insuffi-
cient Angular Aperture, or being imperfectly corrected, or both.
in. All Object-glasses of less than l-5th inch focus may be classed
as high powers ; the focal lengths to which they are ordinarily con-
structed are l-6th, l-8th,_ l-10th, l-12th, l-16th, l-20th, l-25th, and
l-50th of an inch respectively ; the l-16th, l-25th, and l-50th being
made by Messrs. Powell and Lealand, and the l-10th and l-20th
by Messrs. Beck. The magnifying powers which Objectives from
l-6thto l-25th inch focus are fitted to afford, range from about 320
to 1200 diameters with the shallower Eye-piece, and from 480 to
1800 diameters with the deeper ; but by the use of still deeper Eye-
pieces, or by the Objective of l-50th inch, or the l-80th recently
constructed by Messrs. Powell and Lealand, a power of 3500 or more
may be obtained. It is questionable, however, whether anything
is really gained thereby. — The introduction of immersion-lenses
(§ 19) has considerably increased the utility of what may be
called moderately high powers, such as l-8th, l-10th, and 1-1 2th.
These, if really good, can be used when necessary with deep Eye-
pieces ; and very little of importance that is beyond their reach
has yet been seen by higher Objectives, though the latter have,
no doubt, special value in certain circumstances when skilfully em-
ployed. With these and higher powers not intended for exclusive
use upon vexatious Diatoms, the angle of aperture should be so pro-
portioned to focal length, as not to sacrifice the penetration required
to show the internal organs of small Rotifera, large Infusoria, mi-
nute Worms, &c. An Objective that will only show surfaces may
be broadly stated to be of little use for Physiological investigation.
Dry-front l-8ths or l-12ths with an aperture closely approaching
170°, are of very limited utility, from want of penetration, and from
focussing extremely close to their objects ; while with 20° or 30°
less aperture and good corrections, they are much more serviceable.
Of Angular Aperture and Definition, very good tests are afforded
by the lines artificially ruled by M. Robert, and by the more
' difficult' species of Diatoniacese. What is known as NobeH's Test
is a plate of glass, on a small space of which, not exceeding one-
fiftieth of an inch in breadth, are ruled from ten to nineteen series
of lines, forming as many separate bands of equal breadth. In
each of these bands the lines are ruled at a certain known distance ;
and the distances are so adjusted in the successive bands, as to
form a regularly diminishing series, and thus to present a suc-
cession of tests of progressively increasing difiiculty. The distances
of the lines differ on different plates ; all the bands in some series
being resolvable under a good Objective of l-4th inch focus, whilst
the closest bands in others defy the resolving power of a l-12th
inch Objective of large aperture. On the nineteen-band Test-
plate the lines are ruled at the following distances, expressed in
parts of a Paris Line, which, to an English Inch, is usually reck-
oned as -088 to 1-000, or as 11 to 125 :—
p
210
MANAGEMENT OF THE MICEOSCOPE.
and 1.
l-1000th.
Band 8.
l-4500th.
Band 14.
l-7500th.
» 2.
l-1500th.
„ 9.
l-5000th.
„ 15.
l-8000tL.
„ 3.
l-2000th.
„ 10.
l-5500th.
„ 16.
l-8500th.
>i 4-
l-2500th.
," 11.
l-6000th.
„ 17.
l-9000th.
n 5-
l-3000th.
„ 12.
l-6500th.
„ 18.
l-9500th.
„ 6.
l-3500th.
„ 13.
l-7000th.
„ 19.
l-10000th.
* 7.
l-4000th.
In the " Monthly Microscopical Journal" for Feb. 1873, Dr. Pigott
gives some careful estimates of these bands in the following words :
— Robert's New Bands are indicated to be from l-1000th to
1 -10,000th of a Paris line. Now, according to Babbage, the French
foot is equal to 1*0657654 English foot, and the line is the l-12th
of a pouce, which is the l-12th of a French foot. By these data I
find the French line is 0;088813783 English inch, and not 0'088815,
as generally given. This makes some difference in the assigned
English divisions per inch ; and for those who may feel interested in
comparing the visibility of Robert's Bands with rows of spheroids in
contact, of the same category— viz., so many to the inch, I now add
the result of some calculations accurately verified (the decimals
are given merely to show the care taken) : —
Band.
JSTo. of spaces
per inch.
I. 11,259-51358.
III. 22,519-02716.
IV. 33,778-54074.
VII. 45,038-05432.
Band. No' °f.sP«ces
per inch.
IX. 56,297-56790.
XL 67,557-08148.
XIII. 78,816-59506.
Band. ^o. of spaces
per inch.
XV. 90,076-10864.
XVII. 101,335-62222.
XIX. 112,595-13580.
147. In objects like Robert's Test-plate, spurious diffraction
lines are easily mistaken for genuine resolution ; and the difficulty of
resolving the higher bands of his series was supposed to be a
physical impossibility, from the adoption of a certain formula of
Fraunhofer, with regard to the spectra produced when light is
permitted to fall upon closely -ruled parallel lines. This subject
is discussed in a paper by Dr. Woodward, read before the Royal
Microscopical Society (see " Monthly Microscopical Journal," Dec.
1869), in which the optical part of the question is cleared up by
Professor Barnard, while Dr. Woodward gives an account of his
success in photographing up to the 19th band, with a new immersion
l-16th inch of Messrs. Powell and Lealand. He says: "I illu-
minated the Microscope as in my former work on Robert's Plate,
with a pencil of mono-chromatic light obtained by reflecting the
direct rays of the sun from a heliostat upon a mirror, by which
they were thrown through a cell filled with a solution of the
ammonio- sulphate of copj>er upon the achromatic condenser. As
an achromatic condenser I substituted for that belonging to the
large Powell and Lealand stand of the Museum, a l-5th inch
Objective of 148° angle of aperture, and used it without a dia-
phragm. Obliquity of light was obtained by moving the centering
screws of the secondary stage. I also obtained satisfactory re-
solution of the 19th band with the same lens, by using for the
RESOLUTION OF NOBERT'S LINES. 211
illumination violet light obtained by throwing the violet end of the
solar spectrum produced by a large prism upon the achromatic
condenser used as above ; and by subsequently shifting the prism, got
successful resolution of the 19th band, with blue, green, yellow, orange
and red light." In a subsequent paper* Dr. Woodward describes
similar success with a 1-1 8th inch immersion Objective by Tolles ;
and he remarks that " those glasses which were quite under-
corrected as to colour, not merely gave the best photographs but did
the best work by lamplight." This result corresponds with what
he observed with Objectives of Powell and Lealand, Hartnack, and
Gundlach ; and although he claims no novelty for the observation,
he advises purchasers not to require so close an approximation to
perfect Achromatism, as is inconsistent, from the irrationality of
the spectrum, with the best spherical correction. Mr. Wenham,
Dr. Pigott, and others hold the same opinion. The best glass is
that which is one as near Achromatism as is possible without
injuring definition; and it may be remarked that Messrs. Powell
and Lealand have succeeded in improving upon the fine definition
of their older glasses in their new series, and at the same time
lessening, perhaps as far as is prudent, the ordinary chromatic-
error. The best glasses at present made show extremely small beads
as a brilliant reel, upon a blue or greenish ground. Dr. Woodward
resolved the 19th band with No. 8 Gundlach and ]STo. 10 Hart-
nack. Dr. Pigott remarks with respect to " artificial lines on glass,
or Robert's, that being grooves cut or ploughed into glass by a fine
pointed diamond, they cannot offer the same characteristics for
definition, as objects whose lines are caused by small spherical bodies
raised in relief, the complete resolution of which requires, besides
definition, penetration, or less angular aperture than is necessary
to catch the shadows arranged lineally upon glass. "f In the same
paper Dr. Pigott remarks that the residuary error of the best glasses
obscures the definition with a magnification of 1000 linear, of a
string of beads less than 80,000 to the inch. The deviation of a
good l-8th he estimates as not exceeding the 50,000th of an inch. It
is obvious that if cut lines on glass are seen truly, they will present
the appearance of grooved depressions with sharp edges, if the cuts
are sufficiently clean.
148. The value of the minuter Diatomacece, as furnishing in
their surface-markings admirable Test-objects for the highest
powers of the Microscope, was first made known by Messrs.
Harrison and Sollitt, of Hull, in 1841 ; and it cannot be questioned
that this discovery has largely contributed to the success of the
endeavours which have since been so effectually made, to perfect
this class of Objectives, and to find out new methods of using
them to the best advantage. The nature of these markings will
be discussed hereafter (§ 236) ; and it will be sufficient in this place
to give a table of the average distances of the transverse or dia-
* "Monthly Microsc. Journ.," Nov., 1872. t Ibid* Dec, 1869.
P2
34 ..
... 32 — 20
36 ..
... 30
38 ..
... 40 — 20
40 .
... 46 — 35
40 ..
... 45 — 40
44 ..
... 80 — 40
45 ..
... 60 — 35
48
48
52 ..
... 51 — 46
54
64 .
. 90—50
...... 85 ..
... Ill — 60
85
... 130 —120
212 MANAGEMENT OF THE MICROSCOPE.
gonal lineation of different species, which will serve to indicate
their respective degrees of difficulty as 'tests.' The greater part of
those which are now in use for this purpose, are comprehended in
the genus Pleurosigma of Prof. W. Smith ; which includes those
Navicular whose 'frustules' are distinguished by their sigmoid
(S-like) curvature (Fig. 133).
Direction Striae in l-1000tf/t of an inch.
of Stria. Smith. Sollitt.
1. Pleurosigma formosura ... diagonal
2. ■ ■ strigile ... transverse
3. - Balticum ... trans verse
4. attenuatum ... transverse
5. ■■ hippocampus ... transverse
6". strigosurn ... diagonal
7. quadratum ... diagonal
8. — — eiongatum ... diagonal
9. lacustre ... transverse
10. — angulatum ... diagonal
11. . aestuarii ... diagonal
12. ■ fasciola ... transverse
13. Navicula rhomboides ... transverse
14. Mtzschia sigmoidea ... transverse
15. Arnphipleura pellucida ... transverse
{Navicula acus)
Good specimens of the first ten of the foregoing list may be resolved,
with judicious management, by good small-angled l-4th or l-5th
inch Objectives, and even, with very Oblique illumination, by
Objectives of half and 4-10ths inch, having an angular aperture of
90° ; the remainder require a l-8th inch or higher power, of moderate
aperture, for the satisfactory exhibition of their markings.
The first column of measurements in the above table gives the
number stated by Prof. W. Smith as averages ; the second column
gives the numbers more recently assigned as the extremes by Mr.
Sollitt,* who pointed out that great differences exist in the fine-
ness of the markings of specimens of the same species obtained
from different localities — a statement now so abundantly con-
firmed, as to be entitled to rank as an established fact. Mr.
Sollitt remarked of P. fasciola, P. strigosurn, Nitzschia sig-
moidea, and Navicula rhomboides, that individual specimens often
have the strise so fine as to defy all means of resolving them. On
the other hand, it was asserted by Mr. Hendry (" Quart. Journ.
of Microsc. Science," Yol. i. N.S. (1861), p. 231), that the stria? of N.
rhomboides range between 30 and 50 in l-1000th of an inch. — It is
in regard to Arnphipleura pellucida, however, that the greatest
difference of opinioii has existed. By Mr. Hendry it was affirmed
(" Quart. Journ. of Microsc. Science," Yol. viii. 1860, p. 208 ; and
Yol. i. ISLS. 1861, p. 87), that the number of its stria? ranges as
low as 34, and that many specimens present 60, 70, and 80 in
* ' On the Measurement of the Stride of Diatoms,' in " Quart. Journ. of
Microsc. Science," Vol. viii. (I860), p. 48.
DIATOM-TESTS FOE HIGH POWERS. 213
l-1000tli of an inch ; so that in some individuals the striation may
be resolved with a l-5th, a l-4th, a 4-10ths, or even a half -inch
Objective, whilst in others it requires the l-8th, or even higher
powers. On the other hand, Messrs. Snllivant and Wormley
(" Silliman's American Journal," Jan. 1861, and " Qnart. Journ.
of Microsc. Science," Yol. i. IST.S. 1861, p. 112), questioned the
reality of any actual striation in this species, and altogether disputed
the possibility of discerning stria? whose distance is no more than
1-1 30,000th of an inch; pointing ont with reference both to the
Diatom-tests and ISTobert's Test-plate, that when the resolving
power of an Objective is near its limit, ' spectral ' or ' spurious '
lines are to be seen, only to be distinguished from the true by a
practised eye. The question may now be considered, however, as
settled by the skill of Dr. Woodward (U.S.), who has succeeded
not only in resolving the markings with great certainty, but also
in obtaining excellent photographic pictures of them, which enable
the striae to be counted with great accuracy. These confirm the
opinion expressed in former editions of this Manual, that Mr.
Sollitt's estimate was too high. Some specimens of Amplii pleura
pelhicida, resolved with a large-angled l-5th of Tolles, and photo-
graphed by Dr. "Woodward, were found by him to have 96 strias to
the l-1000th of an inch. The same Objective would not resolve be-
yond the loth band of Robert's Plate. Dr.Woodward made another
photograph of this Diatom with Beck's immersion l-10th, which
resolved ISTobert's 16th band. Another photograph sent to the
Eoyal Microscopical Society was made with a 1-1 8th (called l-30th)
of Tolles ; and this, Dr. Woodward says, " exceeds all I have been
able to do in this direction with any Objective, except the im-
mersion 1-1 6th (so called) of Messrs. Powell and Lealand." The
prints show a handsome resolution of the frustules from end to
end, with powers of 1500 and 1650 diameters : one of them,
1 -200th of an inch long, contains 91 striae in the 1 -1000th of
an inch ; while on a smaller frustule Dr. Woodward found the
striae to exceed 100 in the 1 -1000th of an inch * — Dr. Woodward
calls this Diatom " a useful and valuable test for immersion
Objectives of l-8th focal length or less. Lower powers can only
hope to resolve it, if possessed of excessive angular aperture." —
Several very difficult tests of this description have been furnished
by the late Prof. Baileyf of West Point (U.S.), among them the
very beautiful Grammatopliora siibtilissima and the Hyalodiscus
suhtilis ; the latter being of discoid form, and having markings
which radiate in all directions, very much like those of an engine -
turned watch. — To these may be added the Surirella gemma,
which presents appearances of a very deceptive character. These
appearances, as represented by M. Hartnack, are shown in
* " Monthly Microsc. Journ.," April, 1871.
t See his interesting Memoirs in Vols. ii. and vii. of the " Smithsonian Con-
tributions to Knowledge." On Hyalodiscus suhtilis, see Hendry, in " Quart.
Journ. of Microsc. Science," Vol. i. N.S. (1861), p. 179.
214
MANAGEMENT OF THE MICROSCOPE.
Fig. 104, A, b ; the upper part of the valve a being illuminated by-
oblique light in the direction of its axis, and the lower part by
oblique light in a direction transverse to its axis ; while b shows a
portion more highly magnified under the last illumination. This
Fig. 104.
A >
A
Valve of Surirella gemma, with portion (b) more highly magnified,
showing two systems of markings a and &, as represented by Hart-
nack ; while C is copied from a photograph taken by Dr. Woodward.
Diatom, however, has been successfully photographed by Dr.
Woodward (Fig. 104, c), who says of it : — " A careful examina-
tion of specimens mounted dry, has satisfied me that Hartnack's
interpretation is erroneous. The fine striae are, I think, rows of
minute hemispherical beads ; the appearance of hexagons is the
optical result of imperfect definition or of unsuitable illumination.
For photographing this object, I have selected a frustule of some-
what less than the medium size. It measures l-290th of an inch
in length. Longitudinally the fine striae count at the rate of
72^000 to the inch. These striae are resolved into beaded appear-
ances, which count laterally 84,000 to the inch."
149. Determination of Magnifying Poiver. — The last subject to
be here adverted to, is the mode of estimating the magnifying power
of Microscopes, or, in other words, the number of times that any
object is magnified. This will of course depend upon a comparison
DETERMINATION OF MAGNIFYING POWER. 215
of the real size of the Object with the apparent size of the Image ;
but our estimate of the latter will depend upon the distance at
which we assume it to be seen ; since, if it be projected at different
distances from the Eye, it will present very different dimensions.
Opticians generally, however, have agreed to consider ten inches as
the standard of comparison ; and when, therefore, an object is said
to be magnified 100 diameters, it is meant that its visual image
projected at ten inches from the Eye (as when thrown down by the
Camera Lucida, § 81, upon a surface at that distance beneath), has
100 times the actual dimensions of the object. The measurement
of the magnifying power of Simple or Compound Microscopes by
this standard is attended with no difficulty. All that is required
is a Stage-Micrometer accurately divided to a small fraction of an
inch (the l-100th will answer very well for low powers, the
1 -1000th for high), and a common foot-rule divided to tenths of
an inch. The Micrometer being adjusted to the focus of the
Objective, the rule is held parallel with it at the distance of ten
inches from the eye. If the second eye be then opened whilst the
other is looking through the Microscope, the circle of light included
within the field of view crossed by the lines of the Micrometer will
be seen faintly projected upon the rule ; and it will be very easy to
mark upon the latter the apparent distances of the divisions on the
Micrometer, and thence to ascertain the magnifying power. Thus,
supposing each of the divisions of 1 -100th of an inch to correspond
with 1^ inch upon the rule, the linear magnifying power is 150
diameters : if it correspond with half an inch, the magnifying
power is 50 diameters. If, again, each of the divisions of the
l-1000th inch Micrometer correspond to 6-10ths of an inch upon
the rule, the magnifying power is 600 diameters ; and if it corre-
spond to 1*2 inches, the magnifying power is 1200 diameters. In
this mode of measurement the estimate of parts of tenths on the rule
can only be made by guess ; but greater accuracy may be obtained
by the use of the Diagonal scale, or still better, by projecting
the Micrometer-scale with the Camera Lucida at the distance
of ten inches from the eye, marking the intervals on paper,
taking an average of these, and repeating this with the compasses
ten times along the inch-scale. Thus, if the space given by one
of the divisions of the l-1000th-inch Micrometer, repeated ten
times along the rule, amounts to 6 inches and 2\ tenths, the value
of each division will be '625 of an inch, and the magnifying power
625. — It is very important, whenever a high degree of accuracy is
aimed at in Micrometry, to bear in mind the caution already given
(§ 77) in regard to the difference in magnifying power produced in
the adjustment of the Objective to the thickness of the glass that
covers the object.* — -The superficial Magnifying power is of course
estimated by squaring the linear ; but this is a mode of statement
* See Hendry 'On Amphipleura pellucida,' in "Quart. Journ. of Microsc,
Science," Vol. i. N.S. (1861), p. 87.
216 MANAGEMENT OF THE MICROSCOPE.
never adopted by Scientific observers, although often employed to
excite, popular admiration, or to attract customers, by those whose
interest is concerned in doing so *
* It may be well here to remark, that the designations given by Opticians
to their Objectives are often far from representing their real focal length, as
estimated by that of Single Lenses of equivalent magnifying power; a
temptation to wwtferrate them being afforded by the consideration that if an Ob-
jective of a certain focus will show a Test-object as well as another of higher
focus, the former is to be preferred. Thus it happens that what are sold as
Half-inch Objectives are often more nearly 4-10ths ; and that what are sold as
l-4ths are not unfrequently more really l-5ths.
CHAPTEB Y.
PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS.
Under this head it is intended to give such general directions
respecting the preparation, mounting, and collection of Objects, as
will supersede the necessity of frequent repetition when each par-
ticular class is described ; and also to enumerate the materials and
appliances which will be required or found advantageous.
Section 1. Preparation of Objects.
150. Microscopic Dissection. — The separation of the different
parts of an Animal or Vegetable structure by dissection, so as to
prepare any portion for being minutely examined under the
Microscope, should be accomplished, so far as may be found prac-
ticable, with the naked eye ; but the best mode of doing this will
depend in great degree upon the size and character of the object.
Generally speaking, it will be found advantageous to carry on the
dissection under Water, with which Alcohol should be mingled
where the substance has been long immersed in spirit. The size
and depth of the vessel should be proportioned to the dimensions
of the object to be dissected ; since, for the ready access of the
hands and dissecting -instruments, it is convenient that the object
should neither be far from its walls, nor lie under any great depth
of water. Where there is no occasion that the bottom of the
vessel should be transparent, no kind of Dissecting Trough is more
convenient than that which every one may readily make for
himself, of any dimensions he may desire, by taking a piece of sheet
Gutta-Percha of adequate size and stoutness, warming it sufficiently
to render it flexible, and then turning-up its four sides, drawing
out each corner into a sort of spout, which serves to pour away its
contents when it needs emptying. The dark colour of this sub-
stance enables it to furnish a back-ground, which assists the
observer in distinguishing delicate membranes, fibres, &e.s espe-
cially when magnifying lenses are employed ; and it is hard
enough, without being too hard, to allow of pins being fixed into
it, both for securing the object and for keeping apart such por-
tions as it is useful to put on the stretch. When glass or earthen-
ware troughs are employed, a piece of sheet-cork loaded with lead
must be provided, to answer the same purposes. In carrying on
dissections in such a trough, it is frequently desirable to concen-
trate additional light upon the part which is being operated on, by
218 PREPARATION OF OBJECTS.
means of the smaller Condensing Lens (Fig. 75) ; and when a low
magnifying power is wanted, it may be supplied either by a single
lens monnted after the manner of Boss's Simple Microscope
(Fig. 31, b), or by a pair of Sj^ectacles mounted with the Semi-
lenses ordinarily used for Stereoscopes.* Portions of the body
under dissection, being floated off when detached, may be conve-
niently taken up from the trough by placing a slip of glass beneath
them (which is often the only mode in which delicate membranes
can be satisfactorily spread out) ; and may be then placed under
the Microscope for minute examination, being first covered with
thin glass, beneath the edges of which is to be introduced a
little of the liquid wherein the dissection is being carried on.
"Where the body under dissection is so transparent, that more
advantage is gained by transmitting light through it than by
looking at it as an opaque object, the trough should have a glass
bottom ; and for this purpose, unless the body be of unusual size,
some of the Glass Cells to be hereafter described (Figs. 11 7-120)
will usually answer very well. The finest dissections may often be
best made upon ordinary slips of glass ; care being taken to keep
the object sufficiently surrounded by fluid. For work of this kind
no simple instrument is more generally serviceable than Quekett's
Dissecting Microscope (Fig. 32) ; but if higher magnifying powers
be needed than this will conveniently afford, recourse may be had
to JSTachet's Binocular Magnifier (Fig. 34), or to an Erector (§§ 69,
70) fitted to a Compound Microscope. In this case, support may be
provided for the hands on either side, by books or blocks of wood
piled up to the requisite height; but in place of flat 'rests' it is
much more convenient to provide a pair of inclined 'planes
sloping away from the stage at an angle of about 30° below the
horizon, which may be either solid blocks of wood, or made of two
boards hinged together.
151. The instruments used in Microscopic Dissection are for the
most part of the same kind as those which are needed in ordinary
minute Anatomical research, such as scalpels, scissors, forceps,
&c. ; the fine instruments used in Operations upon the Eye, how-
ever, will commonly be found most suitable. A pair of delicate
Scissors, curved to one side, is extremely convenient for cutting open
tubular parts ; these should have their points blunted ; but other
scissors should have fine points. A pair of very fine-pointed
Scissors (Fig. 105), one leg of which is fixed in a light handle, and
the other kept apart from it by a spring, so as to close by the pres-
* The Author can strongly recommend these Spectacles as useful in a great
variety of manipulations which are best performed under a low magnifying
power, with the conjoint use of both Eyes. — To those whose researches would
be specially aided by the conjoint use of both eyes, armed with a somewhat
higher power, he would strongly recommend Smith and Beck's 3-inch Achro-
matic Binocular Magnifier, which is constructed on the same principle, allowing
the object to be brought very near the eyes, without requiring any uncom-
fortable convergence of their axes.
DISSECTING INSTRUMENTS. 219
sure of the finger and to open of itself, will be fonnd (if the blades
be well sharpened on a hone) mnch superior to any kind of knives,
for cutting: through delicate tissues with as little disturbance of
Spring-Scissors.
thern as possible ; Swammerdarn is said to have made great use of
this instrument in his elaborate Insect-dissections. Another cut-
ting instrument much used by some dissectors may be designated
as a miniature of the shears used in shearing sheep, or as a cutting-
forceps ; the blades of such an instrument may be prevented from
springing too far asunder by means of a regulating screw (as in the
Microtome of M. Strauss-Durckheim), or by some other kind of
check ; and the cutting action, being executed by the opposed
pressure of the finger and thumb, may be performed with great
precision. A pair of small straight forceps with fine points, and
another pair of curved forceps, will be found useful in addition to
the ordinary dissecting forceps. — Of all the instruments contrived
for delicate dissections,. however, none are more serviceable than
those which the Microscopist may make for himself out of ordinary
Needles. These should be fixed in light wooden handles* (the cedar
sticks used for camel-hair pencils, or the handles of steel-penholders,
or small Porcupine-quills, will answer extremely well), in such a
manner that their points should not project far,f since they will
otherwise have too much ' spring ; ' much may be done by their
mere tearing action ; but if it be desired to use them as cutting
instruments, all that is necessary is to give them an edge upon a
hone. It will sometimes be desirable to give a finer point to such
needles than they originally possess ; this also may be done upon a
* Special Needle-Holders (like miniature port-crayons) have been made
for this purpose ; and although they afford the facility of lengthening or
shortening the acting point of the needle at will, and also of carrying a reserve
store of needles at the other end, yet the Author would decidedly recommend
the use of the wooden handles, of which a large stock may be obtained for the
cost of a single pah of special Holders.
t The following is the mode in which the Author has found it convenient to
mount his Needles for this and other purposes : the needle being held firmly in
a pair of pliers grasped by the right hand, its point may be forced into the end
of a cedar or other stick held in the left, until it has entered to the depth of
half an inch or more ; the needle is then cut off to the desired length (the eye-
end being thus got rid of) ; and being then drawn out, the truncated end is
forced into the hole previously made by the point, until it cannot be made to
penetrate farther, when it will be found to be very securely fixed. The end of
the handle which embraces it may then be bevelled-away round its point of
insertion.
220
PEEPAEATION OF OBJECTS.
Fig. 106.
hone. A needle with its point bent to a right angle, or nearly so,
is often nsefnl ; and this may be shaped by simply heating the
point in a lamp or candle, giving to it the required turn with a
pair of pliers, and then hardening the point again by re-heating it
and plunging it into cold water or tallow.
152. Cutting Sections of Soft Substances. — Most important
information repecting the structure of many substances, both
Animal and Yegetable, may be obtained by cutting sections of
them, thin enough to be viewed as transparent objects. Where the
substances are soft, no other instrument is necessary for this purpose
than a sharp knife, which may be best made with
a thin two-edged blade like that of a lancet ; con*
siderable practice is needed, however, to make effec-
tual use of it ; and some individuals acquire a
degree of dexterity which others never succeed in
attaining. In cutting sections of Animal tissues,
which, owing to the quantity of water they contain,
do not present a sufficiently firm resistance, it is
often desirable to half-dry these, by exposing small
pieces freely to the air, with the aid of a gentle
warmth if required ; when this desiccating process
has been carried sufficiently far, thinner sections
can be cut than could possibly have been made in
the original state of the tissue ; and the texture,
after a short maceration in water, almost entirely
recovers its pristine characters. There are certain
tissues, however, which will not bear to be thus
treated, and of which it is sufficient to examine an
extremely minute portion ; and for making sections
of these, such a pair of Scissors as is represented
in Fig. 106 will often be found very useful ; since,
owing to the curvature of the blades,* the two ex-
tremities of a section taken from a flat surface will generally be
found to thin away, although the middle of it may be too thick to
exhibit any structure. — Where only a moderate degree of thinness is
required, either in consequence of the transparence of the tissue, or
because it is not desired to exhibit its minutest details, the two-
bladed Knife contrived by Prof. Valentin (Fig. 107) may be employed
with advantage. The blades are attached to each other at their
lower end by a screw, in such a manner that their ' spring' tends
to keep them apart ; and their distance is regulated by pushing
Curved Scis-
sors for cutting
Thin Sections.
* It is difficult to convey by a drawing the idea of the real curvature of this
instrument, the blades of which, when it is held in front view, curve — not to
either side — but towards the observer; these scissors being, as the French
instrument-makers sajr, courbes sur le plat. — As an example of the utility of
such an instrument to the Micruscopist, the Author may cite the curious
demonstration given a few years since, by Dr. Aug. Waller, of the structure of
the Gustative Papillse, by snipping them off from the living Human tongue,
which may be done with no more pain than the prick of a pin would occasion.
SECTIONS OF SOFT SUBSTANCES.
221
tlie little rivet backwards or forwards in the slit through which
it works. The knife should be dipped in water before nsing, or,
Fig. 107.
Valentin's Knife.
still better, the section should be made under water, as the instru-
ment works much better when wet ; after use it should be care-
fully washed and dried, a piece of soft leather being passed between
the blades. If any water have found its way into the part through
which the rivet works, the moveable blade should be detached by
taking out its screw, and each blade should be cleaned separately.
This instrument is now generally constructed on an improved form ;
the blades being made with a convex instead of a straight edge,
their distance from each other being regulated by a milled-head
screw, and their separation for cleaning being more easily accom-
plished. Sections of soft tissues may also be made by imbedding the
substance in melted pa-
raffine, so as when the Fig. 108.
paraffine has hardened
by cooling, to form a
cylindrical plug, which
can be placed in' the
Section instrument
(Fig. 108). >
153. Cutting Sections
of Harder Substances.
— There is a large
class of substances, both
Animal and Vegetable,
which are too hard to
admit of sections being
made in the manner
just described, but of
which extremely thin
slices can be made by
a sharp cutting instru-
ment, if only they be
properly held and sup-
ported,— more especially when the thickness of the section can be
regulated by a mechanical contrivance ; such are, in particular, the
Stems and Boots of Plants, and the Horns, Hoofs, Cartilages, and
similarly firm structures of Animals. Various costly machines have
been devised for this purpose, some of them characterized by great
ingenuity of contrivance and beauty of workmanship ; but every
Section-Instrument.
222 PEEPAKATION OF OBJECTS.
purpose to which these are adapted will be found to be answered
by a very simple and inexpensive little instrument, which may
either be held in the hand, or (which is preferable) may be firmly
attached by means of a T-shaped piece of wood (as in Fig. 108), to
the end of a table or work-bench. This instrument essentially
consists of an upright hollow cylinder of brass, with a kind of
piston which is pushed from below upwards by a fine threaded
screw turned by a large milled-head ; at the upper end the cylinder
terminates in a brass table, which is made to present a perfectly
flat surface. At one side is seen a small milled-head, which acts
upon a ' binding screw,' whose extremity projects into the cavity
of the cylinder, and serves to compress and steady anything that
it holds. A cylindrical stem of wood, a piece of horn.whalebone,
cartilage, &c, is to be fitted to the interior of the cylinder, so as
to project a little above its top, and is to be steadied by the
' binding screw ; ' it is then to be cut to a level by means of a sharp
knife or razor laid flat upon the table. The large milled-head is
next to be moved through such a portion of a turn as may very
slightly elevate the substance to be cut, so as to make it project in
an almost insensible degree above the table, and this projecting
part is to be sliced-off, with a knife previously dipped in water.
The best knife for this purpose is a razor, ground flat (instead of
concave) on one side, but having still a concave surface on the
other ; the flat side is to be laid downwards upon the table ; and
the motion given to the edge should be a combination of drawing
and pressing. (It will be generally found that better sections are
made by working the knife from the operator, than towards him.)
"When one slice has been thus taken off, it should be removed from
the blade by dipping it into water, or by the use of a camel-hair
brush ; the milled-head should be again advanced, and another
section taken ; and so on. Different substances will be found both
to bear and to require different degrees of thickness ; and the
amount that suits each can only be found by trial. It is advan-
tageous to have the large milled-head graduated, and furnished
with a fixed index ; so that this amount having been once deter-
mined, the screw shall be so turned as to always produce the exact
elevation required. — Where the substance of which it is desired to
obtain sections by this instrument is of too small a size or of too
soft a texture to be held firmly in the manner just described, it
may be placed between the two vertical halves of a cork of suitable
size to be pressed into the cylinder ; and the cork, with the object
it grasps, is then to be sliced in the manner already described, the
small section of the latter being carefully taken-off the knife, or
floated-away from it, on each occasion, to prevent it from being lost
among the lamellae of cork which are removed at the same time. —
The special methods of preparation which are required in the case of
the various substances of which sections may be conveniently cut
by this instrument, will be noticed under their several heads.
154. Grinding and Polisliing of Sections. — Substances which are
SECTIONS OF HARD SUBSTANCES. 223
too hard to be sliced with a cutting instrument in the manner last
described,— such as Bones, Teeth, Shells, Corals, Fossils of all
kinds, and even some hard Vegetable Tissues, — can only be
reduced to the requisite thinness for Microscopical examination,
by grinding-down thick sections until they become so thin as to be
transparent. General directions for making such preparations
will be here given ;* but those special details of management which
particular substances may require, will be given when these sub-
stances are respectively described. — The first thing to be done will
usually be to procure a section of the substance, as thin as it can
be safely cut. Most substances not siliceous may be divided by
the fine saws used by artisans for cutting brass ; but there are
some bodies (such as the enamel of teeth, and porcellanous shells),
which, though merely calcareous, are so hard as to make it very
difficult and tedious to divide them in this mode ; and it is much
the quicker operation to slit them with a disc of soft iron (resem-
bling that used by the lapidary) charged at its edge with diamond-
dust, which disk may be driven in an ordinary lathe. Where waste
of material is of no account, a very expeditious method of obtain-
ing pieces fit to grind down is to detach them from the mass with
a strong pair of ' cutting pincers,' or, if it be of small dimensions,
with ' cutting pliers ;' and a flat surface must then be given to it,
either by holding it to the side of an ordinary grindstone, or by
rubbing it on a plate of lead (cast or planed to a perfect level)
charged with emery, or by a strong-toothed file, the former being
the most suitable for the hardest substances, the latter for the
toughest. There are certain substances, especially Calcareous
Fossils of Wood, Bone, and Teeth, in which the greatest care is
required in the performance of these preliminary operations, on
account of their extreme friability ; the vibration produced by the
working of the saw or the file, or by grinding on a rough surface,
being sufficient to disintegrate even a thick mass, so that it falls
to pieces under the hand ; such specimens, therefore, it is requisite
to treat with great caution, dividing them by the smooth action of
the wheel, and then rubbing them down upon nothing rougher
than a very fine ' grit.' Where (as often happens) such specimens
are sufficiently porous to admit of the penetration of Canada
Balsam, it will be desirable, after soaking them in turpentine for
a while, to lay some liquid balsam upon the parts through which
the section is to pass, and then to place the specimen before the
fire or in an oven for some little time, so as first to cause the
balsam to run-in, and then to harden it ; by this means the speci-
men will be rendered much more fit for the processes it has after-
wards to undergo.— It not unfrequently happens that the small
size, awkward shape, or extreme hardness of the body, occasions a
difficulty in holding it either for cutting or grinding ; in such a
* The following directions do not apply to Siliceous substances ; as sections
of these can only be prepared by those who possess a regular Lapidary's ap-
paratus, and who have been specially nstructed in the U6e of it.
224 PEEPAEATION OF OBJECTS.
case, it is uracil better to attach it to the glass in the first instance
by any side that happens to be flattest, and then to rub it down
by means of the ' hold ' of the glass upon it, until the projecting
portion has been brought to a plane, and has been prepared for
permanent attachment to the glass. This is the method which it
is generally most convenient to pursue with regard to small bodies ;
and there are many which can scarcely be treated in any other
way than by attaching a number of them to the glass at once, in
such a manner as to make them mutually support one another.*
155. The mode in which the operation is then to be proceeded
with, depends upon whether the section is to be ultimately set up
in Canada balsam (§ 173), or is to be mounted dry (§ 170), or in
fluid (§ 182). In the former case, the following is the plan to be
pursued : — The flattened surface is to be polished by rubbing it
with water on a ' Water-of-Ayr'-stone, on a hone or ' Turkey'-
stone, or on a new stone latterly introduced under the name of the
' Arkansas'-stone ; the first of the three is the best for all ordinary
purposes, but the two latter, being much harder, may be employed
for substances which resist it.f When this has been sufficiently
accomplished, the section is to be attached with Canada balsam to
a slip of thick well-annealed glass ; and as the success of the final
result will often depend upon the completeness of its adhesion to
this, the means of most effectually securing that adhesion will now
be described in detail. Some Canada balsam, previously rendered
somewhat stiff by the evaporation of part of its turpentine, is to
be melted on the glass slip, so as to form a thick drop, covering a
space somewhat larger than the area of the section ; and it should
then be set aside to cool, during which process the bubbles that
* Thus, in making- horizontal and vertical sections of Foraminifera, as it
would be impossible to slice them through, they must be laid close together in
a bed of hardened Canada Balsam on a slip of glass, in such positions, that,
when rubbed down, the plane of section shall traverse them in the desired
directions ; and one flat surface having been thus obtained for each, this must
be turned downwards, and the other side ground away. The following
ingenious plan has been suggested by Dr. Wallich ("Ann. of Nat. Hist.,"
July, 1861, p. 58), for turning a number of minute objects together, and thus
avoiding the tediousness and difficulty of turning each one separately : — The
specimens are cemented with Canada Balsam, in the first instance, to a thin film
of mica, which is then attached to a glass slide by the same means ; when they
have been ground down as far as may be desired, the slide is gradually heated
just sufficiently to allow of the detachment of the mica-film and the specimens
it carries ; and a clean slide with a thin layer of hardened balsam having been
prepared, the mica-film is transferred to it with the ground surface downwards.
When its adhesion is complete, the grinding may be proceeded with ; and as
the mica-film will be found to yield to the stone without the least difficultj^,
the specimens, now reversed in position, may be reduced to any degree of
thinness that may be found desirable.
f As the flatness of the polished surface is a matter of the first importance,
that of the Stones themselves should be tested from time to time ; and when-
ever they are found to have been rubbed-down on any one part more than on
another, they should be flattened on a paving-stone with fine sand, or on the
lead-plate with emery.
GFJXDIXG AND POLISHING SECTIONS. 225
may have formed in it will usually burst. When cold, its hard-
ness should be tested, which is best done by the edge of the
thumb-nail ; for it should be with difficulty indented by its pressure,
and yet should not be so resinous as to be brittle. If it be too
soft, as indicated by its too ready yielding to the thumb-nail, it
should be boiled a little more ; if too hard, which will be shown
by its chipping, it should be re-melted and diluted with more fluid
balsam, and then set aside to cool as before. When it is found to
be of the right consistence, the section should be laid upon its
surface with the polished side downwards ; the slip of glass is next
to be gradually warmed until the balsam is softened, special care
being taken to avoid the formation of bubbles ; and the section is
then to be gently pressed down upon the liquefied balsam, the
pressure being at first applied rather on one side than over its
whole area, so as to drive the superfluous balsam in a sort of wave
towards the other side, and an equable pressure being finally made
over the whole. If this be carefully done, even a very large section
may be attached to glass without the intervention of any air-
bubbles ; if, however, they should present themselves, and they
cannot be expelled by increasing the pressure over the part beneath
which they are, or by slightly shifting the section from side to
side, it is better to take the section entirely off, to melt a little
frem balsam upon the glass, and then to lay the section upon it as
before.
1 56. When the Section has been thus secured to the glass, and
the attached part thoroughly saturated (if it be porous) with hard
Canada balsam, it may be readily reduced in thickness, either by
grinding or filing, as before, or, if the thickness be excessive, by
taking off the chief part of it at once by the slitting wheel. So
soon, however, as it approaches the thinness of a piece of ordinary
card, it should be rubbed down with water on one of the smooth
stones previously named, the glass slip being held beneath the
fingers with its face downwards, and the pressure being applied
with such equality that the thickness of the section shall be (as
nearly as can be discerned) equal over its entire surface. As soon
as it begins to be translucent, it should be placed under the Micro-
scope (particular regard being had to tbe precaution specified in
§ 131) and note taken of any inequality ; and then when it is
again laid upon the stone, such inequality may be brought down
by making special pressure with the forefinger upon the part of
the slide above it. When the thinness of the section is such as to
cause the water to spread around it between the glass and the
stone, an excess of thickness on either side may often be detected
by noticing the smaller distance to which the liquid extends. In
proportion as the substance attached to the glass is ground away,
the superfluous Balsam, which may have exuded around it, will be
brought into contact with the stone ; and this should be removed
with a knife, care being taken, however, that a margin be still
left round the edge of the section. As the section approaches the
Q
226 PEEBAEATION OF OBJECTS.
degree of thinness which is most suitable for the display of its
organization, great care mnst be taken that the grinding process
be not carried too far ; and frequent recourse should be had to the
Microscope, which it is convenient to have always at hand when
work of this kind is being carried on. There are many substances
whose intimate structure can only be displayed in its highest per-
fection, when a very little more reduction would destroy the section
altogether ; and every Microscopist who has occupied himself in
making such preparations, can tell of the number which he has
sacrificed in order to attain this perfection. Hence, if the amount
of material be limited, it is advisable to stop short as soon as a
good section has been made, and to lay it aside — 'letting well
alone' — whilst the attempt is being made to procure a better one ;
if this should fail, another attempt may be made, and so on, until
either success has been attained, or the whole of the material has
been consumed — the first section, however, still remaining : whereas,
if the first, like every successive section, be sacrificed in the attempt
to obtain perfection, no trace will be left " to show what once has
been." In judging of the appearance of a Section in this stage
under the Microscope, it is to be remembered that its transparence
will subsequently be considerably increased by mounting in Canada
balsam (§ 1 73) : this is particularly the case with Fossils to which a
deep hue has been given by the infiltration of some colouring matter,
and with any substances whose particles have a molecular aggre-
gation that is rather amorphous than crystalline. When a suffi-
cient thinness has been attained, the Section may generally be
mounted in Canada balsam ; and the mode in which this must be
managed will be detailed hereafter (§ 177).
157. As there are certain substances, however, the view of whose
structure is impaired by mounting in Canada balsam, and which
should therefore be mounted either dry or in fluid, a different
method of procedure must be adopted with them. If tolerably
thin sections of them can be cut in the first instance, or if they
are of a size and shape to be held in the hand whilst they are
being roughly ground down, there will be no occasion to attach
them to glass at all : it is frequently convenient to do this at first,
however, for the purpose of obtaining a ' hold' upon the specimen ;
but the surface which has been thus attached must afterwards be
completely rubbed away, in order to bring into view a stratum
which the Canada balsam shall not have penetrated. As none
but substances possessing considerable toughness, such as Bones
and Teeth, can be treated in this manner, and as these are the
substances which are most quickly reduced by a coarse file and
are least liable to be injured by its action, it will be generally
found possible to bring the sections to a considerable thinness, by
laying them upon a pi$ce of _ cork or soft wood held in a vice, and
operating upon them first with a coarser and then with a finer file.
When this cannot safely be carried further, the section must be
rubbed down upon that one of the fine stones already mentioned
POLISHING SECTIONS. — CHEMICAL ACTIONS. 227
(§ 155) which is found best to suit it : as long as the section is
tolerably thick, the finger may be used to press and move it ; but
as soon as the finger itself begins to come into contact with the
stone, it must be guarded by a flat slice of cork or by a piece of
gutta-percha a little larger than the object. Under either of
these, the section may be rubbed down until it has been reduced
to the requisite degree of tenuity ; but even the most careful
working on the finest-grained stone will leave its surface covered
with scratches, which not only detract from its appearance, but
prevent the details of its internal structure from being as readily
made out as they can be in a polished section. This polish may
be imparted by rubbing the section with putty -powder (peroxide of
tin) and water upon a leather strap, made by covering the surface
of a board with buff-leather, having three or four thicknesses of
cloth, flannel, or soft leather beneath it : this operation must be
performed on both sides of the section, until all the marks of the
scratches left by the stone shall have been rubbed out ; when the
specimen will be fit for mounting, after having been carefully
cleansed from any adhering particles of putty-powder.
158. Chemical Actions. — One important part of the preparation
of Microscopic objects is often effected by the use of Chemical
Re-agents. These may be employed either for the sake of removing
substances of which it is desired to get rid, in order to bring some-
thing else into view, or for the sake of detecting the presence of
particular substances in the object under examination. Thus, the
Author has found that he has frequently been better able to bring
into view particular features in the organization of Foraminifera
by removing portions of their shells by the application of diluted
Acid, than by grinding down thin sections. The acid (ISTitric or
Hydrochloric) may be applied with great nicety by means of a fine
pointed camel's hair pencil, the object being attached to a slide, and
placed under the simple Microscope ; and another camel's hair pencil
charged with water should be at hand, to enable the observer to
stop the solvent action whenever he may consider that it has been
carried far enough. Again, in order to obtain the animal basis of
Shell, Bone, Tooth, &c, it is necessary to dissolve away the Cal-
careous portion of these tissues by the use of acids ; a mixture of
Nitric and Hydrochloric acids is preferable, ; and this should be
added, little by little to a considerable bulk of water, until a dis-
engagement of gas be perceived to commence from the surface of
the specimen. Care should always be taken not to hurry the process
by adding too much acid, since, when the animal membrane is of
v^ery delicate consistence, it is liable to be dissolved ; and in some
cases it is better to allow the action to go on for many weeks, add-
ing only a drop or two of acid at a time. When Siliceous particles
are to be removed (such as those which form the loricce of the
DiatomaceEe), for the sake of leaving the organic membrane in a
state adapted to separate examination, Hydrofluoric acid must be
employed as the menstruum. It is sometimes necessary to get rid
Q2
228 PREPARATION OF OBJECTS.
of tlie Organic matter, for the sake of obtaining the Mineral par-
ticles in a separate state, as in the case of the spicules of Sponges,
Gorgonias, &c. : this may be done either by incineration, or by
boiling or macerating for a long time in a solution of caustic
potash. A still better plan is to warm the objects in nitric acid, and
drop in, cautiously, crystals of chlorate of potash. In separating
from Guano, again, the Siliceous skeletons of Diatomacese, &c, which
it may contain, Hydrochloric and Nitric acids are largely used to
dissolve away every part of the mass on which they will act ; the
microscopic organisms for which search is made, being contained in
the few grains of sediment which are left when a pound of pure
guano has been thus treated. — On the other hand, it is often
desirable to harden Animal Tissues, in order that they may be more
readily examined : this is best effected in some instances by macera-
tion in strong Alcohol,* and in others by maceration in a solution
of Chromic Acid, so dilute as to be of a pale straw colour, which is
particularly efficacious in bringing into view the finer ramifications
of ISTerves.
159. In applying Chemical Re-agents to Microscopic objects for
the purpose of testing, it is necessary to use great care not to add
too much at once ; and the Test-Bottle itself may be made to afford
the means of regulating the quantity, in either of the following
modes : — The stopper of the test-bottle may be drawn to a capil-
lary orifice, from which the fluid is caused to flow, drop by drop,
by the warmth of the hand applied to the bottle, which causes an
expansion of the air it may contain : the perforated stopper, when
not in use, is covered by a cap which fits closely around the neck
of the bottle. Or the tubular stopper may be shaped like that of
the bottle represented in Fig. 115, the lower end of the tube being
drawn to a fine point, so that the desired quantity of the test-
liquid, and no more, may be made to flow from it by pressing the
elastic cap of the funnel. Another arrangement consists in the
elongation of the stopper, which is drawn to a fusiform point, so
as to serve as the test-rod for its own bottle,f thereby enabling either
a mere trace or several ordinary drops of the re-agent to be applied
at once ; for the elongated stopper will take up a considerable
quantity, a larger or smaller proportion of which (as desired) may
be left behind, by bringing the lower part of the stopper into con-
tact with the inside of the neck of the bottle as it is being with-
drawn.— The Author is disposed, however, from his own experience,
to recommend the small Syringe formerly described (§ 115), with
its nozzle drawn out to a point, as the most convenient instrument
for applying minute quantities of Test-liquids to Microscopic
objects ; one of its advantages being the very precise regulation
* The Author has found this menstruum especially useful in his researches
into the structure of Comatula, the tissues of which, when fresh, are so ex-
tremely soft that their parts are almost undistinguishable.
t Bottles of this pattern, which was devised by Dr. Griffith, are sold by Mr.
Ferguson, of Giltspur-street.
APPLICATION OF TEST-LIQUIDS. 229
which can be obtained by the dexterous use of it, of the quantity of
the test to be deposited ; whilst another consists in the power of
withdrawing any excess. Care must be taken in the use of it,
to avoid the contact of the test-liquid with the packing of the
piston. Whichever method is employed, great care should be taken
to avoid carrying away from the slide to which the test-liquid is
applied, any loose particles which may be upon it, and which, may
be thus transferred to some other object, to the great perplexity of
the Microscopist. It is better, indeed, not to deposit the drop of
test-liquid on the slide in immediate contact with the substance
to which it is to be applied ; but to bring the two into contact after
the test-bottle, stopper, or syringe has been withdrawn.
160. The following are the Test-Liquids most frequently
needed : —
1. Solution of Iodine in water (1 gr. of iodine, 3 grs. of iodide
of potassium, 1 oz. of distilled water) turns Starch blue and
Cellulose brown ; it also gives an intense brown to Albuminous
substances.
2. Dilute Sulphuric Acid (one of acid to two or three parts of
water), gives to Cellulose that has been previously dyed with iodine
a blue or purple hue ; also, when mixed with a solution of sugar, it
gives a rose-red hue, more or less deep, with Nitrogenous substances
and with, bile (Pettenkofers test).
3. Solution of Chloride of Zinc, Iodine, and Iodide of Potassium,
made in the following way : — Zinc is dissolved in Hydrochloric acid,
and the solution is permitted to evaporate, in contact with metallic
zinc, until it attains the thickness of a syrup ; this syrup is then
saturated with iodide of potassium, and iodine is last added. This
solution (which is known as Schultz's test) serves, like the preceding,
to detect the presence of Cellulose, and has the advantage over
sulphuric acid of being less destructive to the tissues. Each will
sometimes succeed where the other fails ; consequently, in doubtful
cases, both should be employed.
4. Concentrated Nitric Acid gives to Albuminous substances an
intense yellow : when diluted with about four or five parts of
water, it is very useful in separating the elementary parts of many
Animal and Vegetable tissues, when these are boiled or macerated
in it.
5. Acetic Acid (which should be kept both concentrated and also
diluted with from three to five parts of water) is a most useful test-
liquid to the Animal Histologist, from its power of dissolving, or
at least of reducing to a state of such, transparence that they can
no longer be distinguished, certain membranes^ fibres, &c, whilst
others are brought strongly into view.
6. Acid Nitrate of Mercury (Millon's test) colours Albuminous
substances red.
7. Solution of Caustic Potash or Soda (the latter being gene-
rally preferable) has a remarkable solvent effect upon many Organic
substances, both Animal and Vegetable, and is extremely useful in
230 EBEPAEATION OF OBJECTS.
rendering some of their structures transparent, so that others are
brought into view ; whilst it has a special action upon Horny
tissues, which enables their component cells to be more readily
distinguished.
8. Alcohol dissolves Eesinous substances and many Yegetable
Colouring matters, and renders most "Vegetable preparations more
transparent ; on the other hand, by its coagulating action on
Albuminous substances, it renders many Animal Tissues (as
Nerve-fibres) more opaque, and thus brings them into greater dis-
tinctness.
9. Ether dissolves ^.ot only Resins, but Oils and Fats.
10. Chromic Acid hardens many Animal tissues, especially Nerve-
fibres.
11. Osmic Acid dissolved in distilled water in the proportion of
from l-10th to l-5th per cent., is very useful for hardening the
Retina aud Epithelium, which it does in a day or two. When
hardened, the tissue should be placed in distilled water for a few
days, and mounted in a saturated solution of potassic acetate.*
161. Staining Processes. — Much attention has been given of late
years to the effects of another kind of testing, in which advantage
is taken of the various degrees of attraction for certain Organic
Colouring matters, which are possessed by different Tissues; so that
whilst some are stained very quickly when immersed in colouring
solutions, others require a much longer contact with them ; and
thus the former may be distinguished in the midst of the latter,
with a certainty and clearness attainable by no other method.
Although there are particular instances in which Magenta may be
employed with advantage, the colouring substance most generally
serviceable is Carmine ; and the following is given by Dr. Beale,
who had large experience of this process, and has obtained im-
portant results by its use, as the best mode of applying it. Ten
grains of Carmine in small fragments are to be placed in a test-
tube, and half a drachm of strong Liquor Ammonias added ; by
agitation and the heat of a spirit-lamp the carmine is soon dis-
solved, and the liquid, after boiling for a few seconds, is to be
allowed to cool. After the lapse of an hour, much of the excess
of ammonia will have escaped ; and the solution is then to be
mixed with 2 oz. of Distilled Water, 2 oz. of pure Glycerine, and
\ oz. of Alcohol. The whole may be passed through a filter ; or,
after being allowed to stand for some time, the perfectly clear
supernatant fluid may be poured off and kept for use. If, after a
long keeping, a little of the Carmine should be deposited through
the escape of the ammonia, the addition of a droj) or two of Liquor
Ammonias will re-dissolve it. The most valuable result of this
process is the facility with which, when carefully and judiciously
employed, it enables the Microscopist to distinguish what Dr.
Beale terms ' germinal matter,' — which is identical with the
* Dr. Butherford, in " Quart. Journ. of Microsc. Science," Jan,, 1872.
STAINING PROCESSES. 231
' protoplasm ' or ' sarcocle ' of other Physiologists — from the
' formed materials ' or tissue-elements, which are the products of its
activity ; the living formative substance being stained by Carmine
so much sooner than any of those products, that it may be deeply
dyed whilst they remain colourless. " The rapidity," says Dr.
Beale, "with which the colouring of a tissue immersed in this
fluid takes place, depends partly upon the character of the tissue,
and partly upon the excess of ammonia present in the solution.
If the solution be very alkaline, the colouring will be too intense,
and much of the soft tissue or imperfectly -developed formed mate-
rial around the germinal matter is destroyed by the action of the
alkali. If, on the other hand, the reaction of the solution be
neutral, the uniform staining of tissue and germinal matter may
result, and the appearances from which so much may be learned
are not always produced. When the vessels are injected with the
Prussian blue fluid, the Carmine fluid requires to be sufficiently
alkaline to neutralize the free acid present. The permeating power
of the solution is easily increased by the addition of a little more
water and alcohol. In some cases the fluid must be diluted with
water, alcohol, or glycerine ; and the observer must not hastily
condemn the process, or conclude (as some have) that a particular
form of germinal matter is not to be coloured, until he has given
the plan a fair trial, and tried a few experiments."* Of the
special uses of this method, various illustrations will be given
hereafter. — Nitrate of Silver is used by Dr. Klein for blackening
Epithelial cement in capillaries and lymphatics. He directs that
the fresh tissue should be placed in a solution of nitrate of silver
in distilled water of the strength of one -half per cent, for from
one to three minutes ; then in very dilute acetic acid (1 to 2 per
cent.) for a minute or two ; and then in glycerine, with exposure to
light. It should be mounted in glycerine or glycerine-jelly. —
Chloride of Gold is also employed to produce a Violet colour. The
fresh tissue is to be placed in a half per cent, solution of gold chlo-
ride in distilled water for from fifteen to twenty minutes, until it is
of a yellow colour ; then in dilute acetic acid (1 to 2 per cent.) for
a few minutes ; and then in distilled water with exposure to light,
until a tinge which is sufficiently violet appears. It should be
mounted in glycerine jelly.
162. Preparation of Specimens in Viscid Media. — To Dr.
Beale the Microscopist is also indebted for a method of pre-
paring Animal and Vegetable tissues for examination under the
l-12th, l-20th, or l-25th-inch Objectives, which is much supe-
rior to those in ordinary use. This consists in the substitution of
a viscid medium, such as pure Glycerine or strong Syrup, for
the Aqueous fluids with which the object to be examined is usually
treated ; many advantages being thereby gained. Thus in thin-
ning-out tissues by compression, an amount of pressure may be
* " How to Work with the Microscope," 4th edit. p. 109.
232 MOUNTING OF OBJECTS.
applied, winch, would be destructive to specimens mounted in
water. Again, these media have a preservative action, so that if
the tissues be permeated by them soon after death, further
changes are prevented. They have, moreover, the effect of render-
ing the tissues more transparent, and enabling their components
to be more readily distinguished. It has been objected that these
viscid media are unsuitable, as causing the tissues to shrink, and
soft cells to collapse, by the exosmose of their fluid contents ; but
in reply it is stated by Dr. Beale, that though such, shrinkage is
the immediate effect of the use of a viscid medium of conside-
rable density, tissues left in it for a few days recover their
original dimensions. " I have preparations," he says (Op. cit.
p. 294), " from creatures of every class. The smallest Animalcules,
tissues of Entozoa, Polypes, Starfishes, Mollusks, Insects, Crus-
tacea, Infusoria, various Vegetable Tissues, microscopic Fungi and
Algae of the most minute and delicate structure, as well as the
most delicate parts of the higher Vegetable tissues, may all be
preserved in these viscid media ; so also may be preserved the
slowest and the most rapidly-growing, the hardest and the softest
Morbid growths, as well as Embryonic structures at every period
of development, even when in the softest state. All that is
required is, that the strength of the fluid should be increased
very gradually, until the ivhole tissue is thoroughly penetrated by
the strongest that can be obtained." " Minute dissections can be car-
ried on in these viscid media with greater facility and certainty than
in more limpid fluids. I can readily detach the most minute parts
of tissues, separate the different structures in one texture without
tearing or destroying them, unravel convoluted tubes, and perform
with ease a great variety of minute operations, which it would be
impossible to effect with any of the ordinary methods of dissection.
"With care in regulating the temperature, I can soften textures thus
preserved in syrup, to the precise extent required for further
minute dissection ; and even very hard textures (such as Bone and
Teeth) may thus be softened, so that by gradually increased
pressure and careful manipulation exceedingly thin layers can be
obtained, without the relation of the anatomical elements to each
other being much altered, and without any of the tissues being
destroyed." (Op. cit. p. 205.) Dr. Beale recommends that any
Be-agents used in making preparations of this kind, should them-
selves be dissolved in Glycerine.
Section 2. Mounting of Objects.
163. The Microscopist not merely desires to prepare Objects for
examination, but, where possible, to preserve them in such a
manner that they may be inspected at any future time. This may
be so effectually accomplished in regard to many substances, that
they undergo no kind of change, however long they may be re-
tained ; and even delicate structures whose composition renders
MOUNTING OF OBJECTS. 233
them peculiarly liable to decay, may often be kept, by complete
seclusion from the air and by immersion in a preservative fluid, in
a state so nearly resembling that in which they were at first pre-
pared, that they will continue, during an indefinite length of time,
to exhibit their original characters with scarcely any deterioration.
The method of 'mounting' Objects to be thus preserved will
differ, of course, both according to their respective natures and
also according to the mode in which they are to be viewed, whether
as transparent or as opaque objects. Thus they may be setup dry,
or in Canada balsam, or in some preservative liquid ; they may
need to be simply covered with thin glass, or they may require to
be surrounded by a ' cell : ' if they are to be viewed by transmitted
light, they must always have glass below them ; but if they are to
be seen by the light reflected from their surfaces, they may often
be preferably mounted on wood, card, or some other material
which itself affords a black back ground. In almost all cases in
which Transparent objects are to be mounted, use will have to be
made of the slips of Glass technically called slides or sliders, and
covers of thin glass ; and it will therefore be desirable to treat of
these in the first instance.
164. Glass Slides. — The kind of Glass usually employed for
mounting objects, is that which is known as ' flatted crown;' and
it is now almost invariably cut, by the common consent of Micro-
scopists in this country, into slips measuring 3 in. by 1 in. : ior
objects too large to be mounted on these, the size of 3 in. by 1^ in.
may be adopted. Such slips may be purchased, accurately cut to
size and ground at the edges, for so little more than the cost of
the glass, that few persons to whom time is an object, would trouble
themselves to prepare them ; it being only when glass slides of
some unusual dimensions are required, or when it is desired to
construct 'built-up cells' (§188), that a facility of cutting glass
with a glazier's diamond becomes useful. The glass slides prepared
for use should be free from veins, air-bubbles, or other flaws, at
least in the central part on which the object is placed ; and any
whose defects render them unsuitable for ordinary purposes, should
be selected and laid aside for uses to which the working Microscopist
will find no difficulty in putting them. As the slips vary conside-
rably in thickness, it will be advantageous to separate the thick from
the thin, and both from those of medium substance : the last may
be employed for mounting ordinary objects ; the second for mount-
ing delicate objects to be viewed by the high powers with which
the Achromatic Condenser is to be used, so as to avoid any un-
necessary deflection of the illuminating pencil by the thickness of
the plate which it has to traverse beneath the object ; whilst the
first should be set aside for the attachment of objects which are to
be ground-down, and for which, therefore, a stronger mounting
than usual is desirable. Where very hard substances have to be
thus operated on, it is advantageous to attach them in the first
instance to pieces of very thick plate-glass : only transferring them
234 MOUNTING OF OBJECTS.
to the ordinary slides when they have been reduced to nearly the
requisite thinness (§ 155).
165. Thin Glass.- — The older Microscopists were obliged to em-
ploy thin laminas of talc for covering objects to be viewed with
lenses of short foci ; but this material, which was in many respects
objectionable, is now entirely superseded by the thin-glass manu-
factured for this express purpose by Messrs. Chance of Birmingham,
which may be obtained of various degrees of thickness, from l-20th
to l-250th of an inch. This glass, being unannealed, is very hard and
brittle ; and much care and some dexterity are required in cutting
it. This should be done with the writing diamond ; and it is ad-
vantageous to lay the thin glass upon a piece of wetted plate-glass,
as its tendency to crack and ' star' is thereby diminished. For
cutting square or other rectangular covers, nothing but a flat rule
is required. For cutting rounds or ovals, on the other hand, it is
necessary to have ' guides' of some kind. The simplest, which
are as effective as any, consist of pieces of flat brass-plate, per-
forated with holes of the various sizes desired, or curtain-rings,
with a piece of wire soldered on either side : these being held
firmly down on the thin glass with two fingers of the left hand,
the writing-diamond is carried round the inner margin of the aper-
ture with the right ; care being taken that, in so doing, the diamond
be made to revolve on its own axis, which is needful both that it
may mark the glass, and also that the beginning and the end of the
cut may join.* Where a number of such 'rounds' are being cut
at once, it saves much trouble, as well as risk of loss by breakage,
in separating them, to cut the glass first into strips whose breadth
shall equal the diameter of the rounds. But it is very convenient
to use-up for this purpose any odd pieces of glass whose shape may
render them unsuitable for being cut into ' squares' without much
waste. The pieces of thin glass thus prepared for use should be
sorted, not only according to size and shape, but also according to
thickness. The thinnest glass is of course most difficult to handle
safely, and is most liable to fracture from accidents of various
kinds ; and hence it should only be employed for the purpose for
which it is absolutely needed, namely, the mounting of objects
which are to be viewed by the highest powers. The thickest pieces,
* A very elegant little instrument, for the purpose of cutting thin-glass
rounds, contrived by Mr. Shadbolt, and another, of a more substantial
character, invented by Mr. Darker, will be found described in Mr. Quekett's
" Practical Treatise." These instruments, however, are rather adapted for the
use of those who have occasion to prepare such rounds in large quantities,
than for the ordinary working Microscopist, who will find the method above
described answer his requirements sufficiently well. Indeed it is in some
respects superior; since a firm pressure made by the ring or plate on the glass
around, tends to prevent the crack from spreading into it. But to every one
to whom the saving of time is a greater object than the expenditure of a few
shillings, it is strongly recommended that these 'rounds' should be purchased
ready cut ; as they may be obtained of any required size and thinness, at a
very moderate cost.
MEASUREMENT OF THICKNESS OF THIN GLASS.
235
again, may be most advantageously emploj^ed as covers for large
Cells in which objects are mounted in fluid (§§ 186, 187), to be
viewed by the low powers whose performance is not sensibly affected
by the aberration thus produced. And the pieces of medium thin-
ness will be found most serviceable for all ordinary purposes ;
neither being, on the one hand, difficult to handle, nor, on the
other, interfering with the clearness of the image formed by medium
powers of moderate aperture, even when no special adjustment is
made for the aberration they occasion (§ 145, v.).
166. The exact thickness of any piece of glass may be deter-
mined without difficulty, by placing it edgeways on the stage of the
Microscope (holding it in the stage-forceps), and measuring its edge
by the Eye-piece Micrometer (§ 77). A much more ready means is
afforded, however, by the Lever of Contact (Fig. 109) devised by
Mr. Boss for this express purpose. This instrument consists of a
small horizontal table of brass, mounted upon a stand, and having
at one end an arc graduated into 20 divisions, each of which
Fig. 109.
Lever of Contact.
represents 1-1 000th of an inch, so that the entire arc measures
l-50th of an inch ; at the other end is a pivot, on which moves a
long and delicate lever of steel, whose extremity points to the
graduated arc, whilst it has very near its pivot a sort of projecting
tooth, which bears at * against a vertical plate of steel that is
screwed to the horizontal table. The piece of Thin Glass to be
measured, being inserted between the vertical plate and the pro-
jecting tooth of the lever, its thickness in thousandths of an inch is
given by the number on the graduated arc to which the extremity
of the lever points. Thus, if the number be 8, the thickness of
the glass is *008 or l-125th of an inch. — When the glass covers
have been sorted according to their thickness, it will be found con-
venient to employ those of one particular thickness for each par-
ticular class of objects ; since, when one object is being examined
after another, no re-adjustment of the Objective will then be required
for each. This will be found a great saving of time and trouble,
when high powers are in use. It is undesirable to employ glass
covers of greater thickness than l-140th (*007) of an inch, with
236 MOUNTING OF OBJECTS.
any object-glass whose aperture exceeds 75° ; and for object-glasses
of 120° and upwards, the glass cover should not exceed l-250th
(•004) of an inch.
167. On account of the extreme brittleness of the Thin Glass,
it is desirable to keep the pieces, when cut and sorted, in some fine
and soft powder, such as Starch. Before using it, however, the
Microscopist should be careful to clean it thoroughly ; not merely
for the sake of removing foulnesses which would interfere with the
view of the object, but also for the sake of getting rid of adherent
starch-grains, the presence of which might lead to wrong conclu-
sions, and also of freeing the surface from that slight greasiness,
which, by preventing it from being readily wetted by water, fre-
quently occasions great inconvenience in the mounting of objects in
fluid. The thicker pieces may be washed and wiped without much
danger of fracture, if due care be employed ; but the thinner re-
quire much precaution ; and in cleansing these, the simple method
devised by Mr. Spencer will be found very useful. This consists in
the use of a pair of round flat disks, about 1^ inch in diameter,
made of wood or metal covered with chamois leather, and fur-
nished with handles ; for when a piece even of the thinnest glass
is laid upon one of these, it may be rubbed clean with the other,
and any amount of pressure may be used without the least risk of
breaking it. Previously to doing this, however, it will be advan-
tageous to soak the pieces for a time in strong Sulphuric Acid, and
then to wash them in two or three waters ; but if greasiness be
their chief fault, they should be soaked in a strong infusion of
Kutgalls, with which it will be also advantageous to cleanse the sur-
face of glass slides that are to be used for mounting objects in liquid.
168. Varnishes and Cements. — There are three very distinct
purposes for which Cements that possess the power of holding firmly
to Glass, and of resisting not merely water but other preservative
liquids, are required by the Microscopist ; these being (1) the
attachment of the glass covers to the slides or cells containing the
object, (2) the formation of thin cells of cement only, and (3) the
attachment of the glass-plate or tube-cells to the slides. The two
former of these purposes are answered by liquid cements or var-
nishes, which may be applied without heat ; the last requires a
solid cement of greater tenacity, which can only be used in the
melted state. The varnishes used for mounting objects in liquid
should always be such as contain no mixture of solid particles.
This is a principle on which the Author, from an experience of
many years, is disposed to lay great stress; having often made
trial, at the recommendation of friends, of varnishes which were
said to have been greatly improved by thickening with litharge or
lampblack ; and having always found that, although they may
stand well for a few weeks or months, they became porous after a
greater lapse of time, allowing the evaporation of the liquid and
admission of air. He has himself found none more durable than
that known as Japanners' Gold-size, which may be obtained at
GOLD-SIZE AND OTHER CEMENTS. 237
almost every colour shop.* "When this is new and liquid, it dries very
quickly, provided a thin layer only be laid on at once ; and its dis-
position to run in is thus kept in check. WTien the first coat has
completely set, a second may be applied ; and it may be advan-
tageous to lay a third over this, or the slide may be finished off
with Brunswick Black or Asphalte. There are few preservative
liquids with which Gold-size may not be employed ; since it is not
acted on by any Aqueous solution, and resists moderately diluted
Spirit ; Oil of Turpentine being its only true solvent. Damar Varnish
(§ 179) is well spoken of by those who have used it. The solution
of Shell-Lac in Naphtha, which is sold under the name of Liquid
Glue, dries more quickly than gold-size, but is more brittle when
completely hardened, and does not adhere so firmly and enduringly
to glass ; and it is, moreover, more easily acted on by diluted
alcohol than the preceding. Its chief use is in mounting objects
dry (§ 172). Bell's Microscopic Cement, which is made by dis-
solving Shell-Lac in strong Alcohol, is said by Dr. Beale to resist
Glycerine better than ordinary cements. A solution of Asphalte
in drying oil or turpentine, known under the name of Brunswick
Black, has come much into use. It is extremely easy and pleasant
to work with, and dries quickly, so that it may be conveniently
used as a 'finish' over Gold-size, to improve the appearance
of the slide ; but it is brittle when dry, and is disposed to
crack, not merely when subject to any ' jar,' but also (after
some time) spontaneously. This evil may be corrected by
adding to it a little solution of Caoutchouc in Mineral Naphtha ;
or still better, by dissolving half a drachm of Caoutchouc in
10 oz. of Mineral Naphtha, and then adding 4 oz. of Asphaltum,
which must be dissolved by the aid of heat if necessary. It is
requisite to the goodness of this Asphalte varnish, that the Asphal-
tum should be of the best quality. This cement answers well for
making Cement-cells (§ 184) ; as does also the Yarnish termed
Black Japan provided that the glasses to which it has been applied
be exposed to the heat of an oven, not raised so high as to cause the
varnish to ' blister.' — Brushes which have been used either with Gold-
Size or Asphalte may be cleansed by Oil of Turpentine ; those which
have been used with Liquid Glue may be cleansed with Naphtha.
169. Although Canada Balsam has been sometimes used as a
Cement, and has the advantage of being worked with extreme
convenience, yet it is so apt to crack when hardened by time, that
a slight 'jar' will cause the cell to spring away from the glass to
which it has been attached. Hence, if employed at all for affixmo-
Cells to Glass Slides, its use should be limited to those which
afford a large surface of attachment (§§ 185, 186), or to those very
thin King-cells (§ 187) which cannot be so conveniently attached
with marine glue, and of which the cover may be secured to the
* The Author has preparations mounted with Gold-size more than thirty
years ago, which have remained perfectly free from leakage ; the precaution
having been taken to lay on a thin coat of varnish every two or three years.
238 MOUNTING OF OBJECTS.
slide by spreading the ring of gold-size round the margin of the
cell itself (§ 189). Care should "be taken in applying the Canada
Balsam, that it be sufficiently hardened by heat, but that it be not
so heated as to become brittle (§ 155). The general method of
using it for this purpose, is the same as that which must be prac-
tised in the case of Marine Glue. The superfluous balsam left after
pressing down the cell is to be removed, first by scraping with a
heated knife, and then by a rag dipped in oil of turpentine, after
which it is desirable to give the glass surface a final cleansing with
alcohol. — For all kinds of Cells (§§ 185-188) except those just
mentioned, the proper cement is Marine Glue, which is a mixture
of shell-lac, caoutchouc, and naphtha, now extensively employed ;
being distinguished by its extraordinary tenacity, and by its power
of resisting solvents of almost every kind. Different qualities of
this substance are made for the several purposes to which it is
applied ; that which is the most suitable to the wants of the Micro-
scopist is known in commerce as GK 4. As this cement can only be
applied hot, and as it is a great saving of trouble to attach a con-
siderable number of cells at the same time, a Mounting-Plate
should be provided, which will furnish the requisite heat to several
slides at once. Such a surface may be afforded by the top of a
stove ; but it is better to have one which can be used at all seasons,
and the heat of which can be precisely regulated at pleasure. A
very simple apparatus much used for this purpose, consists of a
small table of brass or iron plate, about 6 inches long and 2 broad,
with legs about 4 inches high, either screwed into its four corners,
or so jointed to them as to fold down ; this is set over a small Spirit
Lamp, the flame of which is regulated to give the heat required.*
The Author has found it much preferable, however, to lay the plate
on one of the rings of a small ' retort-stand' (used in Chemical
operations), which admits of being shifted to any height that may
be desired, so that the heat applied may be precisely graduated ;
or, if a Gas-lamp be applied for the ordinary purposes of illumina-
tion, its stem may be fitted with a sliding-ring, which will carry
either a hot plate or a water-bath. It is convenient, moreover, to
have two such plates laid on two rings ; one being allowed to cool
with the slides upon it, whilst the other is being heated. The
Glass Slides and Cells which are to be attached to each other must
first be heated on the mounting-plate ; and some small cuttings of
marine-glue are then to be placed, either upon that surface of the
cell which is to be attached, or upon that portion of the slide on
which it is to lie, the former being perhaps preferable. When
they begin to melt, they may be worked over the surface of attach-
ment by means of a needle-point ; and in this manner the melted
glue may be uniformly spread, care being taken to pick out any of
the small gritty particles which this cement sometimes contains.
* An improvement on the ordinary form of Mounting-Plate has been
described by Mr. Freestone in " Transact, of Microsc. Society," Vol. xii. p. 46.
CEMENTING WITH MAEINE-GLUE. 239
When the surface of attachment is thus completely covered with
liquefied glue, the cell is to be taken up with a pair of forceps,
turned over, and deposited in its proper place on the slide ; and it
is then to be firmly pressed down with a stick (such as the handle
of the needle), or with a piece of flat wood, so as to squeeze out
any superfluous glue from beneath. If any air-bubbles should be
seen between the cell and the slide, these should if possible be got
rid of by pressure, or by slightly moving the cell from side to side ;
but if their presence results, as is sometimes the case, from de-
ficiency of cement at that point, the cell must be lifted off again,
and more glue applied at the required spot. Sometimes, in spite
of care, the glue becomes hardened and blackened by overheating ;
and as it will not then stick well to the glass, it is preferable not
to attempt to proceed, but to lift off the cell from the slide, to let
it cool, and then to repeat the process. When the cementing has
been satisfactorily accomplished, the slides should be allowed to
cool gradually, in order to secure the firm adhesion of the glue ;
and this is readily accomplished, in the first instance, by pushing
each, as it is finished, towards one of the extremities of the plate,
which is of course cooler than the centre. If two plates are in use,
the heated plate may then be readily moved away upon the ring
which supports it, the other being brought down in its place ; and
as the heated plate will be some little time in cooling, the firm
attachment of the cells will be secured. If, on the other hand,
there be only a single plaf e, and the operator desire to proceed at
once in mounting more cells, the slides already completed should be
carefully removed from it, and laid upon a wooden surface, the
slow conduction of which will prevent them from cooling too fast.
Before they are quite cold, the superfluous glue should be scraped
from the glass with a small chisel or awl ; and the surface should
then be carefully cleansed with a solution of Potash, which may be
rubbed upon it with a piece of rag covering a stick shaped like a
chisel. The cells should next be washed with a hard brush and
soap and water, and may be finally cleansed by rubbing with a little
weak spirit and a soft cloth. In cases in which appearance is not
of much consequence, and especially in those in which the cell is
to be used for mounting large opaque objects, it is decidedly pre-
ferable not to scrape off the glue too closely round the edges of
attachment ; as the ' hold ' is much firmer, and the probability of
the penetration of air or fluid much less, if the immediate margin
of glue be left both outside and inside the cell.
170. Mounting Objects Dry. — There are certain objects which,
even when they are to be viewed by transmitted light, are more
advantageously seen when simply laid on glass, than when they
are immersed either in fluid or in balsam. This is the case espe-
cially with sections of bones and teeth, much of whose internal
structure is obliterated by the penetration of fluid ; and also with
the scales of Lepiclopterous and other Insects, whose minute sur-
face-markings are far more distinct when thus examined, than
240 MOUNTING OF OBJECTS.
when treated in any other way. For preserving such objects, it is
of course desirable that they should be protected by a cover ; and
this must be so attached to the glass slide as to keep the object in
place, besides being itself secured. For this purpose, Sealing-wax
varnish is often used", but it is unsuitable on account of its brittle-
ness when dry ; Brunswick Black or Gold- Size mixed with Lamp-
black is much to be preferred, and, if carefully laid on, will not
tend to run in between the cover and the slide. If the object have
any tendency to curl up, or to keep off the cover from the slide by
its own ' spring,' it will be useful, while applying the varnish, to
make use of pressure, such as that afforded by the Spring- Clip*
represented in Fig. 110 ; and this pressure should not be remitted
until the varnish is dry enough to
Fig. 110. hold down the cover by itself. Where
*b=i^r__j, the object is thin, and not liable to
be injured by a gentle heat, the best
/trn— ^S=^\ ~I/~\ method is to use a Cement-cell (§ 184)
^"^ll^^^Br^ thoroughly hardened ; and after the
]g) C^^^¥\ object has been placed in it, and its
V ^ \ cover laid on, the slide is warmed
\ \ sufficiently to soften the ring of
* Cement, on which the cover is then
Spring-Clip. carefully pressed down, so as at the
same time to attach itself and to
fix the object. For mounting delicate objects, the thinner slides
should be selected ; and for very difficult Test-objects, it is advan-
tageous to employ thin glass below as well as above the specimens,
for the sake of diminishing the aberration which the illuminating
pencil sustains in its passage to the object, and for allowing the
Achromatic Condenser to approach the object as closely as pos-
sible. For this purpose the simplest method is to take a slip of
Wood (preferably either mahogany or cedar) of the ordinary size
of the glass slide (3 in. by 1 in.), with a central aperture of from
3 to 5-8ths of an inch ; to cover this aperture with a ' square'
or ' round' of thin glass of sufficient size to project considerably
beyond it ; to lay the object upon this glass, and to protect it
with a cover of rather smaller size, which should be fastened down
all round by varnish to prevent the entrance of moisture ; and
finally to secure both glasses to the wooden slide, by gumming down
over them a piece of paper of the same size as that of the slide
itself, with a perforation for the object.
171. For dry-mounting Opaque objects, the method adopted
must vary with the mode in which the object is to be illuminated.
If a Side- Condenser or Parabolic Beflector is to be employed, which
is the most appropriate method for the great majority of objects,
the whole slide may be opaque ; and the following simple plan
* This very useful little implement is an improvement by Mi*. Jabez Hogg
upon a form originally devised by Dr. Maddox. It is sold at a very cheap rate
by Messrs. Baker, Mr. Collins, and other dealers in Microscopic Apparatus.
DRY-MOUNTING OPAQUE OBJECTS. 241
devised by the Author (whose entire collection of Foraminifera is
thus mounted) will be found to afford peculiar conveniences. Let
there be provided a Wooden slide of the kind just described, a
piece of card of the same dimensions, and a piece of dead-black
paper, rather larger than the aperture of the slide, if a dark
mounting be desired, which is preferable for most objects : this
piece of paper is to be gummed to the middle of the card, and then,
some stiff gum having been previously spread over one side of the
slide (care being taken that there is no superfluity of it imme-
diately around the aperture), this is to be laid down upon the
card, and subjected to pressure.* An extremely neat ' cell ' will
thus be formed for the reception of the object (Fig. Ill), the depth
of which will be determined
by the thickness of the slide, Fig. 111.
and the diameter by the size y
of the perforation ; and it /
will be found convenient to /
provide slides of various /
thicknesses, with apertures '
of different sizes. The Cell Wooden Slide for Opaque Objects,
should always be deep enough
for its wall to rise above the object : but, on the other hand, it should
not be too deep for its walls to interfere with the oblique incidence
of the light upon any object that may be near its periphery. The
Object, if flat or small, may be attached by ordinary Gum -mucilage ;f
if, however, it be large, and the part of it to be attached have an
irregular surface, it is desirable to form a ' bed ' to this by Gum
thickened with Starch. If, on the other hand, it should be desired
to mount the object edgeways (as when the mouth of a Foraminifer
is to be brought into view), the side of the object may be
attached with a little gum to the wall of the cell. — The complete
protection thus given to the Object is the great recommendation of
this method. But this is by no means its only convenience. It
allows the slides not only to range in the ordinary Cabinets, but
also to be laid one against or over another and to be packed closely
in cases or secured by elastic bands ; which plan is extremely conve-
nient not merely for the saving of space, but also for preserving the
objects from dust. Should any more special protection be required,
* It will be found a very convenient plan to prepare a large number of such
slides at once : and this may be done in a marvellously short time, if the slips
of card have been previously cut to the exact size in a bookbinder's press.
The slides, -when put together, should be placed in pairs, back to back ; and
every pair should have each of its ends embraced by a Spring-Press (Fig. 114)
until dry.
f It will be found very advantageous for almost every purpose, to add about
l-10th part of Glycerine to thick Gum-mucilage ; for the gum is thereby pre-
vented from hardening so completely as to become brittle, and the bodies
attached by it are less likely to be separated by a jarring shock ; whilst, on the
other hand, if it should be desired to remove the object from the slide, the gum
is more readily softened and dissolved by the addition of a drop of water.
242 MOUNTING OF OBJECTS.
a Thin Glass coyer may be laid over the top of the cell, and seenred
there either by a rim of gum or by a perforated paper cover
attached to the slide ; and if it should be desired to pack these
covered slides together, it is only necessary to interpose guards of
card somewhat thicker than the glass covers. In cases in which it
is desired to retain the power of examining the object without the
intervention of a glass cover, a thin disk of Bone or Vulcanite may
be attached to the slide (as suggested by Mr. Piper, " Trans, of
Microsc. Soc," Yol. xv. p. 18) by means of a split metal rivet
passing through a hole near its edge, and attached to the slide
near the edge of the cell by clenching it on the under side before
the cardboard-bottom is attached. The rivet acts as a pivot oil
which the disk turns, so that it may either cover the cell or may
be moved to one side ; and the disk may be conveniently made to
carry a label for the description of the object.* For objects which it
is desired to examine under different aspects, Morris's Object-holder
(Fig. 84) will be found very convenient : full advantage can only
be taken of this, however, when the objects are mounted on de-
tached disks ; and in such cases Beck's Dish-holder (Fig. 83) is
decidedly preferable.
172. Objects to be viewed by Lieberkiihn illumination, however,
require a different mode of mounting, in order that the light may
be allowed to pass up around them from the mirror to the speculum.
If they are of moderate size, the Wooden slide may still be conve-
niently employed for them, its aperture being made as large as it
will bear, and its cardboard-bottom being replaced by a thin ordi-
nary glass slide ; and the object may either be mounted on a small
disk punched out of blackened card, or it may be attached directly
to the glass, to the under side of which a spot of black varnish or
a disk of black paper should be then affixed. Small and delicate
objects, however — such as Diatoms and Polycystina — are best
mounted on small disks of thin blackened card attached to Glass
slides ; being protected either by Ring-cells (§ 187) of Glass, Metal,
or Yulcanite,f or by perforated disks cut with punches of suitable
size out of cardboard or kid-leather, which, having been repeatedly
brushed over with Liquid Glue, are attached to the slide, and have
their covers affixed to them with the same material.
173. Mounting Objects in Canada Balsam. — This method of
mounting is suitable to a very large proportion of those Objects
which are to be viewed by transmitted light, and whose texture is
not affected by the loss of the aqueous fluid they may contain ;
and it has many advantages over the mounting of the like objects
dry. For, in the first place, as it fills-up the little inequalities of
* Disks and rivets for this purpose are procurable from Messrs. Baker.
t Ring-cells cut in a lathe from Gutta-percha tubing have been proposed for
this purpose ; but they do not adhere permanently to glass ; and cells of Vul-
canite made in the same manner are greatly to be preferred. Cells cut off
from Pasteboard tubing may also be employed, if treated with Liquid Glue as
mentioned above.
MOUNTING OBJECTS IN CANADA BALSAM. 243
their surface, even where it does not actually penetrate their sub-
stance, it increases their transparence by doing-away with irre-
gular refractions of the light in its way through them, and gives
them the aspect of perfect smoothness ; this is well seen in the
case of sections of Shell, &c, which, when thus mounted, do not
require a high polish (§ 156). But, secondly, where the structure,
although itself hard, is penetrated by internal vacuities, the Balsam,
by filling these, prevents that obscuration resulting from the inter-
position of air-spaces, and from additional internal surfaces of
reflection, by which the transmitted rays are distorted, and a large
proportion of them lost : this is well seen in the case of the Fora-
rninifera, and of sections of the 'test ' and ' spines ' of Echinicla,
whose intimate structure can be far better macle-out when they
are thus mounted, than when mounted dry, although their sub-
stance is (for the most part at least) itself so dense, that the
balsam cannot be imagined to penetrate it ; and likewise with dry
Vegetable preparations, which are perhaps also affected in the
manner to be next described. Thirdly, there are very many struc-
tures of great interest to the Microscopist, whose appearance is
extraordinarily improved by this method of mounting, in conse-
quence of a specific effect which the Balsam has in combining (so
to speak) with their component elements, so as to render them far
more transparent than before : this effect is seen in the case of all
dry preparations of In sect -structure, especially of such as consist of
their hard external tegument or of parts derived from this ; also in
the various Horny tissues (hairs, hoof, horn, &c.) of the higher
animals ; and likewise in many organized substances, both recent
and fossil, which are penetrated by Calcareous matter in an amor-
phous condition. — -Besides these advantages, the mounting of
objects in Canada balsam affords one of the easiest methods of
fixing and preserving them ; and consequently it may be almost
always had recourse-to in the case of such transparent objects as do
not need to be preserved in fluid, save where, in virtue of the action
just described, it impairs the distinctness of surface-markings, or
obliterates internal cavities or canals, which constitute the most
important features of the object.
174 Canada Balsam, being nothing else than a very pure
Turpentine, is a natural combination of Besin with the Essential
Oil of Turpentine. In its fresh state it is a viscid liquid, easily
poured out, but capable of being drawn into fine threads ; and
this is the condition in which the Microscopist will find it most
desirable to use it for the mounting of objects generally. The
Balsam may be conveniently kept in a glass bottle or jar with a
wide mouth, being taken up as required with a small glass rod
drawn to a blunt point, such as is used by Chemists as a ' stirrer ;'
and if, instead of a cork or stopper, this bottle should be provided
with a tall hollow ' cap,' the glass rod may always stand in the
Balsam with its upper end projecting into the cap. In taking out
the Balsam, care should be taken not to drop it prematurely from
fi'2
2U MOUNTING OF OBJECTS.
the rod, and not to let it come into contact with the interior of the
neck or with the month of the jar : both these mischances may
be avoided by not attempting to take-up on the rod more than it
will properly carry, and by holding it in a horizontal position
after drawing it out from the bottle, until the slip on which it is to
deposit the Balsam is just beneath its point. Some recommend
that the Balsam should be kept in the Tin tubes used for Artists'
colours ; but the screw-caps of these are liable to be fixed by the
hardening of the contents : and the Author has himself been in the
habit of employing in preference a Syringe, resembling that repre-
sented in Fig. 96, but with a freer opening. This is most readily
filled with Balsam, in the first instance, by drawing out the piston
and pouring-in balsam previously rendered more liquid by gentle
warmth ; and nothing else is required to enable the operator at any
time to expel precisely the amount of balsam he may require, than
to warm the point of the syringe, if the balsam should have hardened
in it, and to apply a very gentle heat to the syringe generally, if
the piston should not then be readily pressed down. "When a
number of Balsam- Objects are being mounted at one time, the
advantage of this plan in regard to facility and cleanliness (no
superfluous balsam being deposited on the slide) will make itself
sensibly felt. It has, moreover, the further recommendation of
keeping the balsam almost perfectly excluded from the air ; the only
contact between them being at its point, where the balsam soon
hardens so as to protect what is within. — When Balsam has been
kept too long, it becomes, through the loss of part of its volatile
oil, too stiff for convenient use, and may be thinned by mixing it
at a gentle heat with pure Oil of Turpentine ; this mixture, how-
ever, does not produce that thorough incorporation of the consti-
tuents which exists in the fresh Balsam ; and it is consequently
preferable to use in other ways the balsam which has become
somewhat too stiff, and to have recourse to a fresh supply of liquid
balsam for mounting-purposes. — In cases in which the Object
might be injured by the heat required to soften the Balsam, it may
be mounted in a solution of thickened Balsam in Chloroform, from
which the volatile solvent will evaporate in a few hours. — For
mounting very delicate objects, it is advantageous to dissolve
Canada Balsam, first hardened by evaporation, in Benzine. This
solution dries less quickly than the chloroform solution, but more
quickly than that of balsam in turpentine. The Benzine must
be added cautiously ; as, when a certain point of dilution is reached,
the mixture thins very rapidly. This solution should not be used
until its components are thoroughly incorporated. — When Canada
Balsam is to be employed as a cement, as for attaching sections, &c,
to glass-slides (§ 155), it should be in a much stiff er condition ;
since, if it be dropped on the slide in too liquid a state, it will
probably spread much wider and will lie in a thinner stratum
than is desirable. This hardening process may be carried to any
extent that may be desired, by exposing the Balsam in an uncorked
MOUNTING OBJECTS IN CANADA BALSAM. 245
jar (the mouth of which, however, should be covered with paper for
the sake of keeping off dust) to a continual gentle heat, such as
that of a water-bath.
175. In mounting Objects in Canada Balsam, it is convenient to
be provided with certain simple instruments, the use of which will
save much time and trouble. — For the heat required, a Spirit
Lamp is by far the best source, both as admitting of easy regula-
tion, and as being perfectly free from smoke. — Where a number of
Objects are being mounted on the same occasion, it will be found
convenient to employ either a water-bath covered with a flat plate
of metal, or a similar metal plate supported at such a distance
above the lamp-flame (§ 169) as not to become more heated than
it would be through a water-bath* — For holding the slide whilst
it is either being heated over the flame or is being subsequently
cooled, and at the same time applying a gentle pressure to the
covering-glass, an ingenious and convenient Mounting Instrument
has been devised by Mr. James Smith. This consists of a plate of
brass turned up at its edges, of the proper size to allow the ordi-
nary glass slide to lie loosely in the bed thus formed ; this plate
has a large perforation in its centre, in order to allow heat to be
directly applied to the slide from beneath ; and it is attached by a
stout wire to a handle (Fig. 112). Close to this handle there is
Fig. 112.
Smith's Mounting Instrument.
attached by a joint a second wire, which lies nearly parallel to the
first, but makes a downward turn just above the centre of the
slide-plate, and is terminated by an ivory knob ; this wire is
pressed upwards by a spring beneath it, whilst, on the other
hand, it is made to approximate the other by a milled-head
turning on a screw, so as to bring its ivory knob to bear with
greater or less force on the covering glass. The use of this
arrangement will be presently explained. — If such a mounting
* Mr. Frederick Marshall has infomied the Author that he has found the
following very simple apparatus extremely convenient : — A Water-Bath made
of tin, of such a size and shape as to afford a flat Stage for laying the slide
upon, and also to receive into its interior a wide-mouthed bottle holdirjg the
balsam. If this bath be filled with boiling water, the balsam is liquefied
without the risk of the formation of air-bubbles ; and the slide also is kept
sufficiently warm during the mounting process. One supply of hot water will
serve thus to mount from 12 to 20 objects. By marking on the Stage the
outline of the slide and its central point, the right spot for laying the object
upon the glass is indicated.
246
MOUNTING OF OBJECTS.
instrument be not employed, the wooden Slider-Forceps of Mr.
Page (Fig. 113) will be found extremely convenient; this, by its
Fig. 113.
Slider-Forceps.
elasticity, affords a secure grasp to a slide of any ordinary thick-
ness, the wooden blades being separated by pressure upon the brass
studs ; and the lower stud, with the bent piece of brass at the
junction of the blades, affords a level support to the forceps, which
thus, while resting upon the table, keeps the heated glass from
contact with its surface. This instrument will be found par-
ticularly useful when the balsam has to be hardened on the slide,
for the purpose of cementing to it bodies of which thin sections
are to be made. — Besides a pair of fine-pointed steel Forceps for
holding the object to be mounted, there should be another of a
commoner kind for taking-up the glass cover, the former being
liable to be soiled with balsam. — A pair of stout Needles mounted
in handles (§ 151) will be found indispensable, both for manipu-
lating the object, and for breaking or removing air-bubbles ; and
if these handles be cut to a flat surface at the other extremity, they
will serve also to press-down the glass covers, for which purpose
a pointed stick also is useful. — For holding- down these covers
whilst the balsam is cooling, if the elasticity of the objects should
tend to make them spring-up, such as are not provided with
the Mounting Instrument above described may advantageously
employ the Spring Clip (Fig. 110) ; or, if its pressure is not firm
enough, recourse may be had to a simple Spring-press made by a
slight alteration of the ' American clothes-peg' which is now in
general use in this country for a variety of purposes ; all that is
necessary being to rub down
the opposed surfaces of the
' clip' with a flat-file, so that
they shall be parallel to
each other when an ordi-
nary slide with its cover is
interposed between them
(Fig. 114). This contri-
vance, however, is defec-
tive in not allowing of the
graduated pressure which
may be made by the Mounting Instrument. — Great care should
be taken to keep these implements free from soils of Balsam ;
since the slides and glass-covers are certain to receive them. The
Spring Press.
MOUNTING OBJECTS IN CANADA BALSAM. 247
readiest mode of cleansing the Needles (their ' temper' being a
matter of no consequence for these purposes) is to heat them
red-hot in the lamp, so as to burn-off the balsam; and then
carefully to wipe them. The Forceps, both of wood and of metal,
should be cleansed with Oil of TurjDentine or with Methylated
Spirit.
176. Much of the success of mounting Objects in this mode will
depend upon their previous preparation. Such hard objects as
sections of Shells or Echinus-spines, should be first well cleansed
with water, and should then be thoroughly dried. Insect structures,
on the other hand, are best macerated for some time in Oil of Tur-
pentine, which will remove any greasiness they may contain, and
will at the same time increase their transparence. When Forami-
nifera are to be mounted in Canada Balsam, long-continued
maceration in Oil of Turpentine generally causes its entrance into
their cavities ; so that as the Turpentine is afterwards replaced by
the Balsam, air-bubbles (of which it is otherwise very difficult to
get rid) are avoided. "Not only dry but moist objects (such as
Fish-scales, Tongues of Mollusks, or Injected preparations) may be
mounted in Canada Balsam, by soaking them successively for ten
or fifteen minutes in Alcohol, Pyroxylic spirit, and Oil of Turpen-
tine ; the Water they at first contained being finally replaced by the
last of these menstrua, which in its turn gives place to the Balsam.
— In mounting an ordinary Object, a sufficient quantity of liquid
balsam should be laid in* the centre of the slide ; this should be
warmed but not boiled ; and any air-bubbles which may make
their appearance should either be caused to burst by touching them
with the needle-point, or should be drawn to one side. The object,
if it can be held in the fine-pointed forceps, should then be plunged
into the drop of balsam ; and, if it be not completely covered, a
little more balsam should be applied over it, care being taken, as
before, to prevent over -heating, and to get rid of the bubbles as
they rise. — In mounting minute Balsam-objects, such as Diatoms,
Polycystina, or Sponge-spicules, and even objects of larger size,
provided they be not of unusual thickness, great advantage will
be obtained from following the plan suggested by Mr. James Smith,
for which his Mounting Instrument (Fig. 112) is specially adapted.
The slide being placed upon its slide-plate, and the object having
been laid upon the glass in the desired position, the covering-glass
is very gently laid upon this, and the ivory knob is to be brought
down so as by a very slight pressure on the cover to keep it in its
place. The slide is then to be very gently warmed, and the Balsam
to be applied (which may be most conveniently done by means of
the glass Syringe, § 174) at the edge of the cover, from which it
will be drawn-in by capillary attraction, leaving no bubbles if too
much heat be not applied. In this manner the objects are kept
exactly in the places in which they were at first laid ; and scarcely
a particle of superfluous balsam, if due care has been employed.,
remains on the slide. The solution of Canada Balsam in Chloro-
248 MOUNTING OF OBJECTS.
form or Benzine (§ 1 74) may be applied in the same manner without
heat. — If the object contain numerons large air-spaces with free
openings, and be one whose texture is not injured by heat,
the air may often be got-rid-of by boiling it in the Balsam ; for the
heat, causing the air to expand, drives-out a large proportion of it ;
this will be replaced, if it be allowed partly to cool, by the entrance
of balsam ; and then, by a second heating, the balsam being boiled
within the cavities, its vapour expels the remaining air, and, on
the condensation of the vapour, the liquid balsam runs -in and takes
its place. For this method to succeed, however, it is essential that
the balsam be prevented from becoming hard through boiling, by the
addition of fresh liquid balsam, from time to time ; and it will often
be found that, should vacuities remain which boiling does not remove,
these contract or altogether disappear if the slide be kept for a few
days at a gentle heat, the semi-fluid balsam being gradually forced
into their place by the pressure of the surrounding air. There are
many textures, however, which are extremely injured by a very
slight excess of heat, having a tendency to curl-up and to become
stiff and brittle ; and the objects containing these are at once spoiled
by boiling them in balsam. In such cases it is much better to have
recourse to the assistance of the Air-pump ;* for by placing the
slide, with the object immersed in very liquid balsam, upon a tin
or copper vessel filled with hot water, under the receiver, and then
exhausting this, the air-bubbles will be drawn-forth, and, on the
re-admission of the air, the balsam will be forced by its pressure
into the place which they occupied. Some objects, however, retain
the air with such tenacity as to require the repetition of the ex-
hausting process two or three times ; and in this case it is prefe-
rable to use Camphine or Oil of Turpentine instead of balsam, on
account of its greater fluidity, and to warm even this to a tempera-
ture of about 100°. — There are certain cases, on the other hand, in
which it is desirable to retain, instead of expelling, the Air contained
within the cavities of the object. Thus, if minute Insects (such as
Fleas) be disj)layed as transparent objects to show the ramifications
of the Tracheae, or if it be wished that a section of Tooth or Bone
should be so mounted in balsam as to exhibit its canaliculi, the
previous maceration in Oil of Turpentine should be never employed,
and the Balsam employed should be some which has been previously
hardened ; this being melted without the use of more heat than is
necessary, the object should be surrounded by it and the cover put
on as quickly as possible ; and the slide should then be laid upon
a surface of stone or metal, the good conducting power of which,
by causing the balsam to cool rapidly, diminishes its tendency to
penetrate the substance of the object. — If a deep cell has to be
filled with Canada Balsam, it is better to fill it in the first in-
stance with Oil of Turpentine, and to immerse the specimen in
* Small Air-pumps, with a plate and receiver specially adapted for mounting
purposes, are made by Mr. Baker and Mr. Collins.
MOUNTING OBJECTS IN CANADA BALSAM. 249
this ; liquid balsam being poured npon the object at one end, the
Turpentine is to be allowed to flow out at the other by inclining
the slide ; then by laying the glass cover on one edge of the
cell, and gradually lowering it until it lies flat, air may be entirely
excluded.
177. When the Object is already attached to the Glass slide, the
mounting in Canada Balsam is usually a matter of very little diffi-
culty. If it be a soft tissue which has been spread-out and allowed
to dry upon the glass for the purpose of securing it in its place, all
that is necessary in the first instance is to dry it thoroughly, to shave
or scrape it with a sharp knife if it should seem too thick, and to
moisten its surface with Oil of Turpentine if it should not readily
' take ' the balsam. The slide is then very gently warmed, a
sufficient quantity of Balsam is spread over the surface of the speci-
men, with due care that it is ' taken ' in every point, and the glass
cover is put-on. If the preparation cover a large area, great care
should be taken in letting-down the cover gradually from one side,
so as to drive a wave of balsam before it which shall sweep away
air-bubbles ; raising it a little, and introducing a small quantity
of fresh balsam, if any vacuity present itself as it descends. — The
preferable mode of mounting thin sections of hard bodies, however,
will depend in great degree upon the size of the section and the
tenacity of its substance. "Where its area is great and its texture
brittle, its removal from the glass on which it has been ground-down
to another slip cannot be accomplished, even by the most dexterous
management, without considerable risk of breaking it; and al-
though, by the friction of the glass upon the stone, the surface of
the slide will probably have been scratched or roughened, yet this
is a dis- sight about which the scientific Microscopist will care but
little, as it only affects the saleable value of such objects. Nothing
more will in this case be necessary, than to lay some liquid Balsam
on the surface of the section, to warm it gently, and then to
place on it a thin-glass cover of suitable dimensions, gently pressing
this down wherever the balsam happens to be thickest, and en-
deavouring to drive all air-bubbles before a wave of liquid, until
they are entirely expelled, or at any rate are driven beyond the
margin of the section. If this operation be not at once successful, —
either a few large air-bubbles, or a great number of smaller ones,
which cannot be got-rid-of by gentle pressure, being visible between
the surface of the section and the covering-glass, — it is better at
once to remove the cover by gentle warmth applied to its upper sur-
face, and to repeat the operation with an additional supply of balsam,
rather than to attempt to drive -out the bubbles by any manipula-
tion. Whatever treatment be adopted, special care should always
be taken not to apply so much heat as to melt the hard balsam be-
neath the section, or to boil the thin balsam above ; and this may
be best managed by turning the slide with its face downwards, so
that the heat may be applied directly to the thin-glass cover and
to the balsam in contact with it, instead of acting on this through
25GT MOUNTING OF OBJECTS.
tlie slide and the object attached to it. If the heat should unfor-
tunately be carried so far as to boil the cement beneath the section,
there will be little chance, if its area be large, of getting-rid of the
bubbles thus produced, without removing it altogether from the
glass to which it was attached, or, at any rate, without pushing it
along the glass in such a way as to slide it away from the bubbles ;
in that case, the part towards which it is moved should always be
well supplied with balsam, and the bubbles that remain should be
drawn away or broken with the needle-point; after which, the
section being slid-back to its original position, it is probable that
no bubbles may be found beneath it. — In cases, however, in which
the appearance of the preparation is an object of much considera-
tion, and in which the tenacity of the substance and the small size
of the section prevent much risk of its breaking in the transfer, it
may be loosened from the glass to which it was first attached, either
by heat, or by soaking in Ether or Chloroform. The former, being
the simplest and readiest method, is the one most commonly prac-
tised ; the only difficulty lies in lifting-off the specimen without
breaking it ; and this may best be done by means of a camel's hair
brush dipped in Oil of Turpentine. The glass to which the section
is to be transferred should have a large spot of liquid balsam laid
in the proper place ; the object is to be laid on this, and its upper
surface covered with the like balsam ; and then, the thin-glass
cover being placed upon it, this is to be gently pressed down in the
manner already described. If Ether or Chloroform be had recourse
to, the slide should be placed in a wide-mouthed bottle of that
liquid, which should then be corked or stopped ; and after a time
the section will be found lying detached in it, whence it may be
taken-up either by the forceps or by a camel's hair brush. — Such
a transfer will often be found advantageous before the final com-
pletion of the reducing process ; for it will occasionally happen that
we find something in the structure of the specimen, which will be
best displayed by rubbing it down afresh on the side first attached
to the glass ; and, when a number of small sections are being made
at once (which it is often very convenient to do, not only in the
case already mentioned, § 154, but in many others), it not only
saves time, but ensures the accurate flattening of the surface in
grinding, to fix several upon the same slip, and to work them down
together until the requisite thinness has been nearly attained, when
they must be transferred to separate slips, and finished one by
one. In either case, the re-attachment must of course be made,
like the original attachment, with Balsam which has been first
hardened (§ 155).
178. When the Balsam employed in mounting has remained in
the liquid condition here recommended, the glass cover will not be
secure from displacement until the balsam has become harder.
This change it will require a long time to undergo, unless the aid
of a gentle continuous warmth be afforded. Nothing is more
suitable for this purpose than the warmth of a Chimney-piece im-
CANADA BALSAM:— GUM DAMAE. 251
mediately above the fire-place ; as it is quite sufficient to produce
the effect in the course of a few days, whilst there is no danger of
its becoming excessive ; but in default of this convenience, an
oven carefully regulated, or (still better) a water-bath, may be
employed. Whether either of these means be adopted, or the
slides be put aside for the Balsam to be hardened by time, they
should always be laid in the horizontal position, that their covers
may not be caused by gravitation to slip down from their places.—-
It may be better, before submitting the slides to this hardening
process, to scrape from their surface any superfluous Balsam that
does not immediately surround the glass-cover; but the knife
should never be carried so near to the edge of this as to run any
risk of displacing it ; and it is much better to defer the final
cleaning of the slide until the attachment of the cover has become
firm. The remaining Balsam may then be scraped away with a
knife or small chisel, the implement being warmed if the balsam,
be very stiff ; the slide should be rubbed with a rag dipped in Oil
of Turpentine until every perceptible soil of balsam is removed,
especial care being taken to cleanse the surface and edges of the
glass-cover : and as this will itself leave a certain resinous film, it
is better to give the slide a final cleansing with Methylated Spirit.
If its surface should have been considerably smeared with balsam,
it is very convenient, after scraping away all that can be removed
in that manner, to scrub it with a soft tooth-brush or an old nail-
brush, first letting fall on it a few drops of Turpentine or Methylated
Spirit ; and there is less risk of displacing the glass-cover in this
mode, than in rubbing it any other way. — The menstrua which
serve thus to cleanse the slides, of course answer equally well
for cleansing the hands. The most ready solvents for Balsam
are Ether and Chloroform ; but the ordinary use of these being
interdicted by their costliness, and by the quickness with
which they are dissipated by evaporation, Alcohol, Methylated
Spirit, Wood Naphtha, or Oil of Turpentine may be used in
their stead.
179. Gum Damar. — A solution of Gum Damar is much used
both here and on the Continent, for many objects which require a
more delicate or less refractive medium than Canada balsam.
One of the formulae for this preparation is as follows : —
A. Gum Damar i oz.
Oil of Turpentine 1 „
Dissolve and filter.
B. Gum Mastic J oz,
Chloroform 2 „
Dissolve and filter ; add A to B. When thickened by drying,
this may be used as a coating for cells.
Diatoms mounted in the Damar solution are shown better than
in Canada balsam. This solution (which may be obtained from
252 MOUNTING OF OBJECTS.
Mr. Baker) lias been found very suitable for preserving delicate
physiological preparations, especially transparent injections.
180. Bisulphide of Carbon. — Mr. Stephenson has obtained
excellent results from mounting Diatoms in bisulphide of carbon.
Its high refractive power, considerably greater than that of the
diatoms, allows structure that is more or less concealed by Canada
balsam, to be clearly seen. The bisulphide can now be obtained in
a purer state than was formerly known, and with a great reduction
of the disagreeable odour that made its use very unpleasant. The
cement for cells, or for the edges of the covering-glass, to prevent
its escape, can be obtained of Mr. Browning.
181. Preservative Media. — Objects which would lose their
characters in drying, and which cannot be suitably mounted in
Canada Balsam, can of course only be preserved in anything like
their original condition by mounting in fluid ; and the choice of
the fluid to be employed in each case will depend upon the cha-
racter of the object and the purpose aimed-at in its preservation.
As specific directions will be given hereafter in regard to most of
the principal classes of Microscopic preparations, little more will
be required in this place than an enumeration of the preserva-
tive Media, with a notice of their respective qualities. — For very
minute and delicate Vegetable objects, especially those belonging
to the orders Desmidiacese and Diatomacese, nothing seems to
produce less alteration in the disposition of the endochrome, or
serves better to preserve their colour, than Distilled Water ; pro-
vided that, by the complete exclusion of air, the vital processes
and decomposing changes can be alike suspended. This method
of mounting, however, is liable to the objection that Confervoid
growths sometimes make their appearance in the preparation,
which may be best prevented by saturating the water with camphor,
or shaking it up with a few drops of creosote, or (if the preserva-
tion of colour be not an object) by adding about a tenth part of
alcohol, or (where the loss of colour would be objectionable) by
dissolving a grain of alum and a grain of bay-salt in an ounce of
water. — For larger preparations of Algee, &c, what is called
Thwaites's Fluid may be employed ; this is prepared by adding to
one part of Eectified Spirit as many drops of Creosote as will satu-
rate it, and then gradually mixing up with it in a pestle and
mortar some prepared Chalk with 16 parts of "Water ; an equal
quantity of Water saturated with Camphor is then to be added,
and the mixture, after standing for a few days, is to be carefully
filtered. A liquid of this kind also serves well for the preserva-
tion of many Animal preparations, but becomes turbid when thus
employed in large quantity ; and the following modification is
recommended by Dr. Beale. Mix 3 drachms of Creosote with 6
ounces of Wood-Naphtha, and add in a mortar as much prepared
Chalk as may be necessary to form a smooth thick paste ; water
must be gradually added to the extent of 64 ounces, a few lumps
of Camphor thrown in, and the mixture allowed to stand for two or
PEESEEVATIVE MEDIA. 253
three weeks in a lightly-covered vessel, with occasional stirring ;
after which it should be filtered, and preserved in well- stoppered
bottles. — Of late years, diluted Glycerine has been much used as a
preservative fluid ; it allows the colours of Vegetable substances to
be retained, but, as usually employed, it alters the disposition of
the endochrome ; and conf ervoid growths are apt to make their
appearance in it. The best proportion seems to be one part of
Glycerine to two parts of Camphor-water. The following method
of using Glycerine, devised by Herr Hantzsch, of Dresden, is said
to be peculiarly effective for minute Yegetable preparations : — A
mixture is made of 3 parts of pure Alcohol, 2 parts of Distilled
Water, and 1 part of Glycerine ; and the object, laid in a cement-
cell, is to be covered with a drop of this liquid, and then put
aside under a bell-glass. The Alcohol and Water soon evaporate,
so that the Glycerine alone is left ; and another drop of the liquid
is then to be added, and a second evaporation permitted; the
process being repeated, if necessary, until enough Glycerine is left
to fill the cell, which is then to be covered and closed in the usual
mode.* — The preparation known as Dearie's Gelatine is one of the
most convenient media for preserving the larger forms of Confervas
and other Microscopic Algae, as well as sections of such as are still
more bulky. This is prepared by soaking 1 oz. of Gelatine in 4 oz.
of Water until the gelatine is quite soft, and then adding 5 oz. of
Honey previously raised .to boiling heat in another vessel ; the whole
is then to be made boiling hot, and when it has somewhat cooled,
but is still perfectly fluid, 6 drops of Creosote, and \ oz. of Spirit
of Wine, previously mixed together, are to be added, and the
whole is to be filtered through fine flannel. This composition,
when cold, forms a very stiff jelly, but it becomes perfectly fluid
on the application of a very slight warmth, and may then be used
like any other preservative liquid, care being taken, however, that
the slide and the glass cover are themselves gently warmed before
it comes into contact with them. The purpose which the honey
answers in this medium — that of preventing it from becoming too
hard — may be as well, or in some cases better, answered by
Glycerine ; and the Glycerine Jelly, prepared by the following
process (see Lawrance in " Quart. Journ. of Microsc. Science,"
Yol. vii. 1859, p. 257), may be very strongly recommended as good
for a great variety of objects, Animal as well as Yegetable, subject
to a caution to be presently given : — " Take any quantity of
Nelson's Gelatine, and let it soak for two or three hours in cold
water ; pour off the superfluous water, and heat the soaked gela-
tine until melted. To each fluid ounce of the Gelatine add one
drachm of Alcohol, and mix well ; then add a fluid drachm of the
* See the Eev. W. W. Spicer's "Handy-Book to the Collection and Prepara-
tion of Freshwater and Marine Algse, &c," pp. 57-59. " Nothing," says Mr.
Spicer, "can exceed the beauty of the preparations of Desmidiaeece prepared
after Herr Hantzsch's method; the form of the plant and the colouring of the
endochrome having undergone no change whatever."
254 MOUNTING OF OBJECTS.
white of an egg. Mix well while the Gelatine is fluid, bnt cool.
2sTow boil until the albumen coagulates, and the gelatine is quite
clear. Filter through fine flannel, and to each fluid ounce of the
clarified Gelatine add six fluid drachms of Price's pure Glycerine,
and mix well. For the six fluid drachms of Glycerine a mixture,
of two parts of Glycerine to four of Camphor-water may be sub-
stituted. The objects intended to be mounted in this medium are
best prepared by being immersed for some time in a mixture of
one part of Glycerine with one part of diluted Alcohol (1 of alcohol
to 6 of water)."* — For many objects which would be injured by
the small amount of heat required to melt either of the two
last-mentioned media, the Glycerine and Gum medium of Mr.
Farrants will be found very useful. This is made by dissolving
4 parts (by weight) of picked Gum Arabic in 4 parts of cold
Distilled Water, and then adding 2 parts of Glycerine. The
solution must be made without the aid of heat, the mixture
being occasionally stirred, but not shaken, whilst it is pro-
ceeding : after it has been completed, the liquid should be
strained (if not perfectly free from impurity) through fine cam-
bric previously well washed out by a current of clean cold water ;
and it should be kept in a bottle closed with a glass stopper
or cap (not with cork), containing a small piece of Camphor. The
great advantage of this medium is that it can be used cold, and
yet soon viscifies without cracking ; it is well suited to preserve
delicate Animal as well as Vegetable tissues, and in most cases
increases their transparence. — For the preservation of Micro-
scopic preparations of Animal structures, a mixture of one part of
Alcohol and five of Water will generally answer very well, save in
regard to the removal of their colours ; if it should have the effect
of rendering them opaque, this will be neutralized by the addition
of a minute quantity of Soda. A mixture of Glycerine and Cam-
phor-water in about the same proportion answers very well for
many objects, especially when it is desired to increase their trans-
parence, and it is more favourable than Diluted Alcohol to the
preservation of colour ; but in using this menstruum it must be
borne in mind that Glycerine has a solvent power for Carbonate of
Lime, and should not be employed when the object contains any
Calcareous structure.f For preserving very soft and delicate
marine Animals, such as the smaller Medusee and Annelida, the
Author has found a mixture of about one -tenth of Alcohol and the
* A very pure Glycerine jelly, of which the Author has made considerable
use, is prepared by Mr. Piirnmington, chemist, Bradford, Yorkshire.
t In ignorance of this fact, the Author employed Glycerine to preserve a
number of remarkably fine specimens of the Pentacrinoid larva of the Comatula
(Plate xxi.), whose colours he was anxious to retain ; and was extremely
vexed to find, when about to mount them, that their Calcareous skeletons had
so entirely disappeared that the specimens were completely ruined. This
result might perhaps be prevented, if the Glycerine were previously saturated
with Carbonate of Lime, by keeping it for some time in a bottle with chips of
Marble.
PEESEEVATIVE MEDIA. 255
game of Glycerine, with Sea-water, the most effectual in pre-
serving their natural appearance ; and the same mixture, with
increased proportions of alcohol and glycerine, answers very well
for larger objects. — For Zoophytes, and many other marine objects,
again, recourse may be advantageously had to Goadby's Solution,
which is made by dissolving 4 oz. of Bay-salt, 2 oz. of Alum, and 4
grains of Corrosive Sublimate, in 4 pints of boiling water : this
should be carefully filtered before it is used ; and for all delicate
preparations it should be diluted with an equal bulk, or even with
twice its bulk, of water. This solution must not be used where
any Calcareous texture, such as Shell or Bone, forms part of the
preparation ; and one of Mr. Goadby's other solutions (8 oz. of bay-
salt and 2 grs. of corrosive sublimate, to a quart of water, — or,
in cases where the coagulating action of Corrosive Sublimate on
Albuminous matters would be an objection, the substitution of 20
grains of Arsenious acid,) may be used in its stead. — Preparations
of the Animal Tissues to be examined as transparent objects under
high magnifying powers, may usually be advantageously mounted
either in Farrants's medium or in Glycerine-jelly. Carbolic Acid
has recently been employed as a preservative medium ; but the
Author has had no experience of its use. — It is often quite impos-
sible to predicate beforehand what Preservative Medium will answer
best for a particular kind of preparation ; and it is consequently
desirable, where there is no lack of material, always to mount the
same object in two or three different ways, marking on each slide
the method employed, and comparing the specimens from time to
time, so as to judge how each is affected. It may be stated, how-
ever, as a general rule, that objects to be viewed by light reflected
from their surfaces should not be mounted in either of the Gela-
tinous media, but in Diluted Alcohol, Goadby's Solution, or some
other liquid which does not tend to render them transparent.
Objects mounted in Gelatinous media, on the other hand, are often
shown admirably by Black-ground Illumination (§ 93).
182. Of Mounting Objects in Fluid. — As a general rule, it is
desirable that objects which are to be mounted in fluid should be
soaked in the particular fluid to be employed, for some little time
before mounting; since, if this precaution be not taken, air-
bubbles are very apt to present themselves. It is sometimes
necessary, in order to secure the displacement of air contained in
the specimen, to employ the Air-pump in the mode already
directed (§ 176) ; but it will generally be found sufficient to im-
merse the specimen for a few minutes in Alcohol (provided that
this does not do any detriment to its tissues), which will often
penetrate where water will not make its way ; and when the
spirit has driven out the air, the specimen may be removed back
to water, which will gradually displace the spirit. When Deane's
Gelatine or Glycerine- jelly is used, however, all that can be done
will be to drain the object of superfluous water before applying
the liquefied medium; but as air-bubbles are extremely apt to
256
MOUNTING- OF OBJECTS.
arise, they must be removed by means of the Air-pump, the Gela-
tine being kept in a liquid state by the nse of a vessel of hot
water, as in the case of Canada balsam. — In dealing with the
small quantities of fluid required in mounting Microscopic objects,
it is essential for the operator to be provided with the means of
transferring very small quantities from the vessel containing it to
the slide, as well as of taking up from the slide what may be lying
superfluous upon it. Where some one fluid, such as Diluted
Alcohol or Goadby's Solution, is in continual use, it will be found
very convenient to keep it in a small Bottle of the kind repre-
sented in Fig. 115, which is now in general use as a Dropping-
bottle. The stopper is perforated, and is
Fig. 115. elongated below into a fine tube, whilst it
expands above into a bulbous funnel, the
mouth of which is covered with a piece of
thin Vulcanized India-rubber tied firmly
round its lip. If pressure be made on this
cover with the point of the finger, and the
end of the tube be immersed in the liquid
in the bottle, this will rise into it on the
removal of the finger ; if, then, the funnel
be inverted, and the pressure be re -applied,
some of the residual air will be forced out,
so that by again immersing the end of the
tube, and removing the pressure, more fluid
Dropping Bottle. will enter. This operation may be repeated
as often as may be necessary, until the bulb
is entirely filled ; and when it is thus charged with fluid, as much
or as little as may be needed is then readily expelled from it by
the pressure of the finger on the cover, the bulb being always
refilled if care be taken to immerse the lower end of the tube
before the pressure is withdrawn. The Author can speak from
large experience of the value of this little implement ; as he can
also of the utility of the small Glass Syringe (§ 115) for the same
purpose.
183. There are many Objects of extreme thinness, which
require no other provision for mounting them in fluid than an
ordinary Glass slide, a Thin Glass cover, and some Gold- size or
Asphalte (§ 168). The object having been laid in its place, and a
drop of the fluid laid upon it (care being taken that no air-space
remains beneath the under side of the object and the surface of
the slide), the glass cover is then to be laid upon it, one side being
first brought into contact with the slide, and the other held up by
a needle-point, and gradually lowered in such a manner that the
air shall be all displaced before the fluid. If any air-bubbles
remain in the central part of the space between the cover and the
slide, the former must be raised again, and more fluid should be
introduced ; but if the bubbles be near the edge, a slight pressure
on that part of the cover will often suffice to expel them, or the
MOUNTING OBJECTS IN FLUID. 257
cover may be a little shifted so as to bring them, to its margin.
There are some objects, however, whose parts are liable to be
displaced by the slightest shifting of this kind ; and it is more
easy to avoid making air-bubbles by watching the extension of the
fluid as the cover is lowered, and by introducing an additional
supply when and where it may be needed, than it is to get rid of
them afterwards without injury to the object. When this end has
been satisfactorily accomplished, all that is needed is, first to
remove all superfluous fluid from the surface of the slide, and from
around the edge of the cover, with a piece of blotting-paper,
taking care not to draw away any of the fluid from beneath the
cover, or (if any have been removed accidentally) to replace what
may be deficient ; and then to make a circle of Gold- size or
Damar around the cover, taking care that it ' wets' its edges, and
advances a little way upon its upper surface. When this first coat
is dry, another should be applied, particular care being taken that
the cement shall fill the angular furrow at the margin of the cover.
In laying on the second coat, it will be convenient, if the cover be
round, to make use of the Turn-table (Fig. 116) ; and if the slide
be so carefully laid upon it that the glass-cover is exactly con-
centric with its axis, the turn-table may be used even for the first
application of the varnish, though a slight error in this respect
may occasion the displacement of the cover. — By far the greater
number of preparations which are to be preserved in liquid, how-
ever, should be mounted in a Cell of some kind, which forms a
well of suitable depth, wherein the preservative liquid may be
retained. This is absolutely necessary in the case of all objects
whose thickness is such as to prevent the glass-cover from coming
into close approximation with the slide ; and it is desirable when-
ever that approximation is not such as to cause the cover to be
drawn to the glass-slide by capillary attraction, or whenever the
cover is sensibly kept apart from the slide by the thickness of any
portion of the object. Hence it is only in the case of objects of
the most extreme tenuity, that the Cell can be advantageously dis-
pensed with ; the danger of not employing it, in many cases in
which there is no difficulty in mounting the object without it,
being that after a time the cement is apt to run-in beneath the
cover, which process is pretty sure to continue when it may have
once commenced.
184. Cement-Cells. — When the cells are required for mounting
very thin objects, they may be advantageously made of varnish
only, by the use of the Turn-table (Fig. 116) contrived by Mr.
Shadbolt. This consists of a small slab of mahogany, into one
end of which is fixed a pivot, whereon a circular plate of brass,
about three inches in diameter, is made to rotate easily, a rapid
motion being given to it by the application of the forefinger to
the milled-head seen beneath. The Glass slide being laid upon
the Turn-table, in such a manner that its two edges shall be equi-
distant from the centre (a guide to which is afforded by a circle
258 MOUNTING OF OBJECTS.
of an inch in diameter, traced npon the brass), and being held
by the springs with which it is furnished, a camel's hair pencil
dipped in the varnish to be nsed (Asphalte or Black Japan is
the best) is held in the right hand, so that its point comes into
contact with the glass, a little within the gniding circle jnst
named. The Turn-table being then pnt into rotation with the left
hand, a ring of varnish of a suitable breadth is made upon the
glass ; and if the slide be set-aside in a horizontal position, this
Fig. 116.
Shadbolt's Turn-table for making Cement-Cells.
ring will be found, when dry, to have lost the little inequali-
ties it may have at first presented, and to possess a very level
surface. If a greater thickness be desired than a single appli-
cation will conveniently make, a second layer may be laid-on
after the first is dry. It is convenient to prepare a number
of these cells at one time, since, when ' the hand is in,' they
will be made more dexterously than when the operation is per-
formed only once ; and it will be advantageous to subject them
to the warmth of a slightly-heated oven, whereby the flattening
of their surface will be more completely assured. The Microscopist
will find it a matter of great convenience to have a stock of these
cells always by him, ready prepared for use.
185. Thin-Glass Cells. — For the reception of objects too thick
for Cement-cells, but not thicker than ordinary Thin-glass, Cells
may be advantageously constructed by perforating pieces of Thin-
Glass with apertures of the desired size, and cementing these to
glass-slides with marine-glue. For making round cells, the per-
forated pieces that sometimes remain entire after the cutting of
disks (§ 165) may be employed, the disks often falling-out of
themselves when the glass is laid aside for a few days ; and thus
the same piece of thin-glass may afford a plate which, when
cemented to a glass- slide, forms a cell, and a disk suitable as the
cover to a cell of somewhat smaller size. There is great danger,
however, of the cracking of the surrounding glass in the cutting
out of the disk, especially when this is of large size ; and it will
generally be found a saving of trouble to employ the method re-
commended by Dr. L. Beale, which consists in attaching a piece
of thin-glass to one of the glass rings of which the deeper cells are
THIN-GLASS CELLS : — SUNK CELLS.
259
Fig. 117.
made (§ 188), of any form that may be desired, by means of
Marine- Glue first laid upon the latter and melted upon the hot
plate ; when the glue is quite cold, the point of a round or semi-
circular file is sharply thrust through the centre of the thin-glass,
which is then to be carefully filed to the size of the interior of the
ring ; and the ring being then heated a second time on the hot
plate, the thin-glass plate may be readily detached from it, and at
once cemented upon the glass-slide. The success of this simple
process depends upon the very firm and intimate adhesion of the
thin-glass to the ring, which prevents any crack from running into
the part of the thin-glass that is attached to it, however roughly
the file may be used. By having many of the rings on the hot
plate at once, and operating with them in turn, a great number of
cells can be made in a short time ; and such large thin cells may
be made in this mode, as could scarcely be fabricated (on account
of the extreme brittleness of this glass) by any other. After the
thin-glass has been cemented to the slide, it is desirable to roughen
its upper surface by rubbing it upon a leaden or pewter plate
(§ 154) with fine emery ; since the gold-size or other varnish adheres
much more firmly to
a ' ground ' than to
a polished surface.
Instead of thin-glass,
thin rings of Tin may
be employed (§ 189),
provided that the
fluid used in mount-
ing is not one that
acts upon that metal.
186. Sunk and
Plate-Glass Cells. —
For mounting objects
of somewhat greater
thickness than can be
included within thin-
glass cells, shallow
Cells may be made
by grinding - out a
concave (either circu-
lar or oval) in the
thickness of a glass
plate (Fig 117.) An
a, priori objection
naturally suggests it-
self to the use of such
cells, — that the con-
cavity of their bottom
will so deflect the course of the illuminating rays, as to distort
or obscure the image ; but to this it may be replied that when
s2
Sunk Cells.
260
MOUNTING OF OBJECTS.
the cell is filled with water or with some liquid of higher re-
fractive power, such deflection will in effect be fonnd very
small ; and the Author can now say from a large experience
that it is practically inoperative. Such cells until recently were
costly ; but being now made in large quantities, their price has
been so much reduced that they may be obtained more cheaply
than cells of any other kind.* For objects whose shape adapts
them to the form and depth of the concavity, these cells will be
found peculiarly advantageous ; since they do not hold air-bubbles
so tenaciously as do those with perpendicular walls, and there is
no cemented plate or ring to be loosened from its attachment, either
by a sudden ' jar,' or by the lapse of time. When transparent
objects are mounted in them, it is important to take care that the
concave bottom is free from scratches and roughness. — "Where
shallow cells are required with flat bottoms, they may be made by
drilling apertures of
Fig. 118. the desired size in
pieces of plate-glass
of the requisite thick-
ness, and by attaching
these with Marine-
Glue to glass-slides
(Fig. 118). Suchholes
may be made not
merely circular (a),
but oval (c) ; and a
very elongated per-
foration may be made
by drilling two holes
at the required dis-
tance, and then con-
necting them by cut-
ting out the inter-
mediate space (b).
Beep Cells, such as
are required for
mounting prepara-
tions of considerable
thickness, may be
made by drilling through a piece of thick Plate-Glass, and cement-
ing it in the usual way (d). These operations, however, can
scarcely be performed by any but regular glass-cutters, and, being
troublesome, are expensive ; hence the Plate-glass cells have been
generally superseded, either by Tube-Cells or by Built-up Cells.
187. Tube-Cells. — These are made by cutting transverse sections
* They are sold by Messrs. Jackson, Oxford-street, either of round or oval
form, Fig. 117, A, B ; and not only ground-out of slides of the usual size
(3 in. by 1 in.) and substance, but also hollowed in pieces of plate glass of
larger dimensions (c) and much greater thickness.
C3
Plate-Glass Cells.
TUBE-CELLS :— BUILT-UP CELLS.
261
Fig. 119.
of thick-walled Glass tubes of the required size, grinding the sur-
faces of these rings to the desired thinness, and then cementing
them to the glass-slides with Marine-Glue. ISTot only may round
cells (Fig. 119, a, b), of any diameter and any depth that the Mi-
croscopist can possibly
require,* be made by this
simple method, but oval,
square - shaped, or ob-
long-cells (c, d) are now
made of the forms and
sizes that he is most
likely to want, by flat-
tening the round glass-
tube whilst hot, or by
blowing it within a
mould. — Instead of sec-
tions of Glass Tubes, it
is less costly, and not
in other respects disad-
vantageous, to employ
Metallic Rings, which
being cemented to Glass-
slides in the usual way,
form Cells fitted to re-
tain any liquids which
do not act chemically
upon them. After a
trial of different metals,
Tin has been found most
suitable ; and rings of
several different sizes
and thicknesses are now
Tube-Cells, Bound and Quadrangular.
made of this metal for the use of the Microscopist. They are even
preferable to rings of glass in this respect, that a perfectly flat
surface may be given to them by slight friction with water on a
Water-of-Ayr stone, after they have been cemented to the glass-
slides ; and this will be found the best preventive against the run-
ning-in of the Gold-size, which often takes place with Glass-tube
cells in consequence of their inequality of surface.
188. Built-up Cells. — When Cells are required of forms or
dimensions not otherwise procurable, they may be built-up of
separate pieces of Glass cemented together. Large shallow Cells,
suitable for mounting Zoophytes or similar flat objects, may be
easily constructed after the following method : — A piece of Plate -
Glass, of a thickness that shall give the desired depth to the cell, is
* The Author has employed gigantic cells of this construction, 10 inches in
diameter and 1£ inch deep, for the preservation of Star-fish in Glycerine ; but
for such purposes he is disposed to think that rings of Porcelain, which might
be made at a much less cost, would be equally effective.
262
MOUNTING OF OBJECTS.
to be exit to the dimensions of its outside wall ; and a strip is then
to be cut-oft with the diamond from each of its edges, of such
breadth as shall leave the interior piece equal in its dimensions to
the cavity of the cell that is desired. This piece being rejected,
the four strips are then to be cemented upon the glass-slide in their
original position, so that the diamond-cuts shall fit together with
the most exact precision ; and the upper surface is then to be
ground flat with emery upon the pewter plate, and left rough as
before. — The perfect construction of large deep Cells of this kind,
(Fig. 120, a, b), however, requires a nicety of workmanship which
few amateurs pos-
Fig. 120. sess, and the expen-
diture of more time
than Microscopists
generally have to
spare ; and as it is
consequently prefe-
rable to obtain them
ready - made, direc-
tions for making
them need not be
here given. — A plan
of making deep cells,
however, has been
introduced by Dr. L.
Beale ; which, though
it does not give them
side walls possessing the same flatness with those of the built-
up cells, adapts them to serve most of the purposes for which
these are required, and makes them more secure against leakage ;
whilst it has the advantage of being so easy and simple, that any
one may put it into practice. A long strip of Plate-glass is to
be taken, whose breadth is equal to the desired depth of the cell,
and whose length must be equal to the sum of the lengths of all its
sides. This strip is to be carefully bent to a right angle in the
blow-pipe flame, at three points previously indicated by marks so
placed as to show where the angles should fall ; and the two ends,
which will thus be brought into contact at right angles, are to be
fused together. Thus a large square well, slightly rounded at the
angles, will be formed ; and this being very brittle, should be allowed
to cool very gradually, or, still better, should be annealed in an oven.
It must then be ground quite true on its upper and lower edges,
either on the lead-plate with emery, or on a flat stone with fine
sand; and it may then be cemented to a Glass- Slide in the usual
way-
189. Mounting objects in Cells. — In mounting an object in a
Cell, the first attention will of course be given to the cleanness of
the interior of the cell, and of the glass-cover which is to be placed
on it : this having been secured, the cell is to be filled with fluid
Built-up Cells.
MOUNTING OBJECTS IN CELLS. 263
by the Dropping-bottle, or Syringe, and any minute air -bubbles
which may be seen adhering to its bottom or sides must be removed
by the needle ; the object, previously soaked in fluid resembling
that with which the cell is filled, is then to be placed in the cell,
and should be carefully examined for Air-bubbles on all sides, and
also by looking up from beneath. This examination should be
made with a Hand-Magnifier or a Simple Microscope ; Quekett's
Dissecting Microscope (Fig. 41) being so especially suited to the
purpose, that the Author never mounts an object in fluid without
making use of it. When every precaution has been taken to free
the cell from these troublesome intruders, the cover may be placed
on it, one side being first brought down upon its edge, and then the
other : and if the cell have been previously brimming over with
fluid (as it ought to be), it is not likely that any air-space will re-
main. If, however, any bubbles should present themselves beneath
the cover, the slide should be inclined, so as to cause them to rise
towards the highest part of its circumference, and the cover
slipped away from that part, so as to admit of the introduction of a
little additional fluid by the pipette or syringe ; and when this has
taken the place of the air-bubbles, the cover may be slipped back
into its place.* All superfluous fluid is then to be taken up with
blotting-paper ; and particular care should be taken thoroughly
to dry the surface of the cell and the edge of the cover, since
the varnish will not hold to them if they be in the least damp
with water. Care must also be taken, however, that the fluid be
not drawn away from between the cover and the edge of the cell
on which it rests ; since any deficiency here is sure to be filled up
by varnish, the running-in of which is particularly objectionable.
These minutiae having been attended to, the closure of the cell
may be at once effected by carrying a thin layer of Gold-size or
Damar around and upon the edge of the glass-cover, taking care
that it touches every point of it, and fills the angular channel
which is left around its margin. If the wall of the cell be very
thin, it will be very advantageous to include it in the ring of
varnish, so as to make it hold down the cover, not only on the cell,
but on the slide beneath ; and this will help to secure it against
the separation of the ring from the slide, which is apt to be
produced by a ' jar' after the lapse of time. The Author has found
it advantageous, however, to delay closing the cell for some little
time after the superfluous fluid has been drawn off ; for as soon as
evaporation beneath the edges of the cover begins to diminish the
* Mr. Quekett and some other practised Manipulators recommend that the
edges of the cell and that of the disk of glass be smeared with the gold-size or
other varnish employed, before the cell is filled with fluid ; but the Author
has found this practice objectionable, for two reasons, — first, because it prevents
the cover from being slipped to one side (which is often desirable) without its
being soiled by the varnish, — and second, because when the edge of the cell
has been thus made to 'take' the varnish, that which is afterwards applied for
the closure of the cell is more likely to run in, than if the whole of the surface
covered by the glass is moistened with an aqueous fluid.
264 MOUNTING OF OBJECTS.
quantity of flnid in the cell, air-bubbles often begin to make their
appearance, which were previously hidden in the recesses of the
object ; and in the course of half an hour, a considerable number
are often collected. The cover should then be slipped aside, fresh
fluid be introduced, the air -bubbles removed, and the cover put on
again ; and this operation should be repeated until it fails to draw
forth any more air-bubbles. It will of course be observed that if
the evaporation of fluid should proceed far, air-bubbles will enter
beneath the cover ; but these will show themselves on the surface
of the fluid ; whereas those which arise from the object itself are
found in the deeper parts of the cell. Much time may be saved,
however, and the freedom of the preparation from air-bubbles may
be most effectually secured, by placing the cell, after it has been
filled in the first instance, in the vacuum of an Air-Pump (§ 176) ;
and if several objects are being mounted at once, they may all be
subjected to the exhausting process at the same time. The applica-
tion of the varnish should be repeated after the lapse of a few hours,
and may be again renewed with advantage several times in the course
of a week or two ; care being taken that each layer covers the edges,
as well as the whole surface, of that which preceded it. Even when
a considerable length of time has elapsed without the appearance
of air-bubbles, the mounting should not be considered secure ; for
a crack may form in the varnish through which air may find
its way : and thus any one who has a large collection of objects
mounted in fluid is pretty sure to find, on examining them from
time to time, that some of them have undergone deterioration from
this cause. It is well, therefore, to adopt the precautionary mea-
sure of re-varnishing the entire collection periodically (say, once a
year), the slight trouble which this occasions being amply compen-
sated by the preservation of valuable specimens that might other-
wise go to ruin.
190. The presence of Air-bubbles in any preparation mounted
in fluid is to be particularly avoided, not merely on account of its
interference with the view of the object, but also because, when
air-spaces, however small, once exist, they are almost certain to
increase, until at last they take the place of the entire fluid, and
the object remains dry. Even in the hands of the most experienced
manipulators, this misfortune not unfrequently occurs ; being
sometimes due to the obstinate entanglement of air -bubbles in the
object when it was originally mounted, and sometimes to the
perviousness of some part of the cement, which has allowed a
portion of the contained fluid to escape, and air to find admission.
In either case, so soon as an air-bubble is seen in such a prepara-
tion, the attempt should be made to prevent its increase by laying
on an additional coat of varnish ; but if this should not be
successful, the cover should be taken off and the specimen
remounted, so soon as the fluid has escaped to such a degree as to
leave any considerable portion of it uncovered.
191. Importance of Cleanliness. — The success of the result of
LABELLING AND PRESERVING OF OBJECTS. 265
any of the foregoing operations is greatly detracted-from, if, in
consequence of the adhesion of foreign substances to the glasses
whereon the objects are mounted, or to the implements used in
the manipulations, any extraneous particles are brought into view
with the object itself. Some such will occasionally present them-
selves, even under careful management ; especially fibres of silk,
wool, cotton, or linen, from the handkerchiefs, &c, with which the
glass-slides may have been wiped ; and grains of starch, which
often remain obstinately adherent to the thin-glass covers kept
in it. But a careless and uncleanly manipulator will allow his
objects to contract many other impurities than these ; and espe-
cially to be contaminated by particles of dust floating through the
air, the access of which may be readily prevented by proper pre-
cautions. It is desirable to have at hand a well-closed cupboard
furnished with shelves, or a cabinet of well-fitted drawers, or a
number of bell-glasses upon a flat table, for the purpose of
securing glasses, objects, &c, from this contamination in the
intervals of the work of preparation ; and the more readily
accessible these receptacles are, the more use will the Micro scopist
be likely to make of them. Great care ought, of course, to be
taken that the Liquids employed for mounting should be freed
by effectual filtration from all floating particles ; and both these
and the Canada Balsam should be kept in well-closed bottles.
192. Labelling and Preserving of Objects. — -Whenever the
mounting of an object has been completed, its name ought to be
at once marked on it, and the slide should be put away in its
appropriate place. Some inscribe the name on the glass itself
with a writing diamond ; whilst others prefer to gum a label* on
the slide ; and others, again, cover one or both surfaces of the
slide with coloured paper, and attach the label to it. In the case
of objects mounted dry or in balsam, the latter method has the
advantage of rendering the glass-cover more secure from dis-
placement by a slight blow or 'jar,' when the varnish or balsam
may have become brittle by the lapse of years. Instead, how-
ever, of attaching the white label on which the name of the object
is written, to the outside of the coloured paper with which the slide
is covered, it is better to attach the label to the glass, and to punch
a hole out of the coloured paper, sufiiciently large enough to show
the name, in the part corresponding to it : in this manner the
label is prevented from falling off, which it frequently does when
attached to the glass without protection, or to the outside of the
paper cover. When objects are mounted in fluid, either with or
without cells, paper coverings to the slides had better be dispensed
with; and besides the name of the object, it is desirable to inscribe
on the glass that of the fluid in which it is mounted. For the
preservation of objects, the pasteboard boxes now made at a very
* Very neat gummed labels, of various sizes and patterns suitable to the
wants of the Microscopist, are sold by the "Drapers' Stationers" in the City.
266 COLLECTION OF OBJECTS.
reasonable cost, with wooden racks, to contain 6, 12, or 24 slides,
will be found extremely useful. In these, however, the slides
must always stand upon their edges ; a position which, besides
interfering with that ready view of them which is required for
the immediate selection of any particular specimen, is unfavour-
able to the continued soundness of preparations mounted in fluid.
Although such boxes are most useful, indeed almost indispensable,
to the Microscopist, for holding slides which he desires (for what-
ever purpose) to keep for awhile constantly at hand, yet his
regularly-classified series is much more conveniently stored in a
Cabinet containing numerous very shallow drawers, in which they
lie flat and exposed to view. Such cabinets are now prepared for
sale under the direction of our principal Opticians, with all the
improvements that experience has suggested. In order to anta-
gonize the disposition of the slides to slip one over another in the
opening or shutting of the drawers, it has been found preferable
to arrange thern in such a manner that they lie with their ends
(instead of their long sides) towards the front of the drawer, and
to interpose a cross-strip of wood, lying parallel to the front of
the drawer, between each row. It is very convenient, moreover,
for the front of the drawer to be furnished with a little tablet of
porcelain, on which the name of the group of objects it may
contain can be written in pencil, so as to be readily rubbed out ;
or a small frame may be attached to it, into which a slip of card
may be inserted for the same purpose. — The Book-Cabinets con-
structed by Mr. Collins, according to the suggestions of the
Author, supply a very convenient and less costly mode of keeping
a large collection of objects. Each cabinet resembles a quarto
pamphlet-case, and contains a number of very light trays, of
which each holds six slides, laid horizontally, and kept apart from
each other by partitions. These trays may be of different depths,
according to the thickness of the slides they are to receive ; and
thus the same cabinet may be made to hold all the objects belong-
ing to any particular series, though some of them may be mounted
on ordinary slips of glass or wood, whilst others may require
thick cells or deep wooden slides.
Section 3. Collection of Objects.
193. A large proportion of the objects with which the Micro-
scopist is concerned, are derived from the minute parts of those
larger organisms, whether Yegetable or Animal, the collection of
which does not require any other methods than those pursued by
the ordinary Naturalist. With regard to such, therefore, no
special directions are required. But there are several most inte-
resting and important groups both of Plants and Animals, which
are themselves, on account of their minuteness, essentially micro-
scopic ; and the collection of these requires peculiar methods and
implements, which are, however, very simple — the chief element
COLLECTION OF OBJECTS. 267
of success lying in the knowledge where to look and what to
look for. In the present place, general directions only will be
given ; the particular details relating to the several groups being
reserved for the account to be hereafter given of each.
194. Of the Microscopic organisms in question, those which
inhabit fresh water must be sought for in pools, ditches, or
streams, through which some of them freely move ; whilst others
attach themselves to the stems and leaves of aquatic Plants, or
even to pieces of stick or decaying leaves, &c, that may be floating
on the surface or submerged beneath it ; while others, again, are to
be sought for in the muddy sediments at the bottom. Of those
which have the power of free motion, some keep near the surface,
whilst others swim in the deeper waters ; but the situation of
many depends entirely upon the light, since they rise to the
surface in sunshine, and subside again afterwards. The Collector
will therefore require a means of obtaining samples of water at
different depths, and of drawing to himself portions of the larger
bodies to which the microscopic organisms may be attached. For
these purposes nothing is so convenient as the Pond- Stick (sold by
Mr. Baker) which is made in two lengths, one of them sliding
within the other, so as when closed to serve as a walking-stick.
Into the extremity of this may be fitted, by means of a screw
socket, (1) a cutting-hook or curved knife, for bringing up portions
of larger Plants in order to obtain the minute forms of Vegetable
or Animal life that may be parasitic upon them; (2) a broad collar,
with a screw in its interior, into which is fitted one of the screw-
topped Bottles made by the York Glass Company ; (3) a ring or
hoop for a muslin Bing-Net. When the Bottle is used for collect-
ing at the surface, it should be moved sideways with its mouth
partly below the water ; but if it be desired to bring up a sample
of the liquid from below, or to draw into the bottle any bodies
that may be loosely attached to the submerged plants, the bottle is
to be plunged into the water with its mouth downwards, carried
into the situation in which it is desired that it should be filled,
and then suddenly turned with its mouth upwards. By unscrew-
ing the bottle from the collar and screwing on its cover, the con-
tents may be securely preserved. The Net should be a bag of fine
muslin, which may be simply sewn to a ring of stout wire. But it is
desirable for many purposes that the muslin should be made remov-
able ; and this may be provided for (as suggested in the " Micro-
graphic Dictionary," Introduction, p. xxiv.) by the substitution of
a wooden hoop grooved on its outside, for the wire ring ; the muslin
being strained upon it by a ring of vulcanized India-rubber,
which lies in the groove, and which may be readily slipped off and
on, so as -to allow a fresh piece of muslin to be put in the place of
that which has been last used. The collector should also be fur-
nished with a number of Bottles, into which he may transfer the
samples thus obtained : and none are so convenient as the screw-
topped bottles made in all sizes by the York Glass Company. It
268 COLLECTION OF OBJECTS.
is well that the bottles should be fitted into cases, to avoid the
risk of breakage. When Animalcules are being collected, the
bottles should not be above two-thirds filled, so that adequate air-
space may be left. — "Whilst engaged in the search for Microscopic
objects, it is desirable for the Collector to possess a means of at
once recognising the forms which he may gather, where this is
possible, in order that he may decide whether the ' gathering '
is, or is not worth preserving ; for this purpose either a powerful
' Coddington' or ' Stanhope' lens (§ 24), a Beale's Pocket
Microscope (§ 61), or the Travelling Microscope of Messrs. Baker
or of Messrs. Murray and Heath (§ 63), will be found most
useful, according to the class of objects of which the Collector is in
search. The former will answer very well for Zoophytes and the
larger Diatomaceae ; but the latter will be needed for Desmidiaceaa,
the smaller Diatoniaceas, and Animalcules.
195. The same general method is to be followed in the collection
of such marine forms of Yegetable and Animal life as inhabit the
neighbourhood of the shore, and can be reached by the Pond- stick.
But there are many which need to be brought up from the bottom
by means of the Dredge; and many others which swim freely
through the waters of the Ocean, and are only to be captured by the
Tow-Net. As the former is part of the ordinary equipment of every
Marine Naturalist, whether he concern himself with the Microscope
or not, the mode of using it need not be here described ; but the
use of the latter for the purposes of the Microscopist requires special
management. The net should be of fine muslin, firmly sewn to a
ring of strong wire about 10 or 12 inches in diameter. This may
be either fastened by a pair of strings to the stern of a boat, so as
to tow behind it, or it may be fixed to a Stick so held in the hand
as to project from the side of the boat. In either case the net
should be taken in from time to time, and held up to allow the
water it contains to drain through it ; and should then be turned
inside-out and moved about in a bucket of water carried in the
boat, so that any minute organisms adhering to it may be washed
off before it is again immersed. It is by this simple method that
Marine Animalcules, the living forms of Polycystina, the smaller
Medusoids (with their allies, Beroe and Gydijpjpe), Noctiluca, the
free-swimming larva? of Echinodermata, some of the most curious
of the Tunicata, the larvae of Mollusca, Turbellaria, and Annelida,
some curious adult forms of these classes, Entomostraca, and the
larvae of higher Crustacea, are obtained by the Naturalist ; and
the great increase in our knowledge of these forms which has been
gained within recent years, is mainly due to the assiduous use
which has been made of it by qualified observers. — It is important
to bear in mind, that, for the collection of all the more delicate of
the organisms just named (such, for instance, as Ecliinoderm
larvai), it is essential that the boat should be rowed so slowly that
the net may move gently through the water, so as to avoid crushing
its soft contents against its sides. Those of firmer structure (such
COLLECTION OF MAEINE SURFACE-ANIMALS. 269
as the Entomostraca) , on the other hand, may be obtained by the
use of a Tow-Net attached to the stern of a sailing-vessel or even
of a steamer in much more rapid motion. When this method is
employed, it will be found advantageous to make the net of
conical form, and to attach to its deepest part a wide-mouthed
bottle, which may be prevented from sinking too deeply by
suspending it from a cork float ; into this bottle many of the
minute Animals caught by the net will be carried by the current
produced by the motion of the vessel through the water, and they
will be thus removed from liability to injury. It will also be useful
to attach to the ring an inner net, the cone of which, more obtuse
than that of the outer, is cut off at some little distance from the
apex ; this serves as a kind of valve, to prevent "objects once caught
from being washed out again. The net is to be drawn-in from time
to time, and the bottle to be thrust-up through the hole in the
inner cone ; and its contents being transferred to a screw-capped
bottle for examination, the net may be again immersed. This form
of net, however, is less suitable for the most delicate objects than
the simple Stick-Net used in the manner just described. — The
Microscopist on a visit to the sea-side, who prefers a quiet row in
tranquil waters to the trouble (and occasional malaise) of dredging,
will find in the collection of floating Animals by the careful use of
the Stick-Net or Tow-Net a never-ending source of interesting
occupation.
CHAPTEE VI.
MICROSCOPIC FORMS OF VEGETABLE LIFE. — PROTOPHYTES.
196. In commencing our survey of those wonders and beauties
of Life and Organization which are revealed to us by the assistance
of the Microscope, it seems on every account the most appropriate
to turn our attention in the first instance to the Vegetable King-
dom ; and to begin with those of its humblest members whose form
and structure, and whose very existence in many cases, are only
known to us through its use. For such as desire to make them-
selves familiar with Microscopic appearances, and to acquire dex-
terity in Microscopic manipulation, cannot do better than educate
themselves by the study of those comparatively simple forms of
Organization which the Vegetable fabric presents. Again, the scien-
tific Histologist looks to the careful study of the structure of the
simplest forms of Vegetation, as furnishing the key (so to speak)
that opens the right entrance to the study of the elementary Orga-
nization, not merely of the higher Plants, but of the highest
Animals. And in like manner, the scientific Physiologist looks to
the complete knowledge of their Life-history, as furnishing the
surest basis for those general notions of the nature of Vital Action,
which the advance of science has shown to be really well founded
only when they prove equally applicable to both Kingdoms.
197. But, further, a peculiar interest attaches itself at the
present time to everything which throws light upon the debated
question of the boundary between the two Kingdoms ; a question
which is not less keenly debated among Naturalists, than that of
many a disputed frontier has been between adjacent Nations. For
many parts of this border-country have been taken and retaken
several times ; their inhabitants (so to speak) having first been
considered, on account of their general appearance, to belong to the
Vegetable Kingdom, — then, in consequence of some movements
being observed in them, being claimed by the Zoologists, — then, on
the ground of their evidently Plant-like mode of growth, being
transferred back to the Botanical side, — then, owing to the sup-
posed detection of some new feature in their structure or physiology,
being again claimed as members of the Animal Kingdom, — and
lastly, on the discovery of a fallacy in these arguments, being once
more turned over to the Botanist, with whom, for the most part,
they now remain. For the attention which has been given of late
years to the study of the humblest forms of Vegetation, has led to
DISTINCTIONS BETWEEN PLANTS AND ANIMALS. 271
the knowledge, among what must be undoubtedly regarded as
Plants, of so many phenomena which would formerly have been
considered unquestionable marks of Animality, that the discovery
of the like phenomena among the doubtful beings in question, so
far from being any evidence of their Animality, really affords a
probability of the opposite kind.
198. In the present state of Science, it would be impossible to
lay down any definite line of demarcation between the two King-
doms ; since there is no single character by which the Animal or
Vegetable nature of any Organism can be tested. Probably the
one which is most generally applicable among those lowest Or-
ganisms that most closely approximate to one another, is — not, as
formerly supposed, the presence or absence of Spontaneous Mo-
tion,— but the dependence of the Being for nutriment upon Organic
Compounds already formed, which it takes (in some way or other)
into the interior of its body ; or, on the other hand, its possession
of the power of producing the Organic Compounds which it applies
to the increase of its fabric, at the expense of certain Inorganic
Elements (Oxygen, Hydrogen, Carbon, and Nitrogen), which it
obtains by decomposing the Water, Carbonic Acid, and Ammonia
with which it is in external relation. The former, though not an
absolute is a general characteristic of the Animal Kingdom ; the
latter is the prominent attribute of the Vegetable ; and although
certain exceptions exist that are highly important in biological
inquiries, they interfere little with the distinctions most useful to
students. For we shall find that Protozoa (or the simplest animals)
which seem to be composed of nothing else than a mass of living
jelly (Chaps, ix. x.)are supported as exclusively either upon other
Protozoa or upon Protophytes (which are humble Plants of equal
simplicity), as the highest Animals upon the flesh of other Animals
or upon the products of the Vegetable Kingdom : whilst these Pro-
tophytes, in common with the highest Plants, draw their nourish-
ment from the Atmosphere or the Water in which they live, and
are distinguished by their power of liberating Oxygen through the
decomposition of Carbonic Acid under the influence of Sun-light.
And we shall moreover find that even such Protozoa as have
neither stomach nor mouth, receive their alimentary matter direct
into the very substance of their bodies, in which it undergoes
a kind of digestion ; whilst the Protophyta absorb through their
external surface only, and take in no solid particles of any descrip-
tion. With regard to motion, which was formerly considered
the distinctive attribute of Animality, we now know not merely
that many Protophytes (perhaps all at some period or other of
their lives) possess a power of spontaneous movement, but also that
the instruments of motion (when these can be discovered) are of
the very same character in the Plant as in the Animal ; being little
hair -like filaments termed Cilia (from the Latin cilium, an eye-
lash), by whose rhythmical vibration the body of which they form
part is propelled in definite directions. The peculiar contractility
272 MICROSCOPIC FORMS OF VEGETABLE LIFE.
of these Cilia cannot be accounted for in either case, any Letter
than in the other ; all we can say is, that it seems to depend upon
the continued vital activity of the living substance of which these
filaments are prolongations, and that this contractile substance
has a composition essentially the same in the Plant as in the
Animal.
199. While there is so large an amount of general truth in the
preceding statements as to the Nutrition of Plants and Animals,
that they must be constantly borne in mind in forming our con-
ceptions of the two groups, deviations from them must not be for-
o-otten. Fungi appear, in some instances, to approach the Animal
type of nutrition ; and if some of the lowest Organisms of deep-sea-
beds are to be ranked as plants, they must perform their vital
processes in a condition that to our organs would be one of total
darkness. In the Porcupine Expedition, living organisms of
various kinds, including some of the higher Marine Invertebrata,
were brought up from a depth of nearly three miles,* to which Light
can only penetrate in an infinitesimally small degree. It is there-
fore a question of great difficulty, whether the low Protoplasmic
Life which pervades the " Globigerina-ooze," and doubtless sup-
plies food to the higher forms, has the power of self -formation,
at the expense of the Carbonic acid which there exists in very large
quantity — perhaps reduced to a liquid condition by the enormous
pressure of three tons on the square inch ; or whether it simply
absorbs Organic matter, which has been imparted to Ocean-water
by the Vegetable life of its upper stratum, especially near shores,
and by the free floating sea-weeds of the open sea, as in the case
of the Sargasso, or Gulf -weed. The latter idea,' first suggested
by Professor Wyville Thomson, derives confirmation from the
results of chemical analysis ; which show that the water of the
open Ocean, at all depths, is pervaded by Organic matter.
200. The plan of organization throughout the Vegetable kingdom
presents this remarkable feature of uniformity, — that the fabric of
the highest and most complicated Plants consists of nothing else
than an aggregation of the bodies termed Cells; every one of which,
among the lowest and simplest forms of Vegetation, may maintain
an independent existence, and may multiply itself almost inde-
finitely, so as to form vast assemblages of similar bodies. And the
essential difference between the plans of structure in the two cases
lies in this : — that the Cells produced by the self -multiplication of
the primordial cell of the Protophyte are all mere repetitions of it
and of one another, each living by and for itself, — whilst those
produced by the like self-multiplication of the primordial cell in
the Oak or Palm, not only remain in mutual connection, but undergo
a progressive ' differentiation ;' a composite fabric being thereby
developed, which is made up of a number of distinct organs (Stem,
Leaves, Roots, Flowers, &c), each of them characterized by spe-
* " The Depths of the Sea," by Professor Wyville Thomson.
VEGETABLE CELLS IN GENEKAL. 273
cialities not merely of external form but of intimate structure (the
ordinary type of the Cell undergoing various modifications, to be
described in their proper place (Chap. viil), and each performing
actions peculiar to itself, which contribute to the life of the Plant
as a whole. Hence, as was first definitely stated by Schleiden, it
is in the life history of the individual cell that we find the true
basis of the study of Vegetable Life in general. And we shall now
inquire, therefore, what information on this point we derive from
Microscopic research.
201. In its most completely-developed form, the Yegetable-Cell
may be considered as a closed membranous bag or vesicle, contain-
ing a fluid cell- sap ; and thus we have to consider separately the
Cell-wall and the Cell-contents. The Cell-wall is composed of
two layers, of very different composition and properties. The inner
of these, which has received the name of Primordial Utricle, ap-
pears to be the one first formed and most essential to the existence
of the cell ; it is extremely thin and delicate, so that it escapes
attention so long as it remains in contact with the external layer ;
and it is only brought into view when separated from this, either
by developmental changes (Fig. 166), or by the influence of re-
agents which cause it to contract by drawing-forth part of its
contents (Fig. 210). Its composition is indicated, by the effects of
re-agents, to be albuminous ; that is, it agrees with the formative
substance of the Animal tissues, not only in the proportions of
oxygen, hydrogen, carbon, and nitrogen which it contains, but also
in the nature of the compound formed by the union of these ele-
ments. The external layer, on the other hand, though commonly
regarded as the proper Cell-wall, is generated on the surface of the
primordial utricle after the latter has completely enclosed the
cavity and its contents, so that it takes no essential part in the
formation of the cell. It is usually thick and strong in comparison
with the other, and may often be shown to consist of several layers.
In its chemical nature it is altogether dissimilar to the primordial
utricle ; for it is essentially composed of Cellulose, a substance
containing no nitrogen, and nearly identical with starch. The
two constituents are readily distinguished by the action of Carmine
(§ 161), which stains the Protoplasmic substance, without affecting
the Cellulose-wall. The relative offices of these two membranes
are very different ; for whilst there are many indications that the
Primordial Utricle continues to participate actively in the vital
operations of the cell, it seems certain that the Cellulose-wall takes
no concern in them, but is only their product, its function being
simply protective. The contents of the Yegetable cell, being
usually more or less deeply coloured, have received the collective
designation of. Endochrome (or internal colouring-substance) ; and
they essentially consist of a layer of colourless Protoplasm (or
organizable fluid, containing albuminous matter in combination
with dextrine or starch-gum) in immediate contact with the pri-
mordial utricle, within which is the more watery Cell- sap, — particles
274 MICROSCOPIC FORMS OF VEGETABLE LIFE.
of Chlorophyll or colouring-substance and of Oil being diffused
through both, or through the former only.
202. But although these component parts may be made-out
without any difficulty in a large proportion of Yegetable-Cells, yet
they cannot be distinguished in some of those humble organisms
which are nearest to the border-ground between the two kingdoms.
For in them we find the Cell- wall very imperfectly differentiated
from the Cell-contents ; the former not having by any means
the firmness of a perfect membrane, and the latter not possessing
the liquidity which elsewhere characterizes them. And in some
instances the Cell appears to be represented only by a mass of
Endochrome, so viscid as to retain its external form without any
limitary membrane, though the superficial layer seems to have a
firmer consistence than the interior substance ; and this may or
may not be surrounded by a gelatinous-looking envelope, which is
equally far from possessing a membranous firmness, and yet is the
only representative of the Cellulose-wall. This viscid Endochrome
consists, as elsewhere, of a colourless Protoplasm, through which
colouring particles are diffused, sometimes uniformly, sometimes
in local aggregations, leaving parts of the protoplasm uncoloured.
The superficial layer, in particular, is frequently destitute of colour ;
and the Primordial Utricle appears to be formed by its solidifica-
tion. A Cell-nucleus, the ' cytoblast' of Schleiden, is supposed to
occur in the living cells of all Plants, though it cannot always be
distinguished. It may be best observed in loose soft tissues, as
those of cucumbers, leaves, stems of liliaceous plants, or the young
hairs on leaves and sepals. It is usually close to the internal
wall, and sub-globose, or lenticular in shape. In this nucleus lie
one or more ' nucleoli,' which may be strongly coloured by twenty-
four hours' immersion in solution of carmine ; after which the pre-
paration should be washed with water containing a few drops of
acetic acid. Young cells are usually filled with protoplasm, which
is viscid and granular near the cell-wall, but more watery towards
the centre ; and a clearly -marked distinction gradually arises
between the outer protoplasmic layer and the interior ' cell-sap.'
Yacuoles, or small cavities, arise in the denser part, separated by
bars of protoplasm ; and these are occupied by ' cell-sap.' Vfhere the
nucleus is in the centre of the cell, part of the protoplasm collects
around it, while another portion is retracted to the inner surface
of the membrane, the two being connected by the bars or finer
threads of protoplasm, which pass through the cell-sap. " Where
the cell-nucleus is imbedded in wall-plasma, there the separate
vacuoles unite into a single central vacuole, which becomes the whole
inner cavity of the cell occupied by the cell-sap, and only in rare cases
a few fine protoplasm-threads stretch across from wall to wall."*
203. ISFow among the Protophytes or simplest Plants, on the
examination of which we are about to enter, there are many of
* See Dr. Braithwaite "On the Histology of Plants," in the " Journal of the
Quekett Club" for April, 1873.
VEGETABLE CELLS IN GENEEAL.— PEOTOPHYTES. 275
which every single Cell is not only capable of living in a state of
isolation from the rest, but even normally does so ; and thus, in
the ordinary phraseology, every Cell is to be accounted a ' distinct
individual.' There are others, again, in which shapeless masses
are made up by the aggregation of continuous Cells, which, though
quite capable of living independently, remain attached to each
other by the mutual fusion (so to speak) of their gelatinous invest-
ments. And there are others, moreover, in which a definite adhe-
sion exists between the Cells, and in which regular plant-like struc-
tures are thus formed, notwithstanding that every cell is still but
a repetition of every other, and is capable of living independently
if detached, so as to answer to the designation of a Unicellular
or single-celled Plant. These different conditions we shall find to
arise out of the mode in which each particular species multiplies
by binary subdivision (§ 204) : for where the cells of the new pair
that is produced by the segmentation of the previous cell undergo
a complete separation from one another, they will henceforth live
quite separately ; but if, instead of undergoing this complete fusion,
they should be held together by the intervening gelatinous envelope,
a shajDeless mass result's from repeated subdivisions not taking
place on any determinate plan ; and if, moreover, the binary
subdivision should always take place in a determinate direction, a
long narrow filament (Fig. 160, d), or a broad flat leaf -like ex-
pansion (g), may be generated. To such extended fabrics the
term Unicellular Plants "can scarcely be applied with propriety ;
since they may be built-up of many thousands or millions of dis-
tinct Cells, which have no disposition to separate from each other
spontaneously. Still they correspond with those which are strictly
Unicellular, as to the absence of differentiation either in struc-
ture or in actions between their component cells ; each one of
these being a repetition of the rest, and no relation of mutual
dependence existing among them. — -AH such organisms may well
be included under the general term of Pkotophytes, by which it
is convenient to designate these primitive or elementary forms of
Vegetation ; and we shall now enter, in such detail as the nature
of the present Treatise allows, into the history of those forms of
the group which present most of interest to the Microscopist, or
which best serve to illustrate the general doctrines of Physiology.
204. The life-history of one of these Unicellular Plants, in its
most simple form, can scarcely be better exemplified than in the
Palmoglcea macro cocca (Kiitzing) ; one of those humble kinds of
vegetation which spreads itself as a green slime over damp stones,
walls, &c. When this slime is examined with the microscope, it is
found to consist of a multitude of green Cells (Plate VIII., Fig. 1,a),
each surrounded by a gelatinous envelope; the Cell, which does not
seem to have any distinct membranous wall, is filled with granular
particles of a green colour ; and a nucleus, or more solid aggregation
which appears to be the centre of the vital activity of the cell, may
sometimes be distinguished through the midst of these. When
t2
276 MICROSCOPIC FORMS OF VEGETABLE LIFE.
treated with tincture of iodine, however, the green contents of the
cell are turned to a brownish, hue, and a dark-brown nucleus (g) is
distinctly shown. Other cells are seen (b), which are considerably
elongated, some of them beginning to present a sort of hour-glass
contraction across the middle ; in these is commencing that curious
multiplication by binary subdivision, which is the ordinary mode
of increase throughout the Yegetable kingdom ; and when cells in
this condition are treated with tincture of iodine, the nucleus is
seen to be undergoing the like elongation and constriction (h). A
more advanced state of the process of subdivision is seen at c, in
which the constriction has proceeded to the extent of completely
cutting-off the two halves of the cell, as well as of the nucleus (i),
from each other, though they still remain in mutual contact ; but
in a yet later stage they are found detached from each other (d),
though still included within the same gelatinous envelope. Each
new cell then begins to secrete its own gelatinous envelope, so that,
by its intervention, the two are usually soon separated from
one another (e). Sometimes, however, this is not the case ; the
process of subdivision being quickly repeated before there is time
for the production of the gelatinous envelope, so that a series of
cells (f) hanging-on one to another is produced. — There appears
to be no definite limit to this kind of multiplication ; and exten-
sive areas may be quickly covered, in circumstances favourable to
the growth of the plant, by the products of the duplicative sub-
division of one Primordial Cell. This, however, is simply an act
of Grovitli, precisely analogous to that by which any one of the
higher forms of Yegetation extends itself, and differing only in this,
that the cells produced by each act of subdivision in these simplest
Plants exactly resemble that from which they sprang ; whilst in
the case of more highly organized Plants, they gradually become
differentiated to a greater or less degree, so that special ' organs'
are evolved, which take upon themselves dissimilar yet mutually
dependent actions in the economy of the entire Organism (§ 200).
205. The process which represents the Generation of the higher
Plants is here performed in a manner so simple that it would not
be recognised as such, if we were not able to trace it up through a
succession of modes of gradually increasing complexity, until we
arrive at the elaborate operations which are concerned in the pro-
duction and fertilization of the Seeds of Flowering Plants. For it
consists in nothing else than the re-union or fusion-together of any
pair of Cells (Plate VIII., Fig. 1, k), — a process which is termed
Conjugation ; and it is characteristic of this humble Plant, and
shows how imperfect must be the consistence of its Cell-membrane,
that this seems to enter into the fusion no less completely than do
the Cell-contents. The communication is at first usually made by a
narrow neck or bridge (k) ; but before long it extends through a
large part of the contiguous boundaries (l) ; and at last the two
cells are seen to be completely fused into one mass (m), which is
termed the Spore. Each Spore thus formed is the Primordial Cell
PLATE VIII.
irf eft
Fig. 2.
Development of Palmogi^ea and Peotoccocus.
[To face p. 276
CONJUGATION OF PEOTOPHYTES. 277
of a neio generation, into which it evolves itself by successive re-
petitions of the process of binary subdivision. — It is curions to
observe that during this Conjugating process a production of Oil
particles takes place in the cells ; these at first are small and
distant, but gradually become larger and approximate more closely
to each other, and at last coalesce so as to form oil-drops of various
sizes, the green granular matter disappearing ; and the colour of
the conjugated body changes, with the advance of this process,
from green to a light yellowish-brown. When the Spore begins to
vegetate, on the other hand, producing a pair of new cells by binary
subdivision, a converse change occurs ; the oil-globules disappear,
and green granular matter takes their place. Now this is precisely
what happens in the formation of the seed among the higher Plants ;
for Starchy substances are transformed into oil, which is stored up
in the seed for the nutrition of the embryo, and is applied during
Germination to the purposes which are at other times answered by
starch or chlorophyll. — The growth of this little plant appears to
be favoured by cold and damp ; its generation, on the other hand,
is promoted by heat and dryness ; and it is obvious that the Spore-
cell must be endowed with a greater power of resisting this than
the vegetating plant has, since the species would otherwise be
destroyed by every drought.
206. If the preceding sketch really comprehends the whole Life
history of the humble Plant to which it relates, this history is
much more simple than that of other forms of Yegetation, which,
without appearing to possess an essentially-higher structure, pre-
sent themselves under a much greater variety of forms and condi-
tions. One of the most remarkable of these varieties is the motile
condition, which seems to be common, in some stage or other of
their existence, to a very large proportion of the lower forms of
Aquatic Yegetation ; and which usually depends upon the extension
of the Primordial Utricle into one or two thread-like filaments,
endowed with the power of executing rhythmical contractions,
whereby the cell is impelled through the water.
207. As an illustration of this peculiar mode of activity, which
was formerly supposed to betoken Animal life, a sketch will be
given of the history of a plant, the Protococcus pluvialis (Plate
YIIL, Fig. 2), which is not uncommon in collections of Pain-water,*
* The Author had under his own observation, twenty-five years ago, an.
extraordinary abundance of what he now feels satisfied must have been this
Protophyte, in a rain-water cistern which had been newly cleaned-out. His
notice was attracted to it by seeing tbe surface of the water covered with a
green froth, whenever the sun shone upon it. On examining a portion of this
froth under the Microscope, he found that the water was crowded with green
cells in active motion ; and although the only bodies at all resembling them of
which he could find any description, were the so-called Animalcules constituting
the genus Chlamydomonas of Prof. Ehrenberg, and very little was known at
that time of the ' motile' conditions of Plants of this description, yet of the
Vegetable nature of . these bodies he could not entertain the smallest doubt.
They appeared in freshly collected rain-water, and could not, therefore, be
278 MICKOSCOPIC FOEMS OF VEGETABLE LIFE.
and which, in its motile condition, has been very commonly regarded
as an Animalcule, its different states having been described under
several different names. In the first place, the colour of these cells
varies considerably ; since, although they are usually green at the
period of their most active life, they are sometimes red ; and their
red form has received the distinguishing appellation of Hcemato-
coccus. Very commonly the red colouring-matter forms only a
central mass of greater or less size, having the appearance of a
nucleus (as shown at e) ; and sometimes it is reduced to a single
granular point, which has been erroneously represented by Prof.
Ehrenberg as the eye of these so-called Animalcules. It is quite
certain that the red colouring- substance is very nearly related in
its chemical character to the green, and that the one may be con-
verted into the other : though the conditions under which this
conversion takes place are not precisely known. In the still form
of the cell, with which we may commence the history of its life, we
find a mass of Endochrome, consisting of a colourless Protoplasm,
through which red or green-coloured granules are more or less uni-
formly diffused : on the surface of this endochrome the colourless
protoplasm is condensed into a more consistent layer, forming an
imperfect Primordial Utricle ; and this is surrounded by a tolerably
firm layer, which seems to consist of Cellulose or of some modifi-
cation of it. Outside this (as shown at a), when the ' still ' cell is
formed by a change in the condition of a cell that has been previously
' motile,' we find another envelope, which seems to be of the same
nature, but which is separated by the interposition of aqueous
fluid ; this, however, may be altogether wanting. The multiplica-
tion of the ' still ' cells by self -division takes place as in Palmoglcea ;
deriving their support from Organic matter : under the influence of light they
were obviously decomposing Carbonic Acid and liberating Oxygen, and this
influence he found to be essential to the continuance of their growth and
development, which took place entirely upon the Vegetative plan. Not many
days after the Protophyte first appeared in the water, a few Wheel-
Animalcules presented themselves ; these fed greedily upon it, and increased so
rapidly (the weather being very warnf) that they speedily became almost as
crowded as the cells of the Protococcus had been ; and it was probably due in
part to their voracity that the Plant soon became less abundant, and before
long disappeared altogether. Had the Author been then aware of its assump-
tion of the ' still' condition, he might have found it at the bottom of the cistern,
after it had ceased to present itself at the surface. — The account of this Plant
given above, is derived from that of Dr. Cohn, in the " Nova Acta Acad. Nat.
Curios." (Bonn, 1850), Tom. xxii. ; of which an abstract by Mr. George Busk
is contained in the "Botanical and Physiological Memoirs," published by the
Ea}* Society for 1853. This excellent observer states that he kept his plants
for observation in little glass vessels, having the form of a truncated cone,
about two inches deep, and one inch and a quarter in diameter, with a flat
bottom polished on both sides, and filled with water to the depth of from two to
three lines. "It was only in vessels of this kind," he says, " that he was able to
follow the development of a number of various cells throughout its whole
course." Probably he would have found the Tube-Cells represented in
Fig. 119, if he had been acquainted with them, to answer his purpose just as
well as these specially constructed vessels.
LIFE-HISTORY OF PEOTOCOCCUS. 279
the endoclirome enclosed in its primordial utricle, first undergoing
separation into two halves (as seen at b), and each of these halves
subsequently developing a cellulose envelope around itself, and un-
dergoing the same division in its turn. Thus 2, 4, 8, 16 new cells
are successively produced ; and these are sometimes set-free by the
complete dissolution of the envelope of the original cell ; but they
are more commonly held-together by its transformation into a gela-
tinous investment, in which they remain imbedded. Sometimes
the contents of the primordial utricle subdivide at once into four seg-
ments (as at d), of which every one forthwith acquires the charac-
ters of an independent cell ; but this, although an ordinary method
of multiplication among the ' motile ' cells, is comparatively rare
in the ' still ' condition. Sometimes, again, the cell-contents of the
* still ' form subdivide at once into eight portions, which, being of
small size, and endowed with motile power, may be considered as
Zoospores ; it is not quite clear what becomes of these ; but
there is reason to believe that some of them retain their motile
powers, and, after increasing in size, develope an investing cyst,
like the free primordial utricles to be presently described ; that others
produce a firm cellulose envelope, and become ' still ' cells ; and that
others (perhaps the majority) perish without any further change.
208. When the ordinary self-division of the ( still ' cells into
two segments has been repeated four times, so as to produce 16
cells — and sometimes at an earlier period — the new cells thus
produced assume the ' motile ' condition ; being liberated before
the development of the cellulose envelope, and becoming furnished
with two long vibratile filaments, or cilia, which appear to be
extensions of the primordial utricle (h). In this condition it
seems obvious that the colourless protoplasm is more developed
relatively to the colouring-matter, than it is in the ' still ' cells ;
it generally accumulates in the part from which the vibratile fila-
ments or cilia proceed, so as to form a sort of transparent beak
(h, k, l) ; and it usually contains ' vacuoles,' occupied only by
clear aqueous fluid, which are sometimes so numerous as to take
in a large part of the cavity of the cell, so that the coloured con-
tents seem only like a deposit on its walls. Before long, this
' motile ' primordial utricle acquires a peculiar saccular invest-
ment, which seems to correspond with the cellulose envelope of
the 'still' cells, but is not so firm in its consistence (i, k, l).
Thread-like extensions of the protoplasm, sometimes containing
coloured globules, are not unfrequently seen to radiate from the
primordial utricle towards the exterior of this enveloping bag (i) ;
these are rendered more distinct by iodine, and can be made to
retract by means of re-agents ; and their existence seems to show,
on the one hand, that the transparent space through which they
extend themselves is only occupied by a watery liquid, and on the
other, that the layer of protoplasm which constitutes the primor-
dial utricle is far from possessing the tenacity of a completely
formed membrane. — The vibratile cilia pass through the cellulose
280 MICROSCOPIC FORMS OF VEGETABLE LIFE.
envelope, which invests their base with a sort of sheath ; and in
the portion that is within this sheath no movement is seen.
During the active life of the ' motile ' cells, the vibration of these
cilia is so rapid, that it can be recognised only by the currents it
produces in the water through which the cells are quickly pro-
pelled ; but when the motion becomes slacker, the filaments them-
selves are readily distinguishable ; and they may be made more
obvious by the addition of iodine.
209. The Multiplication of these ' motile ' cells may take place
in various modes, giving rise to a great variety of appearances.
Sometimes they undergo a regular binary subdivision, whereby a-
pair of motile cells is produced (c), each resembling its single pre-
decessor in possessing the cellulose investment, the transparent
beak, and the vibratile filaments, before the dissolution of the ori-
ginal investment. Sometimes, again, the contents of the primor-
dial cell undergo a segmentation in the first instance into four
divisions (d) ; which may either become isolated by the dissolution
of their envelope, and may separate from each other in the con-
dition of free primordial utricles (h), developing their cellulose
investments at a future time ; or may acquire their cellulose
investments (as in the preceding case) before the solution of that
of the original cell ; and sometimes, even after the disappearance
of this, and the formation of their own independent investments,
they remain attached to each other at their beaked extremities,
the primordial utricles being connected with each other by pedun-
cular prolongations, and the whole compound body having the form
of a -f-. This quaternary segmentation appears to be a more
frequent mode of multiplication among the ' motile ' cells, than
the subdivision into two ; although, as we have seen, it is less
common in the ' still ' condition. So, also, a primary segmentation
of the entire endochrome of the ' motile ' cells into 8, 16, or even
32 parts, may take place (e, f), thus giving rise to as many minute
primordial cells. These Micro-gonidia, when set free, and possess-
ing active powers of movement, rank as Zoospores (g) : they may
either develope a loose cellulose investment or cyst, so as to attain
the full dimensions of the ordinary motile cells (i, k), or they may
become clothed with a dense envelope and lose their vibratile cilia,
thus passing into the ' still ' condition (a) ; and this last trans-
formation may even take place before they are set free from the
envelope within which they were produced, so that they constitute
a mulberry -like mass, which fills the whole cavity of the original
cell, and is kept in motion by its cilia.
210. All these varieties, whose relation to each other has been
clearly proved by watching the successional changes that make up
the history of this one Plant, have been regarded as constituting,
not merely distinct species, but distinct genera of Animalcules ;
such as Chlamydomonas, Euglena, Trachelomonas, Gyges, Gonium,
Pandorina, Botryocystis, Uvella, Syncryjpta, Monas, Astasia, Bodo,
LIFE-HISTORY OF PROTOCOCCUS. 281
and probably many others.* Certain forms, such as the ' motile'
cells i, k, L, appear in a given infnsion, at first exclusively and then
principally ; they gradually diminish, become more and more rare,
and finally disappear altogether, being replaced by the ' still ' form.
After some time, the number of the ' motile ' cells again increases,
and reaches, as before, an extraordinary amount ; and this alterna-
tion may be repeated several times in the course of a few weeks.
The process of segmentation is often accomplished with great
rapidity. If a number of motile cells be transferred from a larger
glass into a small capsule, it will be found, after the lapse of a few
hours, that most of them have subsided to the bottom ; in the
course of the day, they will all be observed to be upon the point of
subdivision ; on the following morning, the divisional brood will
have become quite free ; and on the next, the bottom of the vessel
will be found covered with a new brood of self-dividing cells, which
again proceed to the formation of a new brood, and so on. — The
activity of Motion and the activity of- Multiplication seem to stand,
in some degree, in a relation of reciprocity to each other ; for the
self -dividing process takes -place with greater rapidity in the ' still '
cells, than it does in the ■ motile.'
211. What are the precise conditions which determine the tran-
sition between the ' still ' and ' motile ' states, cannot yet be pre-
cisely stated ; but the influence of certain agencies can be predicted
with tolerable certainty. Thus it is only necessary to pour the
water containing these organisms from a smaller and deeper into
a larger and shallower vessel, at once to determine segmentation
in numerous cells, — a phenomenon which is observable also in
many other Protophytes. The ' motile ' cells seem to be favourably
affected by Light, for they collect themselves at the surface of the
water and at the edges of the vessel ; but when they are about to
undergo segmentation, or to pass into the 'still' condition, they
sink to the bottom of the vessel, or retreat to that part of it in
which they are least subjected to light. When kept in the dark,
the ' motile ' cells undergo a great diminution of their chlorophyll,
which becomes very pale, and is diffused, instead of forming definite
granules ; they continue their movement, however, uninterruptedly,
without either sinking to the bottom, or passing into the still
form, or undergoing segmentation. A moderate warmth, particu-
larly that of the vernal sun, is favourable to the development of
the ' motile ' cells ; but a temperature of excessive elevation pre-
vents it. Eapid evaporation of the water in which the ' motile '
forms may be contained, kills them at once ; but a more gradual
* In the above sketch, the Author has presented the facts described by
Dr. Oohn, under the relation which they seemed to him naturally to bear, but
which differs from that in which they will be found in the original Memoir;
and he is glad to be able to state, from personal communication with its able
Author, that Dr. Cohn's later observations have led him to adopt a view of the
relationship of the ' still' and ' motile' forms, which is in essential accordance
with his own.
282 MICROSCOPIC FORMS OF VEGETABLE LIFE.
loss, sucli as takes-place in deep glasses, causes them merely to
pass into the - still ' form ; and in this condition, — especially when
they have assumed a red hue, — they may be completely dried-up,
and may remain in a state of dormant vitality for many years. It
is in this state that they are waf ted-about in atmospheric currents,
and that, being brought-down by the rain into pools, cisterns, &c.,
they may present themselves where none had been previously
known to exist ; and there, under favourable circumstances, they
may undergo a very rapid multiplication, and may maintain them-
selves until the water is dried-up, or some other change occurs
which is incompatible with the continuance of their vital activity.
They then very commonly become red throughout, the red colour-
ing-substance extending itself from the centre towards the circum-
ference, and assuming an ap}3earance like that of oil-drops ; and
these red cells, acquiring thick cell-walls and a mucous envelope,
float in flocculent aggregations on the surface of the water. This
state seems to correspond with the ' winter-spores ' of other Proto-
phytes ; and it may continue until warmth, air, and moisture
cause the development of the red cells into the ordinary ' still '
cells, green matter being gradually produced, until the red sub-
stance forms only the central part of the endochrome. After this,
the cycle of changes occurs which has been already described ; and
the Plant may pass through a long series of these, before it returns
to the state of the red thick-walled cell, in which it may again
remain dormant for an unlimited period. — Even this cycle, how-
ever, cannot be regarded as completing the History of the species
before us ; since it does not include the performance of any true
Generative act. There can be little doubt that, in some stage of
its existence, a Conjugation of two cells occurs, as in the pre-
ceding case ; and the attention of observers should be directed to
its discovery, as well as to the detection of other varieties in the
condition of this interesting little Plant, which will be probably
found to present themselves before and after the performance of
that act.
212. From the Composite ' motile ' forms of the preceding type,
the transition is easy to the group of Volvocinece,- — an assemblage of
minute Plants of the greatest interest to the Microscopist, on ac-
count both of the Animalcule-like activity of their movements, and
of the great beauty and regularity of their forms. The most re-
markable example of this group is the well-known Volvox globator
(Fig. 121), which is not uncommon in fresh-water pools, and which,
attaining a diameter of l-30th of an inch, may be seen with the
naked eye when the drop containing it is held-up to the light,
swimming through the water which it inhabits. Its onward motion
is usually of a rolling kind ; but it sometimes glides smoothly along,
without turning on its axis ; whilst sometimes, again, it rotates
like a top, without changing its position. When examined with a
sufficient magnifying power, the Volvox is seen to consist of a hollow
sphere, composed of a very pellucid material, which is studded at
STRUCTUEE OF VOLVOX GLOBATOK. 283
regular intervals with minute green spots, and which is often (bnt
not constantly) traversed by green threads connecting these spots
together. From each of the spots pro-
ceed two long cilia; so that the entire Fig. 121.
surface is beset with these vibratile fila- J-y&$^*ym-r. ,
ments, to whose combined action its *^0?^^0^Mh
movements are dne. Within the ex- ^^^^{^^^&&s;.
ternal sphere may generally be seen from Jg^^^^
two to twenty other globes, of a darker ^^^^^^f:\
colour, and of varying sizes ; the smaller iSl^sf
of these are attached to the inner sur-
face of the investing sphere, and pro-
ject into its cavity ; but the larger lie
freely within the cavity, and may often
be observed to revolve by the agency of
their own ciliary filaments. After a
time, the original sphere bursts, and the Volvox Globator.
contained spherules swim forth and
speedily develope themselves into the likeness of that within which
they have been evolved ; their component particles, which are at
first closely aggregated together, being separated from each other
by the interposition of the transparent pellicle. — It was long sup-
posed that the Volvox was a single Animal ; and it was first shown
to be a composite fabric, made up of a repetition of organisms in
all respects similar to each other, by Prof. Ehrenberg ; who,
however, considered these organisms as Monads, and described
them as each possessing a mouth, several stomachs, and an eye !
Our present knowledge of their nature, however, leaves no doubt
of their Vegetable character ; and the peculiarity of their History
renders it desirable to describe it in some detail.
213. Each of the so-called < Monads' (Plate IX., Figs. 9, 11) is in
reality a somewhat flask- shaped mass of Endochrome, about
l-3000th of an inch in diameter ; consisting, as in the previous in-
stances, of Chlorophyll-granules diffused through a colourless Pro-
toplasm ; and bounded by a layer of condensed protoplasm, which
represents a Primordial Utricle, but is obviously far from having
attained a membranous consistence. It is prolonged outwardly
(or towards the circumference of the sphere) into a sort of colour-
less beak or proboscis, from which proceed two long vibratile cilia
(Fig. 11) ; and it is invested by a pellucid or hyaline envelope
(Fig. 9, d) of considerable thickness, the borders of which are flat-
tened against those of other similar envelopes (Fig. 5, c, c), but
which does not appear to have the tenacity of a true membrane.
It is impossible not to recognise the precise similarity between the
structure of this body and that of the motile ' encysted ' cell of
Protococcus plwvialis (Plate VIII., Fig. 2, k) ; there is not, in fact,
any perceptible difference between them, save that which arises
from the regular aggregation, in Volvox, of the cells which normally
detach themselves from one another in Protococcus. The presence of
284 MICROSCOPIC FORMS OF VEGETABLE LIFE.
Cellulose in the hyaline substance is not indicated, in the ordinary-
condition of Volvox, by the iodine and sulphuric acid test, though
the use of ' Schultz's solution' gives to it a faint blue tinge ; there
can be no doubt of its existen ce, however, in the hyaline envelope
of what has been termed Volvox aureus, which seems to be the
sporangial form of Volvox globator (§ 218). The cilia and endo-
chrome, as in the motile forms of Protococcus, are tinged of a deep
brown by iodine, with the exception of one or two particles in each
cell, which, being turned blue, may be inferred to be Starch ; and
when the contents of the cell are liberated, bluish flocculi, appa-
rently indicative of the presence of Cellulose, are brought into view,
by the action of sulphuric acid and iodine. All these reactions are
characteristically Vegetable in their nature. — When the cell is ap-
proaching maturity, its Endochrome always exhibits one or more
' vacuoles ' (Fig. 9, a a), of a spherical form, and usually about one-
third of its own diameter ; and these ' vacuoles ' (which are the so-
called ' stomachs ' of Prof. Ehrenberg) have been observed by Mr.
G. Busk to undergo a very curious rhythmical contraction and dila-
tation at intervals of about 40 seconds ; the contraction (which
seems to amount to complete obliteration of the cavity of the
vacuole) taking-place rapidly or suddenly, whilst the dilatation is
slow and gradual. This curious action ceases, however, as the cell
arrives at its full maturity ; a condition which seems to be marked
by the greater consolidation of the primordial utricle, by the re-
moval or transformation of some of the chlorophyll, and by the
formation of the red spot (b), which obviously consists, as in Pro-
tococcus, of a peculiar modification of chlorophyll.
214. Each mass of Endochrome normally communicates with
those in nearest proximity with it, by extensions of its own sub-
stance, which are sometimes single and sometimes double (Fig. 5,
b, b) ; and these connecting processes necessarily cross the lines
of division between their respective hyaline investments. The
thickness of these processes varies very considerably ; for some-
times they are broad bands, and in other cases mere threads ; whilst
they are occasionally wanting altogether. This difference seems
partly to depend upon the age of the specimen, and partly upon
the abundance of nutriment which it obtains ; for, as we shall
presently see, the connection is most intimate at an early period,
before the hyaline investments of the cells have increased so much
as to separate the masses of endochrome to a distance from one
another (Figs. 2, 3, 4) ; whilst in a mature individual, in which the
separation has taken place to its full extent, and the nutritive
processes have become less active, the masses of endochrome very
commonly assume an angular form, and the connecting processes
are drawn-out into threads (as seen in Fig. 5), or they retain their
globular form, and the connecting processes altogether disappear.
The influence of re-agents, or the infiltration of water into the
interior of the hyaline investment, will sometimes cause the con-
necting processes (as in Protococcus, § 208) to be drawn back into the
PLATE IX.
fflMK £
<^v
aspy
177^/7
I
9
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Development of Voltos Globatoe
[To face p. 284.
STRUCTURE AND DEVELOPMENT OF VOLVOX. 285
central mass of endochrome ; and they will also retreat on the
mere rupture of the hyaline investment : from these circumstances
it may be inferred that they are not enclosed in any definite
membrane. On the other hand, the connecting threads are some-
times seen as double lines, which -seem like tubular prolonga-
tions of a consistent membrane, without any protoplasmic granules
in their interior. It is obvious, then, that an examination of a
considerable number of specimens, exhibiting various phases
of conformation, is necessary to demonstrate the nature of
these communications ; but this may be best made-out by attend-
ing to the history of their Development, which we shall now
describe.
215. The spherical body of the young Volvox (Plate IX., Fig. 1)
is composed of an aggregation of somewhat angular masses of
Endochrome (&), separated by the interposition of hyaline sub-
stance ; and the whole seems to be enclosed in a distinctly membran-
ous envelope, which is probably the distended hyaline investment
of the Primordial Cell, within which, as will presently appear, the
entire aggregation originated. In the midst of the polygonal
masses of endochrome, one mass (a), rather larger than the rest, is
seen to present a circular form; and this, as will presently appear,
is the originating cell of what is hereafter to become a new sphere.
The growing Volvox at first increases in size, not only by the inter-
position of new hyaline substance between its component masses
of endochrome, but also by an increase in these masses themselves
(Fig. 2, a), which come into continuous connection with each other
by the coalescence of processes (&) which they severally put-forth ;
at the same time an increase is observed in the size of the globular
cell (c), which is preliminary to its binary subdivision. A more
advanced stage of the same developmental process is seen in Fig. 3 ;
in which the connecting processes (a, a) are so much increased
in size, as to establish a most intimate union between the masses of
endochrome, although the increase of the intervening hyaline sub-
stance carries these masses apart from one another ; whilst the
endochrome of the central globular cell has undergone segmentation
into two halves. In the stage represented in Fig. 4, the masses of
endochrome have been still more widely separated by the interposi-
tion of hyaline substance ; each has become furnished with its pair
of ciliary filaments ; and the globular cell has undergone a second
segmentation. Finally, in Fig. 5, which represents a portion of the
spherical wall of a mature Volvox, the endochrome-masses are ob-
served to present a more scattered aspect, partly on account of their
own reduction in size, and partly through the interposition of a
greatly-increased amount of hyaline substance, which is secreted
from the surface of each mass ; and that portion which belongs to
each cell, standing to the endochrome-mass in the relation of the
cellulose coat of ordinary cells to their primordial utricle, is fre-
quently seen to be marked-out from the rest by delicate lines
of hexagonal areolation (c, c), which indicate the boundaries of
286 MICROSCOPIC FOEMS OF VEGETABLE LIFE.
each. Of these it is often difficult to obtain a sight, a nice manage-
ment of the light being usually requisite with fresh specimens ;
but the prolonged action of water (especially when it contains a
trace of iodine), or of glycerine, will often bring them into clear
view. The prolonged action of glycerine, moreover, will often show
that the boundary lines are double, being formed by the coalescence
of two contiguous cell-walls ; and they sometimes retreat from each
other so far that the hexagonal areolae become rounded. As the
primary sphere approaches maturity, the large secondary germ-
mass, or Macro-gonicUum, whose origin has been traced from the
beginning, also advances in development ; its contents undergoing
multiplication by successive segmentations, so that we find it to
consist of 8, 16, 32, 64, and still more numerous divisions, as shown
in Figs. 6, 7, 8. Up to this stage, at which first the sphere appears
to become hollow, it is retained within the hyaline envelope of the
cell within which it has been produced ; a similar envelope can be
easily distinguished, as shown in Fig. 10, just when the segmenta-
tion has been completed, and at that stage the cilia pass into it,
but do not extend beyond it ; and even in the mature Volvox it
continues to form an investment around the hyaline envelopes of
the separate cells, as shown in Fig. 11. It seems to be by the
adhesion of the hyaline investment of the new sphere to that of the
old, that the secondary sphere remains for a time attached to the
interior wall of the primary ; at what exact period, or in what
precise manner, the separation between the two takes place, has
not yet been determined. At the time of the separation, the de-
velopmental process has generally advanced as far as the stage
represented in Fig. 1 ; the foundation of one or more tertiary
spheres being usually distinguishable in the enlargement of certain
of its cells.
216. This development and setting-free of composite Macro-
gonidia seems to be the ordinary and characteristic mode of multi-
plication in Volvox ; but there are other phenomena which must
not be left without mention, although their precise import is as yet
uncertain. Thus, according to Mr. G. Busk, the body designated
by Prof. Ehrenberg Splicerosira volvox, is an ordinary Volvox in a
different phase of development ; its only marked feature of dis-
similarity being that a large proportion of the green cells, instead
of being single (as in the ordinary form of Volvox) save where they
are developing themselves into young spheres, are very commonly
double, quadruple, or multiple ; and the groups of ciliated cells
thus produced, instead of constituting a hollow sphere, form by
their aggregation discoid bodies, of which the separate fusiform cells
are connected at one end, whilst at the other they are free, each being-
furnished with a single cihum. These clusters separate themselves
from the primary sphere, and swim forth freely, under the forms
which have been designated by Prof. Ehrenberg as TJvella and
Syri&ryjota. (According to Mr. Carter, however, Sphcerosira is the
male or spermatic form of Volvox globator. See § 218, note.)
MULTIPLICATION OF VOLVOX.
287
Again, it has been noticed by Dr. Hicks* that towards the end
of the autumn, the bodies formed by the binary subdivision of the
single cells of Volvox, instead of forming spherical ciliated Macro -
gonidia which tend to escape outwards, form clusters of irregular
shape, each composed of an indefinite mass of gelatinous sub-
stance in which the green cells lie separately imbedded. These
clusters, being without motion, may be termed Stoio-spores ; and
it is probable that they constitute one of the forms in which the ex-
istence of this organism is prolonged through the winter, the others
being the product of the true Generative process to be presently
described.
217. Another phenomenon of a very remarkable nature, namely,
the conversion of the contents of an ordinary Yegetable cell into a
free moving mass of Protoplasm that bears a strong resemblance
to the animal Amoeba (Fig. 252), is affirmed by Dr. Hicksf to take
place in Volvox, under circumstances that leave no reasonable
ground for that doubt of its reality which has been raised in regard
to the accounts of similar phenomena occurring elsewhere. The En-
dochrome-mass of one of the ordinary cells increases to nearly
double its usual size ; but instead of undergoing duplicative sub-
division so as to produce a Macro -gonidium as in Fig. 122, b, it
Fig. 122.
Formation of Amoeboid Bodies in Volvox: — a, o, ordinary cells
passing into the amoeboid condition ; b, ordinary macro-gonidium ;
c, c, free amceboids.
loses its colour and its regularity of form, and becomes an irregu-
lar mass of colourless protoplasm containing a number of brown
* " Quart. Jonrn. of Microsc. Science," N.S.,
t " Trans, of Microsc. Society," N.S., Vol. viii.
Jonrn. of Microsc. Science " n.s., Vol. ii. (18G2), p.
Vol. i. (1861), p. 281.
(1860), p. 99, and " Quart
96.
288 MICEOSCOPIC FORMS OF VEGETABLE LIFE.
or reddish-brown granules (a, a), and capable of altering its form
by protruding or retracting any portion of its membranous wall,
exactly like a true Amoeba. By this self -moving power, each of
these bodies, c, c (of which twenty may sometimes be counted
within a single Volvox) glides independently over the inner surface
of the sphere among its unchanged green cells, bending itself
round any one of these with which it may come into contact, pre-
cisely after the manner of an Amoeba. After the Amoeboid has
begun to travel, it is always noticed that for every such moving
body in the Volvox there is the empty space of a missing cell ; and
this confirms the belief founded on observation of the gradational
transition from the one condition to the other, and on the difficulty
of supposing that any such bodies could have entered the sphere
parasitically from without, that the Amoeboid is really the product
of the metamorphosis of a mass of Vegetable protoplasm. This
metamorphosis may take place, according to Dr. Hicks, even after
the process of binary subdivision has commenced. What is the
subsequent destination of these Amoeboid bodies, has not yet been
certainly ascertained ; but from his observations upon similar
bodies developed from the protoplasmic contents of the roots of
Mosses, Dr. Hicks thinks it probable that they become converted
into minute ciliated bodies, which he has found to occur in larger or
smaller groups, enclosed in cavities formed in the mucous layer
just underneath the transparent sphere : of the subsequent history
of these, however, we are at present left entirely in the dark.*
218. But the reproduction of Volvox is not effected only by
processes which consist, under one form or another, in the multi-
plication of cells by subdivision. As already pointed out, the Life
History of no organism can be considered as complete, unless it
includes an act of Conjugation, or some other form of the true
Generative process ; and the observations of Dr. Cohnf fully bear
out this proposition in regard to Volvox. A sexual distinction
between Sperm-cells and Germ-cells, such as is seen in Vaucheria
* The known care and accuracy of Dr. Hicks gives a weight to his state-
ments as to the Amoeboid condition sometimes assumed by the contents of
Vegetable cells, which justifies their provisional reception, notwithstanding
their apparent improbability. It will be seen as we proceed (§ 300), that the
phenomenon is not so exceptional as it at first sight appears ; and it does not
involve any real confusion between the boundaries of Animal and Vegetable
life. For the mere fact of spontaneous motion by the extension and retraction
of processes of an indefinite Protoplasmic mass, no more makes that mass an
animal, than the vibration of the cilia formerly supposed to be exclusively pos-
sessed by Animalcules alters the truly vegetal character of the zoospores of a
Conferva or of the Volvox-siphere itself. Until proof shall have been given that
these Vegetable Amoeboids take into their interior, and appropriate by an act
of digestion, nutrient materials supplied either by the Vegetable or by the
Animal kingdom, the doctrine already stated (§ 198) as to the essential distinction
between the two Kingdoms in this particular holds good ; but recent observa-
tions seem to render it probable that an organism which lives a truly vegetal
life in one phase of its existence, may live a truly animal life in another (§ 864).
f " Annales des Sciences Naturelles," 4i6me Sdr., Botan., Tom. v. p. 323.
SEXUAL GENERATION OF VOLVOX. 289
(§270), shows itself in certain spheres of Volvox; these being
distinguishable by their greater size, and by the larger number of
their component utricles. They are generally monoecious, that is,
each sphere contains both kinds of sexual cells ; the greater number
of cells, however, remain neutral or asexual. The female or Germ-
cells exceed their neighbours in size, acquire a deeper green tint,
and become elongated towards the centre of the sphere ; their endo-
chrome undergoes no division. In the male or Sperm cells, on the
other hand, though resembling the germ-cells in size and form, the
endochrome breaks -up symmetrically into a multitude of linear
corpuscles, aggregated into discoidal bundles. These bundles are
beset with vibratile cilia, and move about within their cells, slowly
at first, afterwards more rapidly, and soon become separated into
their constituent corpuscles. Each of these has a linear body,
thickened at its posterior extremity, and is furnished with two long
cilia, bearing a strong general resemblance to the antherozoids of
Ghara (Fig. 172, h). These Antherozoids, escaping from the sperm-
cells within which they were produced, diffuse themselves through
the cavity of the sphere, and collect about the Germ-cells, which
probably have not yet acquired any distinct cell-wall ; so that the
Antherozoids can come into direct contact with their endochrome-
mass, to which they attach themselves by their prolonged rostrum
or beak. In this situation they seem to dissolve-away, so as to
become incorporated with the endochrome ; and the product of this
fusion (which is obviously only ' conjugation ' under another form)
is a reproductive globule or Spore. This body speedily becomes en-
veloped by an internal smooth membrane, and with a thicker external
coat which is usually beset with conical-pointed processes ; and the
contained Chlorophyll gives-place, as in Palmoglcea (§ 205), to
Starch and a red or orange-coloured Oil. As many as forty of
such Oospores* have been seen by Dr. Cohn in a single sphere of
Volvox, which thus acquires the peculiar appearance that has been
distinguished by Ehrenberg by a different specific name, Volvox
stellatus. Sometimes the Oo-spores are smooth ; and the sphere
charged with such is the V. aureus of Ehrenberg. That these two
reputed species are only different phases of the ordinary Volvox
globator, had been previously pointed out by Mr. G. Busk ; but they
were regarded by him, not as generative products, but as ' still ' or
' winter-spores.' — ISTo observer has yet traced out the develop-
mental history, either of the Stato-spores, or of the Oo-spores of
Volvox stellatus and aureus, or of the detached clusters of Splioe-
rosira; and these points offer themselves as problems of great
interest for any Microscopist whose locality offers ready means for
their solution.f
* The term Oospore (egg-spore) may be conveniently used to designate the
reproductive cell which is the immediate product of the Sexual act or of the
Conjugation -which represents it.
t The doctrine of the Vegetable nature of Volvox, which had been suggested
by Siebold, Braun, and other German Naturalists, was first distinctly enunciated
290 MICROSCOPIC FORMS OF VEGETABLE LIFE.
219. Desmidiacece. — Among the simplest tribes of Protophytes,
there are two which are of such peculiar interest to the
Microscopist, as to need a special notice ; these are the Desmi-
diacece and the Diatomacece. Both of them were ranked by
Ehrenberg and many other Naturalists as Animalcules ; but the
fuller knowledge of their life-history, and the more extended
acquaintance with the parallel histories of other simple forms of
Yegetation, which have been gained during the last twenty years,
are now generally accepted as decisive in regard to their
"Vegetable nature. — The Desmidiacece* are minute plants of a
green colour, growing in fresh water ; generally speaking, the
cells are independent of each other (Figs. 123, 125, 126) ; but
sometimes those which have been produced by binary subdivision
from a single primordial cell, remain adherent one to another in
linear series, so as to form a filament (Fig. 128). This tribe is
distinguished by two peculiar features ; one of these being the
semblance of a subdivision into two symmetrical halves, divided
by a ' sutural line,' which is sometimes so decided as to have led
to the belief that the cell is really double (Fig. 126, a), though in
other cases it is merely indicated by a slight notch ; whilst the
other is the frequency of projections from their surface, which are
sometimes short and inconspicuous (Fig. 126), but are often elon-
gated into spines, presenting a very symmetrical arrangement
by Prof. Williamson, on the basis of the history of its development, in the
"Transactions of the Philosophical Society of Manchester," Vol. ix. Sub-
sequently Mr. (Jr. Busk, whilst adducing additional evidence of the Vegetable
nature of Volvox, in his extremely valuable Memoir in the " Transactions of
the Microscopical Society," N.S., Vol. i. (1853), p. 31, called in question some
of the views of Prof. Williamson, which were justified by that gentleman in
his "Further Elucidations" in the same Transactions. The Author has
endeavoured to state the facts in which both these excellent observers agree
(and which he has himself had the opportunity of verifying), with the interpre-
tation that seems to him most accordant with the phenomena presented by
other Protophytes ; and he believes that this interpretation harmonizes with
what is most essential in the doctrines of both, their differences having been to
a certain degree reconciled by their mutual admissions.— The observations of
Dr. Cohn on the sexuality of Volvox have been confirmed by Mr. Carter
(" Ann. of Nat. Hist.," 3rd Ser., Vol. iii. 1859, p. 1), who, however, does not
accord with the account given above of the relations of its different forms.
According to him, V. globator and V. stellatus are essentially distinct; the
former is not monoecious but dioecious, Sphairosira volvox being its male or
spermatic form ; whilst the latter is monoecious. — An extremely interesting
Volvocine form described by Cohn tinder the name Stephcmosphcera pluvialis
exhibits all the phenomena of reproduction by Macro-gonidia or composite
masses of adherent cells, by Micro-gonidia or active zoospores, by ' still' or
Stato-spores, and by Oospores produced by true sexual action, in a very
characteristic manner ; and his account of its life-history should be consulted
by every one who desires to study that of any of the Protophyta. See "Ann.
of Nat. Hist." 2nd Ser., Vol. x. (1852), p. 321, and " Quart. Journ. of Microsc.
Sci.," Vol. vi. (1858), p. 131.
* Our first accurate knowledge of this group dates from the publication of
Mr. Kalfs's admirable Monograph of it in 1848. For later information see the
sections relating to it in Pritchard's "History of Infusoria," 4th Ed., 1861.
GENERAL CHARACTERS OF DESMIDIACE^E.
291
(Fig. 123). These projections are generally formed by the Cellulose
envelope alone, which jDossesses an almost horny consistence, so as
Fig. 123.
Various species of Staurastrum: — A, S. vestitum; Br S'. aculeatum ,-
C, & paradoxum ; D, E, S. brachiatum.
to retain its form after the discharge of its contents (Figs. 126,
b, d, 130, e), but does not inclnde any Mineral ingredient, either
calcareous or siliceous, in its composition; in other instances, how-
ever, they are formed by a notching of the margin of the cell
(Fig. 125), which may affect only the outer casing, or may extend
into the cell-cavity. The outer coat is surrounded by a very
transparent sheet of gelatinous substance, which is sometimes
very distinct (as shown in Fig. 128), whilst in other cases its
existence is only indicated by its preventing the contact of the
cells. The outer coat encloses an inner membrane or Primordial
Utricle, which is not always, however, closely adherent to it ; and
this immediately surrounds the Endochrome or coloured substance
which occupies the whole interior of the cell, and which in certain
stages of its growth is found to contain Starch-granules. — Many of
these Plants have a power of slowly changing their place, so that
they approach the light side of the vessel in which they are kept,
and will even traverse the field of the Microscope under the eye of
the observer ; by what agency this movement is effected has not
yet been certainly made out.
220. A Circulation of fluid has been observed in Closterium, not
only (as in the cells of higher Plants, § 322) within the Primordial
Utricle, but also (it is asserted) between this and the Cellulose en-
velope. It is not difficult to distinguish this movement along the
u2
292
MICROSCOPIC FOEMS OF VEGETABLE LIFE.
convex and concave edges of the cell of any vigorous specimen of
Closterhim, if it be examined under a magnifying power of 250 or
300 diameters ; and a peculiar whirling movement may also be
distinguished in the large rounded space which is left at each end
of the cell by the retreat of the Endochrome from the Primordial
Utricle (Fig. 124, a, b). By careful focussing, the circulation may
Fig. 124.
Circulation in Closterhim lunula .-—A, frond showing central separa-
tion at a, in which large globules, &, are not seen ; — B, one extre-
mity enlarged, showing at a the appearance of a double row of cilia,
at b the internal current, and at c the external current ; — c, external
jet produced by pressure on the frond (?) ; — D, frond in a state of self-
division.
be seen in broad streams over the whole surface of the endochrome ;
and these streams detach and carry with them, from time to time,
little oval or globular bodies (a, b) which are put-forth from it, and
are carried by the course of the flow to the chambers at the extre-
mities, where they join a crowd of similar bodies. In each of
these chambers (b), a current may be seen from the somewhat
abrupt termination of the Endochrome, towards the obtuse end of
the cell (as indicated by the interior arrows) ; and the globules it
contains are kept in a sort of twisting movement on the inner side
(a) of the primordial utricle. Other currents are seen externally
to it, which form three or four distinct courses of globules, passing
towards and away from c (as indicated by the outer arrows), where
they seem to encounter a fluid jetted towards them as if through an
aperture in the primordial utricle at the apex of the chamber ; and
here some communication between the inner and the outer currents
appears to take place.* This circulation is by no means peculiar
* See Lord S. G. Osborne's communications to the " Quart. Journ. of Microsc.
Sci.," Vol. ii. (1854), p. 234, and Vol. iii. (1855), p. 54.— Although the Circula-
BINARY SUBDIVISION OF DESMIDIACE^. 293
to Closterium, having been seen in many other Desmidiacece. —
Another cnrions movement is often to be witnessed in the interior
of the cells of members of this family, especially the various species
of Cosniarium, which has been described as ' the swarming of the
grannies,' from the extraordinary resemblance which the mass of
particles of Endochrome in active vibratory motion bears to a
swarm of bees. This motion continues for some time after the
particles have been expelled by pressure from the interior of the
cell, and it does not seem to depend (like that of true ' Zoospores')
upon the action of Cilia, but rather to be a more active form of the
molecular movement common to other minute particles freely sus-
pended in fluid (§ 144). It has been supposed that the ' swarming'
is related to the production of Zoospores (§ 209) ; but for this idea
there does not seem any adequate foundation.*
221. When the single Cell has come to its full maturity, it com-
monly multiplies itself by binary subdivision; but the plan on
which this takes place is often peculiarly modified, in order to
maintain the symmetry characteristic of the tribe. In a cell of
the simple cylindrical form of those of Didijmo^rium (Fig. 128),
little more is necessary than the separation of the two halves, which
takes place at the sutural line, and the formation of a partition
between them by the infolding of the primordial utricle according
to the plan already described (§ 204) ; and in this manner, out of
the lowest cell of the filament a, a double cell b is produced. But
it will be observed that each of the simple cells has a bifid wart-
like projection of the cellulose wall on either side, and that the half
of this projection, which has been appropriated by each of the two
new cells, is itself becoming bifid, though not symmetrically; in
process of time, however, the increased development of the sides
of the cells which remain in contiguity with each other brings up
the smaller projections to the dimensions of the larger, and the
symmetry of the cells is restored. — In Closterium (Fig. 124, d),
the two halves of the Endochrome first retreat from one another at
the sutural line, and a constriction takes place round the cellulose
wall ; this constriction deepens until it becomes an hour-glass con-
traction, which proceeds until the cellulose wall entirely closes
round the primordial utricle of the two segments ; in this state,
Hon is an un questionable fact, yet I have no hesitation in regarding the
appearance of ciliary action as an optical illusion due to the play of the peculiar
light employed among the moving particles of the fluid ; the appearance which
has been thus interpreted being producible at will (as Mr. Wenham has shown
in the same journal, Vol. iv. 1856, p. 158) by a particular adjustment of the
illumination, but being undiscoverable when the greatest care is taken to avoid
sources of fallacy. I must confess to a similar scepticism respecting the
external apertures said by Lord S. G. Osborne to exist at the extremities of
Closterium; for whilst their existence is highly improbable on a priori grounds,
Mr. Wenham (than whom no observer is entitled to more credit) states that
" not the slightest break can be discovered in the laminated structure that the
thickened ends display."
* See Archer in " Quart. Joum. of Microsc. Sci.," Vol. viii. (1860), p. 215.
294 MICKOSCOPIC FOEMS OF VEGETABLE LIFE.
one half commonly remains passive, whilst the other has a motion
from side to side, which gradually becomes more active ; and at last
one segment qnits the other with a sort of jerk. At this time a
constriction is seen across the middle of the primordial ntricle of
each segment ; but there is still only a single chamber, which is
that belonging to one of the extremities of the original entire frond.
The globular circulation, for some hours previously to subdivision,
and for a few hours afterwards, runs quite round the obtuse end a
of the endochrome ; but gradually a chamber is formed like that
at the opposite extremity, by a separation between the cellulose
coat and the primordial utricle ; whilst, at the same time, the obtuse
form becomes changed to a more elongated and contracted shape.
Thus, in five or six hours after the separation, the aspect of each
extremity becomes the same, and each half resembles the perfect
frond in whose self -division it originated ; and the globular circu-
lation within the newly-formed chamber comes into connection with
the general circulation, some of the free particles which are moving
over the surface of the primordial utricle being drawn into its
vortex and tossed-about in its eddies.
222. The process is seen to be performed after nearly the same
method in Staurastrum (Fig. 123, d, e) ; the division taking-place
across the central constriction, and each half gradually acquiring
the symmetry of the original. — In such forms as Cosmarium, how-
ever, in which the cell consists of two lobes united together by a
narrow isthmus (Fig. 126), the division takes place after a different
method ; for when the two halves of the outer wall separate at the
sutural line, a semiglobular protrusion of the Endochrome is put
forth from each half ; these protrusions are separated from one an-
other and from the two halves of the original cell (which their in-
terposition carries apart) by a narrow neck ; and they progressively
increase until they assume the appearance of the half-segments of
the original cell. In this state, therefore, the plant consists of
a row of four segments, lying end to end, the two old ones forming
the extremes, and the two new ones (which do not usually acquire
the full size or the characteristic markings of the original before
the division occurs) occupying the intermediate place. At last
the central fission becomes complete, and two bipartite fronds are
formed, each having one old and one young segment ; the young
segment, however, soon acquires the full size and characteristic
aspect of the old one ; and the same process, the whole of which
may take place within twenty -four hours, is repeated ere long.*
The same general plan is followed in Micrasterias denticulata
(Fig. 125) ; but as the small hyaline hemisphere, put-forth in the
first instance from each frustule (a), enlarges with the flo wing-in
of the endochrome, it undergoes progressive subdivision at its
* See the observations of Mrs. Herbert Thomas on Cosmarium margaHti-
ferum, in "Transact, of Microsc. Society," N.S., Vol. iii. 1855, pp. 33-36. —
Several varieties in the mode of subdivision are described in this short record
of long-continued observations, as of occasional occurrence.
BINABY SUBDIVISION OF DESMIDIACE^E.
295
edges, first into three lobes (b), then into five (c), then into seven
(d), then into thirteen (e), and finally at the time of its separation
Fig. 125.
Binary Subdivision of Micrasterias denticulata.
(f) acquires the characteristic notched ontline of its type, being
only distinguishable from the older half by its smaller size. The
whole of this process may take place within three hours and a
half.* — In Sphoerozosma, the cells thus produced remain connected
in rows within a gelatinous sheath, like those of Didymoprium
(Fig. 128) ; and different stages of the process may commonly be
observed in the different parts of any one of the filaments thus
formed. In any such filament, it is obvious that the two oldest
segments are found at its opposite extremities, and that each sub-
division of the intermediate cells must carry them further and
* See Lobb in " Transact, of Microsc. Society," N.S., Vol. ix. (1861), p. 1.
296 MICROSCOPIC FORMS OF VEGETABLE LIFE.
further from each other. This is a very different mode of in-
crease from that of the Gonfervacece, in which the terminal cell
alone undergoes subdivision (§ 273), and is consequently the one
last formed.
223. Although it is probable that the Desmidiacece generally
multiply themselves also by the subdivision of their endochrome
into a number of Zoospores, only one undoubted case of the kind
has yet been recorded (the Pediastrece, § 228, being no longer
ranked within this group) ; that, namely, of Docidium Wirenbergii,
whose elongated cell puts forth from the vicinity of the sutural
line one, two, or three tubular extensions resembling the finger of a
glove, through which there pass out from 20 to 50 motile Micro-
gonidia formed by the breaking-up of the endochrome of the
neighbouring portion of each segment.*
224. Whether there is in this group anything that corresponds
to the Encysting process (§ 207) or the formation of Stato-spores,
(§ 216) in other Protophytes, has not yet been certainly ascer-
tained ; but the following observations may have reference to such
a condition. It is stated by Focke that the entire endochrome of
Closterium sometimes retracts itself from' the cell- wall, and breaks
itself up into a number of globules, every one of which acquires a
very firm envelope. And it is affirmed by Mr. Jenner that " in
all the Desmidiaceas, but especially in Closterium and Micrasterias,
small, compact, seed-like bodies of a blackish colour are at times
to be met with. Their situation is uncertain, and their number
varies from one to four. In their immediate neighbourhood the
endochrome is wanting, as if it had been required to form them ;
but in the rest of the frond it retains its usual colour and appear-
ance." It seems likely that, when thus enclosed in a firm cyst,
the Gonidia are more capable of preserving their vitality, than
they are when destitute of such a protection ; and that in this con-
dition they may be taken-up and wafted through the air, so as to
convey the species into new localities.
225. The proper Generative process in the Desmidiacece is
always accomplished by the act of Conjugation ; and this takes
place after a manner very different from that in which we have
seen it to occur in Palmoglcea (§ 205). For each cell here pos-
sesses, it will be recollected, a firm external envelope, which cannot
enter into coalescence with that of any other ; and this membrane
dehisces more or less completely, so as to separate each of the
conjugating cells into two valves (Fig. 126, c, d; Fig. 127, c).
The contents of each cell, being thus set-free without (as it appears)
any distinct investment, blend with those of the other ; and a mass
is formed by their union, which soon acquires a truly membranous
envelope.f This envelope is at first very delicate, and is filled with
green and granular contents ; by degrees the envelope acquires
* See Archer in " Quart. Jourrt. of Microsc. Sci.," Vol. viii. (1860), p. 227.
f In certain species of Closterium, as in many of the Diatomacece (§ 240), the
act of conjugation gives origin to two Sporangia.
CONJUGATION IN DESMIDIACEJS.
297
increased thickness, and the contents of the spore-cell become
brown or red. The surface of the Sporangium, as this body is now-
termed, is sometimes smooth,
as in Closterium and its allies Fig. 126.
(Fig. 127) ; but in the Gosma-
riece, it acquires a granular, tu-
berculated, or even spinous sur-
face (Fig. 126), the spines being
sometimes simple and sometimes
forked at their extremities.* —
The mode in which Conjugation
takes place in the filamentous
species constituting the Desmi-
diece proper, is, however, in
many respects different. The
filaments first separate into their
component joints ; and when
two cells approach in conjuga-
tion, the outer cell-wall of each
splits or gapes at that part which
adjoins the other cell, and a new
growth takes place, which forms
a sort of connecting tube that
unites the cavities of the two
cells (Fig. 128, d, e). Through
this tube the entire endochrome transverse view
of one cell passes over into the empty fronds,
cavity of the other (d), and the
two are commingled so as to form a single mass (e), as is the
case in many of the Conjugates (§ 276). The joint which con-
tains the Sporangium can scarcely be distinguished at first (after
the separation of the empty cell), save by the greater densit}^
of its contents ; but the proper coats of the sporangium gra-
dually become more distinct, and the enveloping cell- wall dis-
appears.— The subsequent history of the Sporangia has hitherto
been made out in only a few cases. From the observations of Mrs.
H. Thomas (loc. cit.) on Cosmarium, it appeared that each sporan-
gium gives origin, not to a single cell but to a brood of cells ; and
this view is fully confirmed by HofEmeister (" Ann, of INat. Hist.,"
3rd Ser., Yol. i. 1858, p. 2), who speaks of it as beyond doubt that
the contents of the sporangia of Cosmarium are transformed by
repeated binary subdivisions into 8 or 16 cells, which assume the
original form of the parent before they are set free by the rupture
or diffluence of the wall of the sporangium. The observations of
Jenner and Focke render it probable that the same is the case in
Closterium ; but much has still to be learned in regard to the deve-
* Bodies precisely resembling these, and almost certainly to be regarded as
of like kind, are often found fossilized in Flints, and have been described by
Ehrenberg as the remains of Animalcules, under the name of Xanthidia.
Conjugation of Cosmarium botrytis: —
A, mature frond; B, empty frond; c,
D, sporangium with
298 MICEOSCOPIC FOKMS OP VEGETABLE LIFE.
lopment of the products of the Generative process, as it is by no
means certain that they always resemble the parent forms. For
Conjugation of Closterhim striatolum: — A, ordinary frond; B, empty frond;
C, two fronds in conjugation.
it is affirmed by Mr. Ealfs that there are several Desmidiaeeas
which never make their appearance in the same pools for two years
snccessively, althongh their Sporangia are abundantly produced, —
a circumstance which would seem to indicate that their Sporangia
give origin to some different forms. It is a subject, therefore, to
which the attention of Microscopists cannot be too sedulously
directed.
226. The subdivision of this Family into Genera, according to
the method of Mr. Ealfs ("British Desmidieee "), as modified by
Mr. Archer (Pritchard's " Infusoria "), is based in the first instance
upon the connection or disconnection of the individual cells ; two
groups being thus formed, of which one includes all the genera
whose cells, when multiplied by binary subdivision, remain united
into an elongated filament ; whilst the other comprehends all those
in which the cells become separated by the completion of the
fission. The further division of the filamentous group, in which
the Sporangia are always orbicular and smooth, is based on the
fact that in one set of genera the joints are many times longer
than they are broad, and that they are neither constricted nor
furnished with lateral teeth or projections ; whilst in the other set
(of which Didijmoprium, Fig. 128, is an example) the length and
breadth of each joint are nearly equal, and the joints are more or
less constricted, or have lateral teeth or projecting angles, or are
otherwise figured ; and it is for the most part upon the variations
CLASSIFICATION OF DESMIDIACE^E.
299
in these last particulars, that the generic characters are based.
The solitary group presents a similar basis for primary division
Fig. 128.
Binary subdivision and Conjugation of Didymoprium Grevillii: —
A, portion of filament, surrounded by gelatinous envelope ; B, dividing
joint; c, single joint viewed transversely; D, two cells in conjuga-
tion ; E, formation of sporangium.
in the marked difference in the proportions of its cells ; such elon-
gated forms as Glosterium (Figs. 124, 127), in which the length of
the frond is many times its breadth, being thus separated from
those in which, as in Micrasterias (Fig. 125), Gosmarium (Fig. 126),
and Staurastrum (Fig. 123), the breadth of the frond more nearly
equals the length. In the former the Sporangia are smooth,
whilst in the latter they are very commonly spinous and are some-
times quadrate. In this group, the chief secondary characters are
derived from the degree of constriction between the two halves of
300 MICROSCOPIC FORMS OF VEGETABLE LIFE.
tlie frond, the division of its margin into segments by incisions
more or less deep, and its extension into teeth or spines.
227. The Desmidiacece are not fonnd in running streams, unless
the motion of the water be very slow ; but are to be looked-for in
standing though not stagnant waters. Small shallow pools that
do not dry-up in summer, especially in open exposed situations,
such as boggy moors, are most productive. The larger and heavier
species commonly lie at the bottom of the pools, either spread-out
as a thin gelatinous stratum, or collected into finger-like tufts.
By gently passing the fingers beneath these, they may be caused
to rise towards the surface of the water, and may then be rifted
out by a tin-box or scoop. Other species form a greenish or dirty
cloud upon the stems and leaves of other aquatic plants ; and
these also are best detached by passing the hand beneath them,
and ' stripping ' the plant between the fingers, so as to carry off
upon them what adhered to it. If, on the other hand, the bodies
of which we are in search should be much diffused through the
water, there is no other course than to take it up in large quantities
by the box or scoop, and to separate them by straining through a
piece of linen. At first nothing appears on the linen but a mere
stain or a little dirt ; but by the straining of repeated quantities,
a considerable accumulation may be gradually made. This should
be then scraped off with a knife, and transferred into bottles with
fresh water. If what has been brought up by hand be richly
charged with these forms, it should be at once deposited in a
bottle ; this at first seems only to contain foul water ; but by
allowing it to remain undisturbed for a little time, the Desmi-
diaceae will sink to the bottom, and most of the water may then be
poured-off, to be replaced by a fresh supply. If the bottles be
freely exposed to solar light, these little plants will flourish,
apparently as well as in their native pools ; and their various
phases of multiplication and reproduction may be observed during
successive months or even years. — If the pools be too deep for the
use of the hand and the scoop, a Collecting-Bottle attached to a
stick (§194) may be employed in its stead. The King-Net (§ 194)
may also be advantageously employed, especially if it be so con-
structed as to allow of the ready substitution of one piece of muslin
for another. For by using several pieces of previously wetted
muslin in succession, a large number of these minute organisms
may be separated from the water ; the pieces of muslin may be
brought home f'olded-up in wide-mouthed bottles, either sepa-
rately, or several in one, according as the organisms are obtained
from one or from several waters ; and they are then to be opened
out in jars of filtered river-water, and exposed to the light, when
the Desmidiaceas will detach themselves.
228. Pediastrem. — The members of this family were formerly
included in the preceding group ; but, though doubtless related
to the true Desmidiacece in certain particulars, they present too
many points of difference to be properly associated with them.
MULTIPLICATION OF PEDIASTEEiE.
301
Their chief point of resemblance consists in the firmness of the
outer casing, and in the frequent interruption of its margin either
by the protrusion of ' horns ' (Fig. 129, a), or by a notching more
or less deep (Fig. 130, b) ; but they differ in these two important
Fig. 129.
Various phases of development of Pediastrum granulatum.
particulars, that the cells are not made up of two symmetrical
halves, and that they are always found in aggregation, which is
not — except in such genera as Scenodesmus (Arthrodesmus, Ehr.)
which connect this group with the preceding — in linear series, but
in the form of discoidal fronds. In this tribe we meet with a form of
multiplication by Zoospores aggregated into Macro-gonidia* which
reminds us of the formation of the motile spheres of Volvox (§ 215),
and which takes place in such a manner that the resultant product
may vary greatly in number of its cells, and consequently both in
size and in form. Thus in Pediastrum granulatum (Fig. 129), the
zoospores formed by the subdivision of the endochrome of one cell
into gonidia, which may be 4, 8, 16, 32, or 64 in number, escape
from the parent frond still enclosed in the inner tunic of the cell ;
and it is within this that they develope themselves into a cluster
resembling that in which they originated, so that whilst the frond
normally consists of 16 cells, it may be composed of either of the
just-mentioned multiples or sub-multiples of that number. At a
is seen an old disk, of irregular shape, nearly emptied by the
* Solitary zoospores or micro -gonidia have been observed by Braun to make
their way out and swim away ; but their subsequent history is unknown.
302 MICROSCOPIC FORMS OF VEGETABLE LIFE.
emission of its macro -gonidia, which had been seen to take-place
within a few honrs previously from the cells a, b, c, d, e ; most of the
empty cells exhibit the cross slit through which their contents had
been discharged ; and where this does not present itself on the
side next the observer, it occurs on the other. Three of the cells
still possess their coloured contents, but in different conditions.
One of them exhibits an early stage of the subdivision of the endo-
chrome, namely, into two halves, one of which already appears
halved again. Two others are filled by sixteen very closely-crowded
gonidia, only half of which are visible, as they form a double layer.
Besides these, one cell is in the very act of discharging its gonidia ;
nine of which have passed forth from its cavity, though still enve-
loped in a vesicle formed by the extension of its innermost mem-
brane ; whilst seven yet remain in its interior. Ths new-born
family, as it appears immediately on its complete emersion, is
shown at b ; the gonidia are actively moving within the vesicle ;
and they do not as yet show any indication either of symmetrical
arrangement, or of the peculiar form which they are subsequently
to assume. Within a quarter of an hour, however, the gonidia are
observed to settle-down into one plane, and to assume some kind of
regular arrangement, most commonly that seen at c, in which
there is a single central body surrounded by a circle of five, and
this again by a circle of ten ; they do not, however, as yet adhere
firmly together. The gonidia now begin to develope themselves
into new cells, increase in size, and come into closer approximation
(d) ; and the edge of each, especially in the marginal row, presents
a notch, which foreshadows the production of its characteristic
' horns.' Within about four or five hours after the escape of the
gonidia, the cluster has come to assume much more of the distinc-
tive aspect of the species, the marginal cells having grown-out into
horns (e) ; still, however, they are not very closely connected with
each other ; and between the cells of the inner row considerable
spaces yet intervene. It is in the course of the second day that the
cells become closely applied to each other, and that the growth of
the horns is completed, so as to constitute a perfect disk like that
seen at f, in which, however, the arrangement of the interior cells
does not follow the typical plan.*
229. The varieties which present themselves, indeed, both as
to the number of cells in each cluster, and the plan on which
they are disposed, are such as to baffle all attempts to base spe-
cific distinctions on such grounds ; and the more attentively the
Life-history of any one of these Plants is studied, the more evident
does it appear that many reputed Species have no real existence.
Some of these, indeed, are nothing else than mere transitory
forms ; thus it can scarcely be doubted that the specimen repre-
sented in Fig. 130, D, under the name of Pediastrum 'pertusum,
* See Prof. Braun on "The Phenomenon of Rejuvenescence in Nature,"
published by the Ray Society in 1853 ; and his subsequent Memoir, " Algarum
Unicellularum Genera nova aut minus cognita," 1855.
VARIATION AMONG PEDIASTRE,E. 303
is in reality nothing else than a yonng frond of P. granulatum,
in the stage represented in Fig. 129, e, hut consisting of 32 cells.
On the other hand, in Fig. 130, e, we see an emptied frond of P.
granulatum, exhibiting the peculiar surface-marking from which
Fig. 130.
lllf
&
rtfel*!*-
%m
Various species (?) of Pediastrum: — A. P. tetras; B, c, P. biradiatum ;
D, P. pertusum ; E, empty frond of P. granidatum.
the name of the species is derived, but composed of no more than
8 cells. And instances every now and then occur in which the frond
consists of only 4 cells, each of them presenting the two-horned
shape. So, again, in Fig. 130, b and c, are shown two varieties of
Pediastrum biradiatum, whose frond is normally composed of six-
teen cells ; whilst at a is figured a form which is designated as P.
tetras, but which may be strongly suspected to be merely a 4-celled
variety of b and c. Many similar cases might be cited ; and the
Author would strongly urge those Microscopists who have the
requisite time and opportunities, to apply themselves to the deter-
mination of the real species of these groups, by studying the
entire life history of whatever forms may happen to He within their
reach, and noting all the varieties which present themselves among
the offsets from any one stock. It must not be forgotten that this
process of multiplication is analogous to the propagation of the
higher Plants by Gemmation or ' budding,' and to the subsequent
separation of the buds, either spontaneously, or by the artificial
operations of grafting, layering, &c. ; and just as in all these cases
the particular variety is propagated, whilst only the characters of
the species are transmitted by the true Generative operation to the
descendants raised from Seed, so does it come to pass that the cha-
304 MICEOSOOPIC FOEMS OF VEGETABLE LIFE.
racters of any particular variety which may arise among these
Unicellular Plants, are diffused by the process of binary subdivi-
sion amongst vast multitudes of so-called individuals. Thus it
happens that, as Mr. Kalfs has remarked, " one pool may abound
with individuals of Staurastrum dejectum or Arthrodesmus incus,
having the mucro curved outwards ; in a neighbouring pool, every
specimen may have it curved inwards ; and in another it may be
straight. The cause of the similarity in each pool no doubt is,
that all its plants are offsets from a few primary fronds." Hence
the universality of any particular character, in all the specimens of
one gathering, is by no means sufficient to entitle these to take
rank as a distinct species ; since they are, properly speaking, but
repetitions of the same form by a process of simple multiplication,
really representing in their entire aggregate the one Plant or Tree
that grows from a single seed.
230. Diatomacece. — Notwithstanding the very close affinity which,
as will be presently shown, exists between this group and the
Desmidiacece, some Naturalists who do not hesitate in regarding
the members of the last-named family as Plants, persist in referring
the Diatomacece to the Animal kingdom. For this separation,
however, no adequate reason can be assigned ; the curious move-
ments which the Diatomacese exhibit being certainly not of a nature
to indicate the possession of any truly Animal endowment, and all
their other characters being unmistakably Vegetable. Like the Des-
midiacese they are simple Cells, having a firm external coating,
within which is included a mass of Endochrome whose superficial
layer seems to be consolidated into a sort of ' primordial utricle.'
The external coat is consolidated by silex, the presence of which
in this situation is one of the most distinctive characters of the
group ; and in some Diatoms — as Goscinodiscus — this siliceous
envelope is composed of two layers. It is a mistake, however, to
suppose that the casing is composed of Silex alone. For a Mem-
brane bearing all the markings of the siliceous envelope has been
found by Prof. Bailey to remain after the removal of the silex by
hydrofluoric acid ; and although this Membrane seems to have been
presumed by him, as also by Prof. W. Smith, to lie beneath the
siliceous envelope, and to secrete this on its surface as a sort of
epidermis, yet the Author agrees with the authors of the " Micro-
graphic Dictionary," in considering it much more likely that it is
the proper Cellulose wall interpenetrated by silex ; especially since
it has been found by Schmidt, that after removing the protoplasm
of Frustulia salina by potash, and the oil by ether, a substance
remains identical in composition with the Cellulose of Lichens.
Moreover, there are several Diatoms in which, as in Arachnoidiscus
(§ 252), a pellicle of vegetable membrane of horny consistence,
having markings of its own quite independent of those of the
silicified layer, overlies the latter ; and it is probably never entirely
absent, although it is sometimes thin enough to be removed by a
few seconds' immersion in boiling nitric acid. Hence, as Prof.
GENERAL CHARACTERS OF DIATOMACEJS. 305
"Walker Arnott lias justly observed,* the appearances presented by
individuals of the same species vary greatly, according to the treat-
ment to which they have been respectively subjected ; and no cer-
tainty can be obtained in the discrimination of Species, except by
the comparison of recent specimens, 1st, after being immersed for
a short time in cold nitric acid, or simply washed in boiling water ;
2nd, after being boiled in acid for about half a minute, or a whole
minute at most ; 3rd, after being boiled for a considerable time. Thus
it is obvious that specimens obtained from Guano or from Fossilized
deposits can only be rightly compared with recent specimens, when
the latter have been subjected to a treatment whereby their Organic
matter shall be removed as completely as possible.
231. The Endochrome of Diatomaceas, instead of being bright
green, is of a yellowish brown ; and its peculiar colour seems to
be in some degree dependent upon the presence of iron, which
is assimilated by the plants of this group, and may be detected
even in their colourless silicified envelopes. The colouring sub-
stance appears to be a modification of ordinary chlorophyll ; it
takes a green or greenish-blue tint with sulphuric acid ; and often
assumes this hue in drying. The Endochrome consists, as in other
plants, of a viscid protoplasm, in which float the granules of
colouring matter. In the ordinary condition of the cell, these
granules are diffused through it with tolerable uniformity, except
in the central spot, which is occupied by a nucleus ; round this
nucleus they commonly form a ring, from which radiating lines
of granules may be seen to diverge into the cell-cavity. At certain
times, Oil-globules are observable in the protoplasm ; these seem
to represent the starch-granules of the Desmidiaceas (§ 219) and the
oil-globules of other Protophytes (§ 201). A distinct movement
of the granular particles of the endochrome, closely resembling
the circulation of the cell-contents of the Desmidiacese (§ 220), has
been noticed by Prof. W. Smithf in some of the larger species of
Diatomaceas, such as Surirella biseriata, Nitzschia scalaris, and
Oampylodiscus spiralis, and by Prof. Max Schultzeiin Coscinodiscus,
Denticella,2bnARhizosolenia ; and although this movement has not the
regularity so remarkable in the preceding group, yet its existence is
important as confirming the conclusion that each Diatom is a single
Cell (the endochrome moving freely from one part of its cavity to
another), and that it does not contain in its interior the aggregation
of separate organs which have been imagined to exist in it.
232. The Diatomacece seem to have received their name from the
* " Quarterly Journal of Microscopical Science," Vol. vi. (1858), p. 163.
t The account of the Diatomacece given in this manual is chiefly based on
the valuable " Synopsis of the British Diatomacese," by the late Prof. W. Smith ;
of which, and of its beautiful illustrations by Mr. Tuffen West, the Author has
been enabled to make free use by the liberality of Messrs. Smith and Beck.
He has, however, entirely redrawn the sketch which he has given of the
Systematic arrangement of the group, in accordance with the more recent
classification of Mr. Balfs (Pritchard's " Infusoria," 4th Edition).
| " Quart. Journ. of Microsc. Science," Vol. vii. (1859), p. 13.
306 MICKOSCOPIC FOEMS OF VEGETABLE LIFE.
readiness with which those forms that grow in coherent masses
(which were those with which Naturalists first became acquainted)
may be cut or broken-through ; hence they have been also desig-
nated by the vernacular term 'brittle-worts.' Of this we have an
example in the common Diatoma (Fig. 140), whose component
cells (which in this tribe are usually designated as frustules) are
sometimes found adherent side by side (as at b) so as to form fila-
ments, but are more commonly met-with in a state of partial
separation, remaining connected at their angles only (usually the
alternate angles of the contiguous frustules) so as to form a zig-zag
chain. A similar cohesion at the angles is seen in the allied genus
Grammatophora (Fig. 141), in Isthmia (Fig. 147), and in many
other Diatoms ; in Biddulphia (Fig. 134), there even seems to be a
special organ of attachment at these points. In some Diatoms,
however, the frustules produced by successive acts of binary sub-
division habitually remain coherent one to another, and thus are
produced filaments or clusters of various shapes. Thus it is
obvious that when each frustule is a short cylinder, an aggrega-
tion of such cylinders, end to end, must form a rounded filament,
as in Melosira (Figs. 144 and 145) ; and whatever may be the
form of the sides of the frustules, if they be parallel one to the
other, a straight filament will be produced, as in Achnanthes
(Fig. 151). But if, instead of being parallel, the sides be some-
what inclined towards each other, a curved band will be the result ;
this may not continue entire, but may so divide itself as to form
fan-shaped expansions, as those of Lichmophora flabellata (Fig.
139) ; or the cohesion may be sufficient to occasion the band to
wind itself (as it were) round a central axis, and thus to form, not
merely a complete circle, but a spiral of several turns, as in
Meridion circulare (Fig. 137). Many Diatoms, again, possess a
stipes, or stalk -like appendage, by which aggregations of frustules
are attached to other plants, or to stones, pieces of wood, &c. ; and
this may be a simple foot-like appendage, as in Achnanthes lon-
gipes (Fig. 151), or it may be a composite Plant-like structure, as in
Lichmophora (Fig. 139), Gomphonema (Fig. 152), and Mastogloia
(Fig. 155). Little is known respecting the nature of this stipes;
it is, however, quite flexible, and may be conceived to be an exten-
sion of the cellulose coat unconsolidated by silex, analogous to the
prolongations which have been seen in the Desmidiacece (§ 219),
and to the filaments which sometimes connect the cells of the
Palmellacecs (§ 263). Some Diatoms, again, have a mucous or
gelatinous investment, which may even be so substantial that their
frustules lie as it were in a bed of it, as in Mastogloia (Figs. 155,
156), or which may form a sort of tubular sheath to them, as in
Schizonema (Fig. 154). In a large proportion of the group, how-
ever, the frustules are always met with entirely free; neither
remaining in the least degree coherent one to another after the
process of binary subdivision has once been completed, nor being
in any way connected either by a stipes or by a gelatinous invest-
SILICEOUS ENVELOPE OF DIATOMACE.E. 307
ment. This is the case, for example, with Triceratium (Fig. 132),
Pleurosigma (Fig. 133), Adinocyclus (Fig. 157, b, b), AdmoptydhuB
(Fig. 146), Arachnoidiscus (Plate X.), Campylodiscus (Fig. 143),
Surirella (Fig. 142), Coscinodiscus (Fig. 157, a, a, a), Heliopelta
(Plate i., fig. 3), and many others. The solitary discoidal forms,
however, when obtained in their living state, are commonly found
cohering to the surface of Seaweeds.
233. We have now to examine more minutely into the curious
structure of the Siliceous envelope which constitutes the charac-
teristic feature of the Diatoinaceas, and the presence of which im-
parts a peculiar interest to the group, not merely on account of
the elaborately-marked pattern which it often exhibits, but also
through the perpetuation of the minutest details of that pattern in
the specimens obtained from Fossilized deposits (Figs. 157, 158).
The siliceous envelope of every Diatomaceous cell or ' f rustule ' con-
sists of two valves or plates, usually of the most perfect symmetry,
closely applied to each other, like the two valves of a Mussel or
other bivalve shell, along a line of junction or suture ; and each
valve being more or less concavo-convex, a cavity is left between
the two, which is occupied by the cell-contents. The form of this
cavity, however, varies widely in different Diatoms ; for sometimes
each valve is hemispherical, so that the cavity is globular ; some-
times it is a smaller segment of a sphere resembling a watch-glass,
so that the cavity is lenticular ; sometimes the central portion is
completely flattened and the sides abruptly turned-up, so that the
valve resembles the cover of a pill-box, in which case the cavity
will be cylindrical ; and these and other varieties may co-exist
with any modifications of the contour of the valves, which may be
square, triangular (Fig. 132), heart-shaped (Fig. 143), boat-shaped
(Fig. 142, a), or very much elongated (Fig. 138), and may be
furnished (though this is rare among the Diatomaceae), with pro-
jecting out-growths (Figs. 148, 149). Hence the shape presented
by the frustule differs completely with the aspect under which it
is seen. In all instances, the frustule is considered to present its
'front' view when its suture is turned towards the eye, as in
Fig. 142, b, c ; whilst its ' side ' view is seen when the centre of
either valve is directly beneath the eye (a). Although the two
valves meet along the suture in those newly-formed frustules which
have been just produced by binary subdivision (as shown in Fig.
134, a, e), yet as soon as they begin to undergo any increase the
valves separate from one another, and the cell-membrane which is
thus left exposed immediately becomes consolidated by silex, and
thus forms a sort of hoop that intervenes between the valves (as
seen at c) ; this hoop becomes broader and broader with the increase
of the cell in length ; and it sometimes attains a very considerable
width (a, b). As growth and self -division are continually going-on
when the frustules are in a healthy vigorous condition, it is rare to
find a specimen in which the valves are not in some degree sepa-
rated by the interposition of the hoop.
x2
308 MICROSCOPIC FORMS OF VEGETABLE LIFE.
234. The impermeability of the Siliceous envelope renders neces-
sary some special aperture, through which the surrounding water
may come into relation with the contents of the cell. Such aper-
tures are found along the whole line of suture in disk-like frustules ;
but when the Diatom is of an elongated form, they are found at the
extremities of the frustules only. They do not appear to be abso-
lute perforations in the envelope, but are merely points at which
its siliceous impregnation is wanting ; and these are usually indi-
cated by slight depressions of its surface. In some Diatoms, as
Surirella (Fig. 142) and Gampylodiscus (Fig. 143), these inter-
ru|)tions are connected with what have been thought to be minute
canals hollowed out between the siliceous envelope and the mem-
brane investing the endochrome ; but it seems probable (§ 246) that
the apparent canals are really internal ribs, or projections of the
shell. — In many genera the surface of each valve is distinguished
by the presence of a longitudinal band on which the usual mark-
ings are deficient ; and this is widened into small expansions at
the extremities, and sometimes at the centre also, as we see in
Pleurosigma (Fig. 133) and Gomphonema (Fig. 153). This band
seems to be merely a portion in which the siliceous envelope is
thicker than it is elsewhere, forming a sort of rib that seems de-
signed to give firmness to the valve ; and its expansions are solid
nodules of the same substance. These nodules were mistaken by
Prof. Ehrenberg for apertures ; and in this error he has been fol-
lowed by Kutzing. There cannot any longer, however, be a doubt
as to their real nature. As Prof. W. Smith has justly remarked:—
" The internal contents of the frustule never escape at these points
when the frustule is subjected to pressure, but invariably at the
suture or at the extremities, where the foramina already described
exist. Nor does the valve, when fractured, show any disposition
to break at the expansions of the central line, as would necessarily
be the case were such points perforations and not nodules." And
Prof. Bailey has arrived at the same conclusion from watching the
results of the action of hydrofluoric acid on the silicified valves,
the thinnest parts of which are of course the first to be dissolved,
whilst the parts which have been described as apertures are found
to be the last to disappear. (See § 250).
235. The nature of the delicate and regular markings with which
probably every Diatomaceous valve is beset, has been of late years
a subject of much discussion among Microscopists ; but on certain
points there is now a general convergence of opinion. There can be
no question as to the nature of the comparatively coarse areolation
seen in the larger forms, such as Isthmia (Fig. 131), Triceratium
(Fig. 132), andiBiddulpMa (Fig. 134) ; in all of which that structure
can be distinctly seen under a low magnifying power and with
ordinary light, whilst with good immersion-lenses and careful illu-
mination a fine beading may be shown in the depressions. In each
of these instances we see a number of symmetrically disposed
areolae, rounded, oval, or hexagonal, with intervening boundaries ;
SUEFACE-MAEKIXGS OF DI ATOM ACE J3.
309
and the idea at once suggests itself, that tEese areola? are portions
of the surface either elevated above or depressed below the rest.
Fig. 131.
Portion of valve of Isthwia nervosa, highly magnified, as usually seen.
That the areola? are really depressions, is suggested by the appear -
ances presented by the surface when the light is obliquely directed ;
and it may also be inferred from their aspect when viewed by the
Fig. 132.
Triceratium favus : — A, side view; B, front view.
Black-ground Illumination (§ 94), since the areola? are then less
bright than their boundaries, less Eght being stopped by their
thinner substance. The view of these objects under the Binocular
Microscope fully confirms the inferences drawn from the phenomena
they present to the single eye ; presenting the network in unmis-
takable relief, and showing the areola? to be really depressions.
Moreover, when a valve is broken, the line of fracture corresponds
to what, on this view of its structure, is its weakest portion ; since
it passes through the areola? instead of through the intervening
310 MICEOSCOPIC FOEMS OF VEGETABLE LIFE.
network, which last, instead of forming the thick framework of tEe
valve, would be its weaker portion if the areola? were prominences.
But the most satisfactory proof that the areolae are depressions is
perhaps that which is afforded by a side-view of them, such as may
be obtained by examining the curved edges of the valves in Isthmia;
this, it may be safely affirmed, can leave no doubt in the mind of
any competent and unprejudiced observer as to the nature of the
markings in that genus ; and analogy would seem to justify the
extension of the same view to the other cases in which the mi-
croscopic appearances correspond.* — Both the depressed areolae
and the intervening network of Diatoms presenting these cha-
racters seem to be composed of minute spherules closely approxi-
mated. Such appearances are easily observed in favourable speci-
mens mounted in damar, or in bisulphide of carbon, using careful
unilateral illumination ; and parts of diatoms that still appear
plane, may look so merely because their spherules are too minute
and too close to each other for resolution. An examination of the
Diatoms in Moller's Type Slide will show insensible gradations
from coarser to finer forms ; and no prudent observer will be in a
hurry to assert that elevations, depressions, or headings cease just
at the point at which his optical apparatus fails to show them.
We shall presently see that Dr. Woodward (U.S.) has established the
existence of beading in the depressions of Triceratium fimbriatiim.
236. It is with regard to the more delicate markings on the minuter
Diatoms, and especially as to the nature of those on the valves of
the various species of Pleurosigma and other forms used as Test-
objects (§ 146), that some observers are still in doubt. These valves
were commonly spoken of as marked by striae, longitudinal, trans-
verse, or oblique, as the case may be ; but this term does not
express the real nature of the markings (the apjmrent lines being
resolvable by Objectives of sufficient magnifying power and angular
aperture into roivs of dots), and should only be used for the sake of
concisely indicating the degree of their approximation. If we examine
Pleiorosigma angulatum, one of the easier tests, with an objective of
l-4th inch focus (having an angular aperture of 90° and a magni-
* When specimens of Diatoms which exhibit this Areolation are examined by
the test of Focal Adjustment (§ 141), it is found that if they axe mounted in
Canada balsam, the optical effects are reversed ; the areolae being made to look
bright (like elevations) when the distance of the objective is increased, and dark
when it is diminished. This, however, is readily explicable by the fact that
the refractive power of the Balsam is greater than that of the Siliceous valve ;
so that the predominant effect will be produced by the convexities formed in
the medium by the concavities of the object. (See Schultze in " Quart. Journ.
of Microsc. Science," Vol. hi. N.S., 1863, p. 131.) It is maintained by Mr. By-
lands ("Quart. Journ. of Microsc. Science," Vol. viii. 1860, p. 27) that the
honeycomb structure is completed in many instances, as in Triceratium and
Coscinocliscus, by the closing-in of its cells or depressed areolae with siliceous
facets on their outer as well as on their inner side. The Author has not been
able to satisfy himself, however, that such is the case ; and he prefers to leave
the question to be resolved by such observers as specially occupy themselves
with this group.
SUEFACE-MAEKINGS OF DIATOMACE.E.
311
fying power of 500 diameters), we shall see very much what is re-
presented in Fig. 133, e ; namely, a double series of somewhat
Fig. 133.
Outline of Pleurosigma quadratum, as seen under a power of 400
diameters :— at A, B, d, are shown the directions of the lines seen under
a power of 1,300, the illuminating rays falling obliquely (in each case)
in a direction at right angles to the lines ; at E are shown two sets of
lines, as seen when the oblique rays fall in the direction of the midrib ;
and at c is shown the appearance of the markings when illuminated
with an Achromatic Condenser of large angular aperture, the spherules
being loithin the focus, and the portion left blank showing the oblitera-
tion of the markings by moisture.
interrupted lines, crossing each other at an angle of 60 degrees, so
as to have between them imperfectly-defined lozenge- shaped spaces.
"When, however, the valve is examined with an objective of higher
power, having an angular aperture of 120° or more, and a magni-
fying power of 1,200 diameters, an appearance like that represented
in Fig. 103, namely, an hexagonal areolation somewhat resembling
that of Triceratium (Fig. 132), in which the areola? can be made to
appear light, and the dividing network dark, or vice versa, accord-
ing to the adjustment of the focus, may be obtained. Analogy
312 MICROSCOPIC FORMS OF VEGETABLE LIFE.
would obviously favour the idea that this apparent hexagonal
areolation of Pleurosigma is of the same kind as that of Tricera-
tium, and that the areolas are depressions in the former, as they
certainly are in the latter ; but the fact that in certain species of
Triceratium, Coscinodiscus, and Actinocyclus, the floors of the
hexagonal depressions are studded with markings resembling those
of a Pleurosigma, these being particularly conspicuous in the
beautiful Heliopelta (Plate I., fig. 3), seems to indicate that these two
forms of structure are essentially different. There is reason to be-
lieve, indeed, that in these and other instances there are two sets
of markings belonging to two distinct layers.* Dr. Woodward has
succeeded in photographing the fine markings on the floor of the
depressions of Triceratium fimbriatum. He found with the best
objectives and white light illumination, rows of minute beads
presenting a greenish colour upon a greenish ground, approximat-
ing to the beading of Pleurosigma angulatum. When specimens of
Pleurosigma mounted beneath glass have had their markings ob-
scured by moisture, the obscurity is dissipated by the application
of a gentle heat, in a way that is readily explicable on the suppo-
sition that the markings are elevations, but is wholly unintelligible
on the idea of their being depressions.f — Further, in the case of the
Triceratium, the hexagonal depressions may be made, by manage-
ment of the focussing and illumination, to assume the aspect of
rounded elevations ; and in like manner the apparent hexagons of
Pleurosigma vanish and are replaced by rows of beads, when the
focus is changed and the illumination suitable. The simplest way
of deciding which appearance is to be accepted in each case, is to
examine fractured valves. In Triceratium the fractures pass
through the apparent depressions, and coincide with various optical
indications in establishing their reality. Fractured valves of
P. angulatum and allied species show that the weakest parts are
between the bead-rows ; and single beads may often be seen termi-
nating a sharp angular portion. The supposition derived from
analogy, that there is a common plan of structure between
Triceratium, Pleurosigma, and Diatoms in general, may neverthe-
less be correct, if, as there is some reason to believe, siliceous
spherules are in all cases the units of their formation.^
* See Mr. C. Stodder (of Boston, TJ. S.), " On the Structure of the Valve of
the Diatomacece," in "Quart. Journ. of Mici-osc. Science," Vol. iii. N.S. (1863),
p. 214 ; also Balfs, Op. cit., Vol. vi. (1858), p. 214; and Bylands, Op. cit., Vol.
viii. (I860), p. 27.
f See Mr. G. Hunt in " Quart. Journ. of Microsc. Sci." Vol. iii. (1855), p. 174.
t See Dr. Wallich's Papers on this subject in " Quart. Journ. of Microsc.
Science," Vol. vi. (1858), p. 247 ; " Annals of Nat. Hist.," Vol. v. Ser. 4 (Feb.
1860), p. 122 ; and '-Trans, of Micr. Boa," Vol. viii., N.S. (1860), p. 129. See
also Norman in "Quart. Journ. of Microsc. Sci.," Vol. ii., N.S. (1862), p. 212. —
Mr." Wenhani, who at one time inclined to the belief that the areolae are de-
pressions, stated (when Dr. Wallich's Paper was read before the Microscopical
Society), as the result of observations made with an Objective of l-50th inch
focus and large aperture, that the valves are composed wholly of spherical
MULTIPLICATION OF DIATOMACE^E.
313
237. The process of Multiplication by binary subdivision takes
place among the Diatomacece on the same general plan as in the
Desniidiaceas, but with
some modifications in-
cident to the peculiari-
ties of the structure of
the former grou]3. — The
first stage consists in
the elongation of the cell,
and the increase in the
breadth of the 'hoop,'
which is well seen in
Fig. 134, a ; for in the
newly formed cell e, the
two valves are in imme-
diate apposition, in d a
hoop intervenes, in a
this hoop has become
much wider, and in b
the increase has gone-on
until the original form
of the cell is completely
changed. At the same
time, the endochrome se-
parates into two halves,
so tbat its granules form
two layers applied to
the opposite sides of the
frustule ; the nucleus
also subdivides, in the
manner formerly shown , Biddulphia pulchdla :-a, chain of cells indif-
/p-. . fi i \ ferent states ; a, full size ; b, elongating prepa-
(riate \IIL, tig 1 G, H, I) ; ratory to subdivision ; c, formation of two new
and (although the pro- cells; rf,e, young cells ;-b, end-view;— c, side-
cess has not been clearly view of a cell more highly magnified.
particles of silex, possessing high refractive power ; and he showed how all
the various optical appearances presented by the different species could be
reconciled with the supposition that their structure is universally the same.
Mr. W. has succeeded in obtaining distinct impressions of the surface-markings
by the Galvano-plastic process. (See "Quart. Joum. of Microsc. Science,"
Vol. iii., 1855, p. 244). — The opinion of Prof. Max Schultze, however, by whom
this subject has been very elaborately investigated, does not harmonize with
the foregoing. He affirms that "neither spherical, conical, nor pyramidal
elevations are the cause of the punctated appearance, although the decussating
sets of ridges may at the points of intersection afford an appearance resembling
that of tubercular elevations." And he considers that the sculpturing, both in
the coarsely and in the finely marked Diatom-valves, though at first sight
allied to what is seen on the surface of artificial siliceous pellicles, is in reality
due to wholly different conditions. (See his Memoir " Die Structur der Diato-
meenschale," and the Abstract of it in " Quart. Joum. of Microsc. Science,"
Vol. iii. N.S., 1863, p. 120.)
314 MICROSCOPIC FORMS OF VEGETABLE LIFE.
made-out in this group) it may be pretty certainly concluded
that the primordial utricle folds-in, first forming a mere con-
striction, then an hour-glass contraction, and finally a complete
double partition, as in other instances (§ 204). From each of
these two surfaces a new siliceous valve is formed, as shown at
Fig. 134, a, c, just as a new cellulose-wall is generated in the
subdivision of other cells; and this valve is usually the exact
counterpart of the one to which it is opposed, and forms with it a
complete cell, so that the original frustule is replaced by two
frustules. Sometimes, however, the new valves seem to be a little
larger than their predecessors ; so that, in the filamentous species,
there may be an increase sufficient to occasion a gradual widening
of the filament, although not perceptible when two contiguous
frustules are compared; whilst, in the free forms, frustules of
different size may be met with, of which the larger are more
numerous than the smaller, the increase in number having taken
place in geometrical progression, whilst that of size was uniform.
It is not always clear what becomes of the 'hoop.' In Melosira
(Figs. 144, 145), and perhaps in the filamentous species generally,
the ' hoops' appear to keep the new frustules united together for
some time. This is at first the case also in Biddulpliia and
Isthmia (Fig. 147), in which the continued connection of the two
frustules by its means gives rise to an appearance of two complete
frustules having been developed within the original (Fig. 134, a, c);
subsequently, however, the two new frustules slip out of the hoop,
which then becomes completely detached; and the same thing
happens with many other Diatoms, so that the ' hoops' are to be
found in large numbers in the settlings of water in which these
plants have long been growing. But in some other cases all trace
of the hoop is lost ; so that it may be questioned whether it has
ever been properly silicified, and whether it does not become fused
(as it were) into the gelatinous envelope. — During the healthy life
of the Diatom, the process of self-division is continually being re-
peated ; and a very rapid multiplication of frustules thus takes
place, all of which (as in the cases already cited, §§ 221, 229,) must
be considered to be repetitions of one and the same individual
form. Hence it may happen that myriads of frustules may be
found in one locality, uniformly distinguished by some peculiarity
of form, size, or marking ; which may yet have had the same remote
origin as another collection of frustules found in some different
locality, and alike distinguished by some peculiarity of its own.
For there is strong reason to believe that such differences spring-up
among the progeny of any true generative act (§ 239) ; and that
when that progeny is dispersed by currents into different localities,
each will continue to multiply its own special type so long as the
process of self-division goes on.
238. It is uncertain whether the DiatomaceaB also multiply by
the breaking-up of their endochrome into Gonidia, and by the
liberation of these, either in the active condition of ' zoospores,' or
MULTIPLICATION OF DIATOMACEiE. 315
in the state of ' still ' or ' resting * spores. Certain observations by
Focke,* however, taken in connection with the analogy of other
Protophytes, and with the fact that the Sporangial frustnles un-
doubtedly thus multiply by gonidia (§ 241), seem to justify the
conclusion that such a method of multiplication does obtain in
this group. And it is not at all improbable that very considerable
differences in the size, form, and markings of the frustules, such
as many consider sufficient to establish a diversity of species, have
their origin in this mode of propagation. It is probable that, so
long as the vegetating processes are in full activity, multiplication
takes place in preference by self-division ; and that it is when
deficiency of warmth, of moisture, or of some other condition,
gives a check to these, that the formation of encysted Gonidia,
having a greater power of resisting unfavourable influences, will
take-place ; whereby the species is maintained in a dormant state
until the external conditions favour a renewal of active vegetation
(§ 211).
239. Prof. W. H. Smith (U.S.), in the second part of his " Memoir
on the Diatomaceas," published in the Lens, considers the Diatom-
frustules as siliceous boxes, with one portion (the cover) slipping
over another, as in Pinnularice, or with edges simply opposed, as in
Frag Maria. In the formation of a new valve, the new part, which
slips out from the older, is somewhat smaller. In the contents of
the " box " he sees, in the larger forms, a distinct nucleus, or some-
times two nuclei, and sometimes a "germinal dot," with numerous
fine threads radiating from the nucleus or the germinal dot. As
the frustule widens, one portion slips from out the other, and
siliceous additions are made to the margin of the box, somewhat
after the manner of those made to the edge of the shell of a Mollusk.
He believes that a double membrane of extreme tenuity commences
its growth at the nucleus (which itself divides), and extends to the
margins of the cell, and folds in as the fission progresses. He has
watched the whole process in large Pinnularice. The actual fission
occurs in fifteen or twenty minutes, but the whole process of self-
division occupies about six days. The part which slips out carries
away one of the old valves ; and by further self-division the new
valve becomes the old one for a second formation ; and so the frus-
tules become smaller and smaller, as stated by Braun. At this
period conjugation occurs, and a return to the normal condition of
the original large frustule, by the formation of a sporangial frustule
double the size of the parent frustules.
240. The process of Conjugation or true Generation has been
observed to take-place among the ordinary DiatomaceaB, almost
exactly as among the Desmidiaceae. Thus in Surirella (Fig. 142) the
valves of two free and adjacent frustules separate from each other
at the sutures, and the two endochromes (probably included in their
primordial utricle) are discharged ; these coalesce to form a single
* " Physiologisch. Studien," Heft ii. 1853.
316
MICROSCOPIC FORMS OF VEGETABLE LIFE.
Sporangial mass, which becomes enclosed in a gelatinous envelope ;
and in due time this mass shapes itself into a frnstnle resembling
that of its parent, but of larger size. In UJpithemia (Fig. 135, a, b),
Conjugation of Epithemia turgida: — A, front view of single frustule ;
B, side view of the same ; C, two frustules with their concave surfaces
in close apposition ; D, front view of one of the frustules, showing the
separation of its valves along the suture ; E, F, side and front views
after the formation of the sporangia.
however — the first Diatom in which the conjugating process was
observed by Mr. Thwaites* — the endochrome of each of the. con-
jugating frustules (c, d) appears to divide at the time of its dis-
charge into two halves ; each half coalesces with half of the other
endochrome ; and thus two sporangial frustules (e, f) are formed,
which, as in the preceding case, become invested with a gelatinous
envelope, and gradually assume the form and markings of the
parent-frustules, but grow to a very much larger size, the sporan-
gial masses having obviously a power of self -increase up to the
time when their envelopes are consolidated. This doubling of the
sporangial product of conjugation seems to be the ordinary type
of the process among the Diatoms. A curious departure from the
usual plan is observed in some of the filamentous species ; for their
component cells, instead of conjugating with those of another
* See "Annals of Natural History," Ser. 1, Vol. xx. (1847), pp. 9, 343, and
Ser. 2, Vol. i. (1848), p. 161.
CONJUGATION OF DIATOMACE^L
317
filament (as is the case with the filamentous Besmidiacece, § 225,
and usually but not invariably with the Zygnemacece, § 276),
conjugate with each other ; and this may take place even before
they have been completely separated by self-division. Thus in
Melosira (§ 248) and its allies, the endochrome of particular
frustules, after separating as if for the formation of a pair of new
cells, moves-back from the extremities towards the centre, rapidly
increasing in quantity, and aggregating into a sporangia! mass
(Fig. 136, 2, a, b, c) ; and around this a new envelope is developed,
Fig. 136.
Self-Conjugation of Melosira Italica (Aulacoseira crenulata,
Thwaites) : — 1, simple filament ; 2, filament developing sporan-
gia ; a, 6, c, successive stages in the formation of sporangia; 3,
embryonic frustules, in successive stages, a, 6, c, of multiplication.
which may or may not resemble that of the ordinary frustules, but
which remains in continuity with them, giving rise to a strange
inequality in the size of the different parts of the filaments (Figs.
144, 145).
241. Of the subsequent history of the Sporangial frustule, much
remains to be learned ; and it is probably not the same in all cases.
It has been already shown that the sporangial frustule, even where
it precisely resembles its parent in form and marking, greatly ex-
ceeds it in size ; and this excess seems to render it improbable that
it should reproduce the race by ordinary self -division. Appearances
have been seen which make it probable that the contents of each
sporangial frustule break-up into a brood of Gonidia, and that it
is from these that the new generation originates. These gonidia,
if each be surrounded (as in many other cases) by a distinct cyst,
may remain undeveloped for a considerable period ; and thev must
augment considerably in size, before they obtain the dimensions of
the parent frustule. — It is in this stage of the process, that the
modifying influence of external agencies is most likely to exert its
318 MICROSCOPIC FOEMS OF VEGETABLE LIFE.
effects ; and it may be easily conceived that (as in higher Plants
and Animals) this influence may give rise to various diversities
among the respective individuals of the same brood ; which diver-
sities, as we have seen, will be transmitted to all the repetitions of
each, that are produced by the seff-dividing process. Hence a very
considerable latitude is to be allowed to the limits of Species, when
the different forms of Diatoniaceas are compared ; and here, as in
many other cases, a most important question arises as to what are
those limits, — a question which can only be answered by such a
careful study of the entire life-history of every single type, as may
advantageously occupy the attention of many a Microscopist who
is at present devoting himself to the mere detection of differences
and to the multiplication of reputed species.*
242. Most of the Diatoms which are not fixed by a stipes possess
some power of spontaneous movement ; and this is especially seen
in those whose frustules are of a long narrow form, such as that of
the Navicular generally. The motion is of a peculiar kind, being
usually a series of jerks, which carry forward the frustule in the
direction of its length, and then carry it back through nearly the
same path. Sometimes, however, the motion is smooth and
equable ; and this is especially the case with the curious Bacillaria
paradoxa (Fig. 138), whose frustules slide over each other in one
direction until they are all but detached, and then slide as far in
the opposite direction, repeating this alternate movement at very
regular intervals.f In either case the motion is obviously quite of
a different nature from that of beings possessed of a power of self-
direction. " An obstacle in the path," says Prof. W. Smith, " is
not avoided, but pushed-aside ; or, if it be sufficient to avert the
onward course of the frustule, the latter is detained for a time equal
to that which it would have occupied in its forward progression,
and then retires from the impediment as if it had accomplished its
full course." The character of the movement is obviously similar
to that of those motile forms of Protophyta which have been already
described ; but it has not yet been definitely traced to any organ of
impulsion ; and the cause of it is still obscure. J By Prof. W. Smith
* See on this subject a valuable paper by Prof. W. Smith 'On the Detennina-
tion of Species in the Diatomacece,' in the "Quart. Joum. of Microsc. Science,"
Vol. iii. (1855), p. 130 ; a Memoir by Prof. W. Gregory ' On shape of Outline
as a specific character of Diatomacece,'' in " Trans, of Microsc. Soc," 2nd Series,
Vol. iii. (1855), p. 10; and the Author's Presidential Address in the same
volume, pp. 44-50.
t This curious phenomenon the Author has himself repeatedly had the
opportunity of witnessing.
% Prof. Smith says: — "Among the hundreds of species which I have ex-
amined in every stage of growth and phase of movement, aided by glasses
which have never been surpassed for clearness and definition, I have never
been able to detect any semblance of a motile organ ; nor have I, by colouring
the fluid with carmine or indigo, been able to detect in the coloured particles
surrounding the Diatom, those rotatory movements which indicate, in the
various species of true Infusorial animalcules, the presence of cilia." (" Synopsis
of British DiatomaceEe," Introduction, p. xxiv.)
MOVEMENTS OF DIATOMACE^. 319
it is referred to forces operating within the f rastnle, and originating
in the vital operations of growth, &c, which may canse the sur-
rounding fluid to be drawn-in through one set of apertures, and
expelled through the other* "If," as he remarks, " the motion
be produced by the exosmose taking-place alternately at one and the
other extremity, while endosmose is proceeding at the other, an
alternating movement would be the result in frustules of a linear
form ; whilst in others of an elliptical or orbicular outline, in which
foramina exist along the entire line of suture, the movements, if
any, must be irregular or slowly lateral. Such is precisely the case.
The backward and forward movements of the Navicular have been
already described ; in Surirella (Fig. 1 42) and Campylodiscus
(Fig. 143), the motion never proceeds further than a languid roll
from one side to the other ; and in Gomphonema (Fig. 153), in
which a foramen fulfilling the nutritive office is found at the larger
extremity only, the movement (which is only seen when the f rustule
is separated from its stipes) is a hardly perceptible advance in
intermitted jerks in the direction of the narrow end."
243. The principles upon which this interesting group should be
classified, cannot be properly determined, until the history of the
Generative process — of which nothing whatever is yet known in a
large proportion of Diatoms, and very little in any of them, — shall
have been thoroughly followed-out. The observations of Fockef
render it highly probable that many of the forms at present con-
sidered as distinct from each other, would prove to be but different
states of the same, if their ivhole history were ascertained. On the
other hand, it is by no means impossible that some which appear
to be nearly related in the structure of their frustules and in their
mode of growth, may prove to have quite different modes of repro-
duction. At present, therefore, any classification must be merely
provisional ; and in the notice now to be taken of some of the most
interesting forms of the Diatomacece, the method of Prof. Kiitzing,
which is based upon the characters of the individual frustules, is
followed in preference to that of Prof. W. Smith, which was founded
on the degree of connection remaining between the several frustules
* It has been objected to this view, by the authors of the " Micrographic
Dictionary," that, if such were the case, the like movements would be frequently
met with in other minute unicellular organisms. They seem to have forgotten,
however, that there are no other such organisms in which the cell is almost
entirely enclosed in an impermeable envelope, the imbibition and expulsion of
fluid being thus limited to a small number of definite points, instead of being
allowed to take place ecpually (as in other unicellular organisms) over the entire
surface.
f According to this observer ("Ann. of Nat. Hist.," 2nd Ser., Vol. xv., 1855,
p. 237) Navicula bifrons forms, by the spontaneous fission of its internal sub-
stance, spherical bodies which, like gemmules, give rise to Surirella microcora.
These by conjugation produce A. splendida, which gives rise to A. bifrons by the
same process. He is only able to speak positively, however, as to the pro-
duction of A", bifrons from A. splendida; that of Surirella microcora from Ar. bifrons,
and that of A. splendida from Surirella microcora, being matters of inference
from the phenomena witnessed by him.
320 MICEOSCOPIC FORMS OF VEaETABLE LIFE.
after self-division.* — In each Family the frnstnles may exist under
four conditions ; (a) free, the self-division being entire, so that the
frustules separate as soon as the process has been completed ; (&)
stipitate, the frustules being implanted upon a common stem
(Fig. 139), which keeps them in mutual connection after they have
themselves undergone a complete self -division ; (c) united in a fila-
ment, which will be continuous (Fig. 144) if the cohesion extend
to the entire surfaces of the sides of the frustules, but may be a
mere zig-zag chain (Fig. 140) if the cohesion be limited to their
angles ; (d) aggregated into a frond (Fig. 154), which consists of
numerous frustules more or less regularly enclosed in a gelatinous
investment. It is not in every Family, however, that these four
conditions are at present known to exist; but they have been
noticed in so many, that they may be fairly presumed to be capable
of occurring in all. — Excluding the family Adiniscece (of whose
siliceous skeletons we have examples in Fig. 157, c, d), which seem
to have no adequate title to rank am6ng Diatoms (their true alli-
ance being apparently with the Polycystina), the entire group may
be divided into two principal Sections : one (B) containing those
forms in which the valves possess a true central nodule and median
longitudinal line (as Pleurosigma, Fig. 133, and Gomplionema,
Fig. 153, a) ; and the other (A) including all those in which the
valves are destitute of a central nodule (as Surirella, Fig. 142, a).
Among the latter, however, we find some (b) in which there is an
umbilicus or pseudo-nodule with radiating lines or cellules, whilst
there are others (a) which have no central marking whatever.
244. Commencing with the last-named division (a), the first
Family is that of Eunotiece, of which we have already seen a cha-
racteristic example in Epithemia turgida (Fig. 135). The essential
characters of this family consist in the more or less lunate form of
the frustules in the lateral view (Fig. 135, b), and in the striae
being continuous across the valves without any interruption by a
longitudinal line. In the Genus Eunotia the frustules are free ;
in Epithemia they are very commonly adherent by the flat or
concave surface of the connecting zone ; and in Himantidium they
are usually united into ribbon-like filaments. — In the Family
Meridiem we find a similar union of the transversely-striated indi-
vidual frustules ; but these are narrower at one end than at the
other, so as to have a cuneate or wedge-like form ; and are
regularly disposed with their corresponding extremities always
pointing in the same direction, so that the filament is curved
instead of straight, as in the he&utilul Meridiem circulare (Fig. 137).
Although this plant, when gathered and placed under the micro-
scope, presents the appearance of circles overlying one another,
* The method of Kiitzing is the one followed, with some modification, by
Mr. Ralfs in his revision of the group for '.' Pritchard's History of Infusoria,"
4th Edition ; and to his systematic airangenient the Author would refer such
as desire more detailed information than the necessary limits of the present
treatise permit him to give.
DIATOMACE^E : — MEEIDION : LICMOPHORE.E.
321
it really grows in a helical (screw-like) form, making several conti-
nuous turns. This Diatom abounds in many localities in this
Fig. 137.
Fig. 137. — Meridian circu'lare.
Fig. 138. — BaciUaria paradoxa.
country ; but there is none in which it presents itself in such
rich luxuriance as in the mountain-brooks about West Point in the
United States, the bottoms of which, according to Prof. Bailey,
" are literally covered in the first warm days of spring with a fer-
ruginous-coloured mucous matter, about a quarter of an inch thick,
which, on examination by the microscope, proves to be filled with
millions and millions of these exquisitely -beautiful siliceous bodies.
Every submerged stone, twig, and spear of grass is enveloped by
them ; and the waving plume-like appearance of a filamentous
body covered in this way is often very elegant." The frustules of
Meridion are attached when young to a gelatinous cushion ;
but this disappears with the advance of age. — In the family
IA&moyliorecB also the frustules are wedge-shaped ; in some genera
they have transverse markings, whilst in others these are deficient ;
but in most instances there are to be observed two longitudinal
suture-like lines on each valve (which have received the special
designation of vittce) connecting the puncta at their two extremities.
The newly-formed part of the stipes in the Genus Licmoplwra,
instead of itself becoming double with each act of self-division of
the frustule, increases in breadth, while the frustules themselves
remain coherent ; so that a beautiful fan-like arrangement is pro-
duced (Fig. 139). A splitting- away of a few frustules seems occa-
sionally to take place, from one side or the other, before the
elongation of the stipes ; so that the entire plant presents us with
322
MICROSCOPIC FORMS OF VEGETABLE LIFE.
a more or less complete flabella or fan upon the summit of the
branches, with imperfect flabellee or single frustules irregularly
scattered throughout the
Fig. 139. entire length of the foot-
stalk. This beautiful
plant is marine, and is
parasitic upon Sea-
weeds and Zoophytes.
245. In the next Fa-
mily, that of Fragilla-
riem, the frustules are
of the same breadth at
each end, so that if they
unite into a filament
they form a straight
band. In some genera
they are smooth, in
others transversely stri-
ated, with a central no-
dule ; when striae are
present, they run across
the valves without in-
terruption. To this fa-
mily belongs the Genus
Diatoma, which gives
its name to the entire
group ; that name (which
means cutting through)
being suggested by the
curious habit of the ge-
nus, in which the frus-
tules after self-division separate from each other along their lines
of junction, but remain connected at their angles, so as to form zig-
zag chains (Fig. 140). The valves of Diatoma, when turned side-
ways (a), are seen to be strongly marked by transverse striae, which
extend into the front view. The proportion between the length
and the breadth of each valve is found to vary so considerably,
that, if the extreme forms only were compared, there would seem
adequate ground for regarding them as belonging to different
species. The genus inhabits fresh water, preferring gently-running
streams, in which it is sometimes very abundant. The Genus Fra-
gillaria is nearly* allied to Diatoma, the difference between them
consisting chiefly in the mode of adhesion of the frustules, which
in Fragillaria form long straight filaments with parallel sides ; the
filaments, however, as the name of the genus implies, very readily
break-up into their component frustules, often separating at the
slightest touch. Its various species are very common in pools and
ditches. This family is connected with the next by the Genus
Nitzschia, which is a somewhat aberrant form distinguished by the
Licmophora flabellata.
DIATOMACE^E :—- -BACILLARIA ; SYNEDEE^.
323
presence of a prominent keel on each valve, dividing it into two
portions which are usually unequal, while the entire valve is some-
times curved, as in N. sigmoidea, which is sometimes used as a
Test-object, but which
is not suitable for that Fig. 140. Fig. 141.
purpose on account of
the extreme variability
of its striation. Nearly
allied to this is the
genus Bacillaria, so
named from the elon-
gated staff -like form of
its frustules ; its valves
have a longitudinal
punctated keel, and
their transverse striae
are interrupted in the
median line. The prin-
cipal species of this
genus is the B. paro-
doxa, whose remark-
able movement has
been already described
(§ 242). Owing to this
displacement of the
frustules, its filaments
seldom present them-
selves with straight pa-
rallel sides, but nearly
in forms more
less oblique, such
as those represented in
Fig. 138. This curious
object is an inhabitant
of salt or of brackish
water. Many of the
species formerly ranked under this genus are now referred to the
genus Biatoma. The Genera Nitzschia and Bacillaria are now
associated by Mr. Ralfs,* with some other genera which agree with
them in the bacillar or staff -like form of the frustules and in the
presence of a longitudinal keel, in the Sub-family Nitzschiew, which
ranks as a section of the Surirellece. — Another Sub-family, Syne-
drew, consists of the genus Synedra and its allies, in which the
bacillar form is retained (Fig. 158, I), but the keel is wanting, and
the valves are but little broader than the front of the frustule.
Fig. 140. — Biatoma vulgare: — «, side view of
frustule; 6, frustule undergoing self- division.
Fig. 141. — Grammatophora serpentina : — a, front
and side views of single frustule ; 6, 6, front and
end views of divided frustule ; c, a frustule about
to undergo self-division ; d, a frustule com-
pletely divided.
* See Pritchard's "Infusoria," 4th Ed. p. 940. The genus Nitzschia was in
the first instance placed by Mr. Ealfs in the family Fragillariece, and the genus
acillaria in the family Surirelleai.
Y 2
324
MICROSCOPIC FORMS OF VEGETABLE LIFE.
246. In the Surirellece proper, the frustules are no longer bacillar,
and the breadth of the valves is usually (though not always) greater
than the front view.
FlG- U2' The Genus Surirella
35 (Fig. 142) is one of
those in which the
supposed ' canalicu-
lar system' of Prof.
W. Smith is most
strongly marked; it
is not, however, by
any means equally
conspicuous in all
the species, and the
appearance is pro-
bably due to imper-
fect lenses or illumi-
nation, some of the
supposed canals be-
ing resolvable into
beads with recent
Objectives. The dis-
tinctive character of this genus, in addition to the presence of the
' canaliculi,' is derived from the longitudinal line down the centre of
each valve (a), and the prolongation of the margins into ' alas.' Nu-
merous species are known, which are mostly of a somewhat ovate
form, some being broader and others narrower than S. constricta ;
the greater part of them are inhabitants of fresh or brackish water,
though some few are marine ; and several occur in those Infusorial
earths which seem to have been deposited at the bottoms of lakes,
Fig. 143.
Surirella constricta : A, side view ; B
c, binary subdivision.
front view
Campylodiscus costatus : — A, front view ;
B, side view.
such as that of the Mourne mountains in Ireland (Fig. 158, b, c, h).
In the Genus Campylodiscus (Fig. 143) the valves are so greatly
increased in breadth as to present almost the form of disks (a), and
DIATOMACE^ :— CAMPYLODISCUS ; GEAMMATOPHOEA. 325
at tlie same time have more or less of a peculiar twist or saddle-
shaped curvature (b). It is in this genus that the supposed ' canali-
culi' are most developed, and it is consequently here that they may
be best studied ; and of their being here really costce or internally
projecting ribs, no reasonable doubt can remain after examination of
them under the Binocular microscope, especially with the Black-
ground illumination. The form of the valves in most of the
species is circular or nearly so ; some are nearly flat, whilst in
others the twist is greater than in the species here represented.
Some of the species are marine, whilst others occur in fresh water ;
a very beautiful form, the G. chfpeus, exists in such abundance in
the Infusorial stratum discovered by Prof. Ehrenberg at Soos near
Ezerin Bohemia, that the earth seems almost entirely composed of it.
247. The next Family, Striatettece, forms a very distinct group,
differentiated from every other by having longitudinal costse on
the connecting portions of the frustules ; these costas being formed
by the inward projection of annular siliceous plates (which do not,
nowever, reach to the centre), so as to form septa dividing the
cavity of the cell into imperfectly-separated chambers. In some
instances these annular septa are only formed during the produc-
tion of the valves in the act of self-division, and on each repetition
of such production, and thus are always definite in number ; whilst
in other cases the formation of the septa is continued after the
production of the valves', and is repeated an uncertain number of
times before the recurrence of a new valve-production, so that the
annuli are indefinite in number. In the curious Grammato'phora,
serpentina (Fig. 141) the septa have several undulations and
incurved ends, so as to form serpentine curves, the number of
which seems to vary with the length of the frustule. The lateral
surfaces of the valves in Grammatophora are very finely striated ;
and some species, as G. suhtilissima and G. marina are used as
Test objects (§146). The frustules in most of the genera, of this
family separate into ziz-zag chains, as in Diatoma ; but in a few
instances they cohere into a filament, and still more rarely they
are furnished with a stipes.— The small Family Terpsindece is
separated by Mr. Half s from the Striatelleae with which it is nearly
allied in general characters, because its septa (which in the latter
are longitudinal and divide the central portions into chambers) are
transverse and are confined to the lateral portions of the frustules,
which appear in the front view as in Biddulphiece (§ 253). The
typical form of this family is the Terpsinoemusica, so named from the
resemblance which the markings of its costee bear to musical notes.
248. We next come to two Families in which the lateral surfaces
of the Frustules are circular, so that according to the flatness or
convexity of the valves and the breadth of the intervening hoop,
the frustules may have the form either of thin disks, short
cylinders, bi-convex lenses, oblate spheroids, or even of spheres.
Looking at the structure of the individual frustules, the
line of demarcation between these two families, Melosirece
and Coscinodiscece, is by no means distinct ; the principal difference
326
MICROSCOPIC FORMS OF VEGETABLE LIFE.
between them being that the valves of the latter are commonly
cellulated, whilst those of the former are smooth. Another im-
portant difference, however, lies in this, that the frustules of the
Goscinodisceoe are always free, whilst those of the Melosi/rece remain
coherent into filaments, which often so strongly resemble those of
the simple Confervacece as to be readily distinguishable only by
the effect of heat. Of these last the most important Genus is
Melosira (Figs. 144, 145), long since characterized as a Plant by
the Swedish algologist Agardh, bnt ranked in the Animal kingdom
with other Diatoms by Prof. Ehrenberg, who inclnded it in his
genus Gallionella. Some of its species are marine, others fresh-
water ; one of the latter, the M. ochracea, seems to grow best in
Fig. 144.
Fig. 145.
Melosira subflexilis.
Melosira varians.
boggy pools containing a ferrnginons impregnation ; and it is
stated by Prof. Ehrenberg to take np from the water, and to in-
corporate with its own snbstance, a considerable quantity of iron.
The filaments of Melosira very commonly fall-apart at the slightest
touch ; and in the Infusorial earths, in which some species abound,
the frustules are always found detached (Fig. 158, a a, d d). The
meaning of the remarkable difference in the sizes and forms of the
frustules of the same filaments (Figs. 144, 145) has not yet been
fully ascertained ; but it seems to be related to the curious process
of self -conjugation already described (§ 240). The sides of the
valves are often marked with radiating striee (Fig. 158, d d) ; and
in some species they have toothed or serrated margins, by which
DIATOMACEjE :— HYALODISCUS ; COSCINODISCUS. 327
the frustules lock-together. To this family belongs the Genus
Hyalodiscus, of which the H. subtilis was first brought into notice
by the late Prof. Bailey as a Test-object, its disk being marked,
like the engine-turned back of a watch, with lines of exceeding
delicacy, only visible by the highest magnifying powers and the
most careful illumination.
249. The Family Coscinodiscece includes a large proportion of
the most beautiful of those discoidal Diatoms, of which the valves
do not present any considerable convexity, and are connected by
a narrow zone. The Genus Goscinodiscus, which is easily distin-
guished from most of the genera of this family by not having its
disk divided into compartments, is of great interest from the vast
abundance of its valves in certain fossil deposits (Fig. 157, a, a, a),
especially the Infusorial earth of Eichmond in Virginia, of Ber-
muda, and of Oran, as also in Guano. Each frustule is of discoidal
shape, being composed of two delicately undulating valves, united
by a hoop ; so that, if the frustules remained in adhesion, they
would form a filament resembling that of Melosira (Fig. 144).
The regularity of the hexagonal divisions on the valves renders
them beautiful microscopic objects ; in some species the areolae
are smallest near the centre, and gradually increase in size towards
the margin ; in others a few of the central areolae are the largest,
and the rest are of nearly uniform size ; while in others, again,
there are radiating lines formed by areolae of a size different
from the rest. Most of the species are either marine, or are inha-
bitants of brackish water ; when living they are most commonly
found adherent to Sea-weeds or Zoophytes ; but when dead, the
valves fall as a sediment to the bottom of the water. In both
these conditions, they were found by Prof. J. Quekett in connec-
tion with Zoophytes which had been brought home from Melville
Island by Sir E. Parry ; and the species seemed to be identical
with those of the Richmond earth.
250. The recent investigations of Mr. J. W. Stephenson* on Cos-
cinodiscus oculus Iridis show that the peculiar " eye-like " appear-
ance in the centre of each of the hexagons, arises from the mixture
of two distinct layers, differing considerably in structure ; the
markings of the lower layer being partially seen through those of
the upper. By fracturing these Diatoms, Mr. Stephenson has suc-
ceeded in separating portions of two layers, so that each could be
examined singly. He has also mounted them in bisulphide of car-
bon, the refractive power of which is very high ; and also in a solu-
tion of phosphorus in bisulphide of carbon, which has a still higher
refractive index. If we suppose a diatom to be marked with
concave depressions, they would act as concave lenses in air,
which is less refractive than their own silex ; but when such lenses
are immersed in bisulphide of carbon, or in the phosphorus solution,
they would be converted into convex lenses of the more refrac-
tive substance, and have their action in air reversed. Analogous
* "Monthly Microscopical Journal," July, 1873.
328 MICKOSCOPIC FORMS OF VEGETABLE LIFE.
but opposite changes must take place, when convex Diatom, lenses
are viewed first in air, and then in the more refractive media.
Applying these and other tests to Coscinodiscus oculus Iriclis,
Mr. Stephenson considers both layers to be composed of hexagons,
represented in Plate XI. figs. 1, 2, from drawings by Mr. Stewart.
The upper layer is much stronger and thicker than the lower
one ; and the framework of its hexagons more readily exhibits
its beaded appearance. The lower layer is nearly transparent,
and little conspicuous when seen in bisulphide of carbon, except,
as the figure shews, when the frame-work of the hexagons, and the
rings in the midst of them, appear thickened and more refractive.
In both layers the balance of observations tends to the belief that
the hexagons have no floors, and are in fact perforated by foramina
like those of minute Polycystina. The cells formed by the hexagons
of the upper layer are of considerable depth ; those of the lower
layer are shallower. In both layers, fractured edges shew the hexagon
frames to be the strongest parts ; and in neither has Mr. Stephenson
been able to detect any broken remnants of floors, which might be
expected to be visible with high powers if they existed at all. — If
further observations should confirm Mr. Stephenson's belief that
Goscinodisci are perforated by numerous foramina, a similar struc-
ture will be sought for in other Diatoms, and the views of naturalists
as to the character of the group may be materially modified. At
present the chief difference in minute structure that has been
recognised, may be seen by comparing the apparently simple
beading of Pleurosigma with the hexagonal formations in Coscino-
discus, &c. ; but a far more important divergence will have to be
considered, if some Diatom-valves have a multiplicity of foramina,
and others either none, or only a few at certain spots. It is very
desirable that living forms of Goscinodisci should be carefully
examined ; since, if they really have foramina, some minute organs
may be protruded through them.
251. The Genus Actinocyclus* closely resembles the preceding
in form, but differs in the markings of its valvular disks, which
are minutely and densely punctated or cellulated, and are divided
radially by single or double dotted lines, which, however, are not
continuous but interrupted — (Plate I., fig 1). The disks are gene-
rally iridescent ; and, when mounted in balsam, they present
various shades of brown, green, blue, purple, and red ; blue or
purple, however, being the most frequent. An immense number
of Species have been erected by Prof. Ehrenberg on minute diffe-
rences presented by the rays as to number and distribution ; but
since scarcely two specimens can be found in which there is a
perfect identity as to these particulars, it is evident that such
minute differences between organisms otherwise similar are not of
* The Author concurs with Mr. Ealfs in thinking it preferable to limit the
genus Actinocyclus to the forms originally included in it by Ehrenberg, and to
restore the genus Actinoplychus of Ehrenberg, which had baen improperly united
with Actinocyclus by Profs. Kutzing and W. Smith.
DIATOMACE^E : — FAMILY COSCIXODISCEJ3.
329
sufficient account to serve for the separation of species. This form
is very common in Guano from Ichaboe. Allied to the preceding
are the two Genera Asterolamyra and Ast&romphalus, both of which
have circular disks of which the marginal portion is minutely
areolated, whilst the central area is smooth and perfectly hyaline
in appearance, but is divided by lines into radial compartments
which extend from the central umbilicus towards the periphery.
The difference between them simply consists in this ; that in
Ast&rolampra all the compartments are similar and equidistant,
and the rays equal (Plate I., fig. 2) ; whilst in Asteromphalus two
of the compartments are closer together than the rest, and the
enclosed hyaline ray (which is distinguished as the median or basal
ray) differs in form from the others, and is sometimes specially
continuous with the umbilicus (Plate I., fig. 4). The excentricity of
the other rays which is thus produced has been made the basis of
another Generic designation, Spatangidium ; but it may be doubted
whether this is founded on a valid distinction.* These beautiful
disks are for the most part obtainable from Guano, and from
Soundings in tropical, and antarctic seas. From these we pass on
to the Genus Actinopfychus (Fig. 146), of which also the frustules
are discoidal in form, but of
which each valve, instead Fig. 146.
of being flat, has an undu-
lating surface, as is seen in
front view (b) ; giving to the
side view (a) the appearance
of being marked by radiat-
ing bands. Owing to this
peculiarity of shape, the
whole surface cannot be
brought into focus at once
except with a low power;
and the difference of aspect
which the different radial
divisions present in Fig. 146, is simply due to the fact that one set
is out of focus whilst the other is in it, since the appearances are
reversed by merely altering the focal adjustment. The number of
radial divisions has been considered a character of sufficient im-
portance to serve for the distinction of species ; but this is probably
subject to variation ; since we not unfrequently meet with disks, of
which one has (say) 8 and another 10 such divisions, but which are
so precisely alike in every other particular that they can scarcely
be accounted as specifically different. The valves of this genus also
are very abundant in the Infusorial earth of Richmond, Bermuda,
.and Oran (Fig. 157, b, b, b) ; and many of the same species have
been found recently in Guano, and in the seas of various parts of
* See Greville in " Quart. Joivrn. of Microsc. Science," Vol. vii. (1859), p. 158,
and in " Transact, of Microsc. Soc." Vol. viii. N.S. (I860), p. 102, and Vol. x.
(1862), p. 41 ; also Wallich in the same Transactions, Vol. viii. (1860), p. 44.
Actinoptyclius undukitus.—^ side view;
B, front view.
330 MICROSCOPIC FORMS OF- VEGETABLE LIFE.
the world. The frustules in their living state appear to be gene-
rally attached to Sea-weeds or Zoophytes.
252. The Bermuda earth also contains the very beautiful form
(Plate i., fig. 3), which, though scarcely separable from Actinop-
tychus except by its marginal spines, has received from Prof.
Ehrenberg the distinctive appellation of Heliopelta (sun-shield).
The object is represented as seen on its internal aspect by the
Parabolic Illuminator (§ 94), which brings into view certain fea-
tures that can scarcely be seen by ordinary transmitted light. Five
of the radial divisions are seen to be marked-out into circular
areola? ; but in the five which alternate with them, a minute granular
structure is observable. This may be shown by careful adjustment
of the focus to exist over the whole interior of the valve, even on
the divisions in which the circular areolation is here displayed ; and
it hence appears that this marking belongs to the internal layer*
(§ 235), and that the circular areolation exists in the outer layer of
the siliceous lorica. In the alternating divisions whose surface is
here displayed, the areolation of the outer layer, when brought into
view by focussing down to it, is seen to be formed by equilateral
triangles ; it is not, however, nearly so well marked as the circular
areolation of the first-mentioned divisions. The dark spots seen at
the ends of the rays, like the dark centre, appear to be solid tuber-
cles of silex not traversed by markings, as in many other Diatoms ;
most assuredly they are not orifices, as supposed by Prof. Ehrenberg.
Of this type, again, specimens are found presenting 6, 8, 10, or 12
radial divisions, but in other respects exactly similar ; on the other
hand, two specimens agreeing in their number of divisions may
exhibit minute differences of other kinds ; in fact, it is rare to find
two that are precisely alike. It seems probable, then, that we
must allow a considerable latitude of variation in these forms, before
attempting to separate any of them as distinct species. — Another
very beautiful discoidal Diatom, which occurs in Guano, and is also
found attached to Sea- weeds from different parts of the world
(especially to a species employed by the Japanese in making soup)
is the Arachnoidiscus (Plate X.), so named from the resemblance
which the beautiful markings on its disk cause it to bear to a
spider's web. According to Mr. Shadbolt,f who has carefully ex-
amined its structure, each valve consists of two layers ; the outer
one, a thin flexible horny membrane, indestructible by boiling
in nitric acid ; the inner one, siliceous. It is the former which has
upon it the peculiar spider's web-like markings : whilst it is the
latter that forms the supporting frame-work, which bears a very
strong resemblance to that of a circular Gothic window. The two
* It is stated by Mr. Stodder (" Quart. Journ. of Microsc. Science," Vol. iii.
N.S., p. 215), that not only has he seen, in broken specimens, the inner granu-
lated plate projecting beyond the outer, but that he has found the inner plate
altogether separated from the outer. The Author is indebted to this gentleman
for pointing out that his figure represents the inner surface of the valve.
t "Transact, of Microsc. Society," First Series, Vol. iii. p. 49.
DIATOMACEiE: — AULACODISCUS ; BIDDULPHIE^. 331
can occasionally be separated entire, by first boiling the disks for
a considerable time in nitric acid, and then carefully washing them
in distilled water. Even without such separation, however, the
distinctness of the two layers can be made-out by focussing for
each separately under a l-4th or l-5th inch objective; or by look-
ing at a valve as an opaque object (either by the Parabolic Illu-
minator, or by the Lieberkiihn, or by a side light) with a 4-10ths
inch objective, first from one side, and then from the other.* — This
family is connected with the succeeding by the small group of
Eupodiscece, the members of which agree with the Coscinodisceae
in the general character of their discoid frustules, and with the
Biddulphiece in having tubercular processes on their lateral surfaces.
In the beautiful Aidacodiscus (Plate I., fig. 5) these tubercles
are situated near the margin, and are connected with bands ra-
diating from the centre ; the surface also is frequently inflated in a
manner that reminds us of Actinoptychus. These forms are for
the most part obtained from Guano.
253. The members of the next Family Fig. 147.
Biddulphiece differ greatly in their ge-
neral form from the preceding ; being
remarkable for the great development
of the lateral valves, which, instead of
being nearly flat or discoidal, so as only
to present a thin edge in front view, are
so convex or inflated as always to enter
largely into the front view, causing the
central zone to appear like a band be-
tween them. This band is very narrow
when the new frustules are first pro-
duced by self-division (§ 237) ; but it
increases gradually in breadth until the
new frustule is fully formed and is
itself undergoing the same duplicative
change. In Buldidphia (Fig. 134) the
frustules have a quadrilateral form,
and remain coherent by their alternate
angles (which are elongated into tooth-
like projections), so as to form a ziz-
zag chain. They are marked externally
by ribbings which seem to be indica-
tive of internal costce partially sub-
dividing the cavity. Nearly allied to
this is the beautiful Genus Isthmia
(Fig. 147), in which the frustules have a Isthmia nervosa.
trapezoidal form owing to the oblique
prolongation of the valves ; the lower angle of each frustule is
coherent to the middle of the next one beneath, and from the basal
* These valves afford admirable objects for showing the 'conversion of
relief ' j)a Nachet's Stereo-Pseudoscopic Microscope (§ 35).
332 MICROSCOPIC FORMS OF VEGETABLE LIFE.
frastule proceeds a stipes by wliicli the filament is attached. Like
the preceding, this Genus is marine, and is fonnd attached to the
Algce of our own shores. The areolated structure of its surface is very-
conspicuous (Fig. 131) "both in the valves and in the connecting
'hoojD;' and this hoop, being silicified, not only connects the
two new frustules (as at b, Fig. 147), until they have separated
from each other, but, after such separation, remains for a time round
one of the frustules, so as to give it a truncated appearance (a, c).
254. The Family Anguliferece, distinguished by the angular
form of its valves in tbeir lateral aspect, is in many respects closely
allied to the preceding ; but in the comparative flattening of their
valves its members more resemble the Coscinodiscese and Eupo-
disceas. Of this family we have a characteristic example in the
Genus Triceratium ; of which striking form a considerable number
of species are met with in the Bermuda and other Infusorial
earths, while others are inhabitants of the existing ocean and of
tidal rivers. The T. favus (Fig. 132), which is one of the largest
and most regularly-marked of any of these, occurs in the mud of
the Thames and in various other estuaries on our own coast ; it
has been found, also, on the surface of large Sea- Shells from
various parts of the world, such as those of Hippopus and Haliotis,
before they have been cleaned ; and it presents itself likewise in
the Infusorial earth of Petersburg (U.S.). The projections at the
angles which are shown in that species, are prolonged in some
other species into ' horns,' whilst in others, again, they are mere
tubercular elevations. Although the triangular form of the
frustule when looked at sideways is that which is characteristic of
the genus, yet in some of the species there seems a tendency to
produce quadrangular and even pentagonal forms ; these being
marked as varieties by their exact correspondence in sculpture,
colour, &c, with the normal triangular forms.* This departure is
extremely remarkable, since it breaks down what seems at first to
be the most distinctive character of the genus ; and its occurrence
is an indication of the degree of latitude which we ought to allow
in other cases. It is difficult, in fact, to distinguish the square
forms of Triceratium from those included in the Genus Amphitetras,
which is chiefly characterized by the cubiform shape of its frus-
tules. In the latter the frustules cohere at their angles so as to
form ziz-zag filaments, whilst in the former the frustules are
usually free, though they have occasionally been found catenated. —
Another group that seems allied to the Biddulphiese is the curious
assemblage of forms brought together in the Family Cluetocereoe,
some of the filamentous types of which seem also allied to the
MelosireoB. The peculiar distinction of this group consists in the
presence of tubular ' awns,' frequently proceeding from the con-
* See Mr. Brightwell's excellent memoirs ' On the genus Triceratium,' in
"Quart. Journ. of Microsc. Science," Vol. i. (1853), p. 245, Vol. iv. (1856), p. 272,
Vol. vi. (1858), p. 153 ; also Wallich in the same journal, Vol. iv. (1858), p. 242 ;
and Greville in "Transact, of Microsc. Soc," N.S., Vol. ix. (1861), pp. 43, 69.
DIATOMACE.E : — CH^ETOCEKEjE ; EHIZOSOLENIA. 333
necting hoop, sometimes spinous and serrated, and often of great
length (Fig. 148), by the interlacing of which the frustules are
united into filaments, whose continuity, however, is easily broken.
In the Genus Baderiastrum (Fig. 149) there are sometimes as
Fig. 148.
Fig. 149.
Baderiastrum fur atum.
^ja_
Chcetoceros Wighamii: — a. front view, and b, side view of frustule; c, side view
of connecting Loop and awns ; c?, entire filament.
many as twelve of these awns, radiating from each frustule like
the spokes of a wheel, and in some instances regularly bifurcating.
"With this group is associated the Genus Rhizosolenia, of which
several species are distinguished by the extraordinary length of
the frustule (which may be from 6 to 20 times its breadth), giving
it the aspect of a filament (Fig. 150), by a transverse annulation
that imparts to this filament a jointed appearance, and by the
termination of the frustule at each end in a cone from the apex of
which a straight awn proceeds. It is not a little remarkable that
the greater number of the examples of this curious family are
obtained from the stomachs of Ascidians, Salpae, Holothurias, and
other Marine animals.*
255. The second principal division (B) of the Diatomaceas con-
sists, it will be remembered, of those in which the frustules have a
median longitudinal Hue and a central nodule. In the first of the
Families which it includes, that of Cocconeidece, the central
nodule is obscure or altogether wanting on one of the valves,
* See Brightwell in " Quart. Joum. of Microsc Science," Vol. iv. (1856)
p. 105, Vol. .vi. (1858), p. 93: Wallich in "Trans, of Microsc. Soc," N.S.,
Vol. viii. (1860), p. 48 ; and West in the same, p. 151.
334
MICROSCOPIC FOEMS OF VEGETABLE LIFE.
which is distinguished as the inferior. This family consists but of
a single Genus Gocconeis, which includes, however, a great number
of species, some or other of them occurring in every part of the
globe. Their form is usually that of ellipsoidal disks, with surfaces
more or less exactly parallel, plane, or slightly curved ; and they
are very commonly found adherent to each other. The frustules
in this genus are frequently found invested by a membranous
envelope which forms a border to them ; but this seems to belong
Fig. 150.
Fig. 151.
Fig. 152.
MMzosolenia
imbricata.
Achnanthes longipes: a ; 6,
c, d, e, successive frustules
in different stages of self-
division.
Gomphonema geminatum : its frus-
tules connected by a dichotomons
stipes.
to the immature state, subsequently disappearing more or less
completely. Another Family in which there is a dissimilarity in
the two lateral surfaces, is that of Achnanthece ; the frustules of
which are remarkable for the bend they show in the direction of
DIATOMACE^:— ACHNAXTHE^; GOMPHONEMEJ3. 335
their length, often more conspicuously than in the example here
represented. This family contains free, adherent, and stipitate
forms ; one of the most common of the latter being the Aclinanthes
longipes (Fig. 152), which is often found growing on Marine Algae.
The difference between the markings of the upper and lower valves
is here distinctly seen; for while both are traversed by striae,
which are resolvable under a sufficient power into rows of dots, as
well as by a longitudinal line, which sometimes has a nodule at
each end (as in Xavicula), the lower valve (a) has also a transverse
line, forming a stauros or cross, which is wanting in the upper
valve (e). A persistence of the connecting membrane, so as to
form an additional connection between the cells, may sometimes
be observed in this genus ; thus, in Fig. 151, it not only holds
together the two new frustules resulting from the subdivision of
the lowest cell, a, which are not yet completely separated the one
from the other, but it may be observed to invest the two frustules
h and c, which have not merely separated, but are themselves
beginning to undergo binary subdivision ; and it may also be per-
ceived to invest the frustule d, from which the frustule e, being the
terminal one, has more completely freed itself. — In the Family
Cymbellece, on the other hand, both valves possess the longitudinal
line with a nodule in the middle of its length ; but the valves have
the general form of those of the Eimotii " . and the line is so much
nearer one margin than the other, that the nodule is sometimes
rather marginal than central, as we see in Cocconema (Fig 158,/). —
The Gomphonemece, like the Meridieae and Licmophoreae, have
frustules which are cuneate or wedge-shaped in their front view
(Figs. 152, 153), but are distinguished from those forms by the
Fig. 153.
Gomphonema geminatum, more highly magnified: — A. side view of frustule;
B, front view; c, frustule in the act of self -division.
presence of the longitudinal line and central nodule. Although
there are some free forms in this family, the greater part of them,
336 MICEOSCOPIC FOKMS OF VEGETABLE LIFE.
included in the genus Gom-phonema, have their frustules either
affixed at their bases or attached to a stipes. This Stipes seems
to be formed by an exudation from the frustule, which is secreted
only during the process of self-division : hence when this process
has been completed, the extension of the single filament below the
frustule ceases ; but when it recommences, a sort of joint or arti-
culation is formed, from which a new filament begins to sprout for
each of the half-frustules ; and when these separate, they carry
apart the peduncles which support them, as far as their divergence
can take place. It is in this manner that the dichotomous
character is given to the entire stipes (Fig. 152). The species
of Gomphonema are, with scarcely an exception, inhabitants of
fresh water, and are among the commonest forms of Diatomacese.
256. Lastly, we come to the large family Naviculece, the mem-
bers of which are distinguished by the symmetry of their frustules
as well in the lateral as in the front view, and by the presence of
a median longitudinal line and central nodule in both valves. In
the Genus Navicula and its allies, the frustules are free or simply
adherent to each other ; whilst in another large section they are
included within a gelatinous envelope, or are enclosed in a definite
tubular or gelatinous frond. Of the genus Navicula an immense
number of species have been described, the grounds of separation
being often extremely trivial ; those which have a lateral sigmoid
curvature (Fig. 133) have been separated by Prof. W. Smith under
the designation Pleurosigma, which is now generally adopted ; but
his separation of another set of species under the name Pinnularia
(which had been previously applied by Ehrenberg to designate the
striated species), on the ground that its striae (costae) are not
resolvable into dots, was not considered valid by Mr. Ralfs, on the
ground that in many of the more minute species it is impossible
to distinguish with certainty between striae and costae. Mr. Slack
has given an account of the resolution of the so-called costas of
twelve species of Pinnularioe into beaded structures.*
257. The multitudinous species of the genus Navicula are for
the most part inhabitants of Fresh water ; and they constitute a
large part of most of the so-called ' Infusorial Earths' which were
deposited at the bottoms of lakes. Among the most remarkable of
such deposits are the substances largely used in the arts for the
polishing of metals, under the names of Tripoli and rotten-stone :
these consist in great part of the frustules of JSTaviculas and Pinnu-
larias. The Polierschiefer, or polishing slate, of Bilin in Bohemia,
the powder of which is largely used in Germany for the same pur-
pose, and which also furnishes the fine sand used for the most
delicate castings in iron, occurs in a series of beds averaging four-
teen feet in thickness ; and these present appearances which indi-
cate that they have been at some time exposed to a high tempera-
ture. The well-known Turkey stone, so generally employed for the
* " Monthly Microscopical Journal."
DIATOMACE.E :— N AYICUL.E ; SCHIZOXEME.E.
337
sharpening of edge-tools, seems to be essentially composed of a
similar aggregation of frustules of jSTaviculae, &c. which has been
consolidated by heat. The species of Plewosigma, on the other
hand, are for the most part either marine or are inhabitants of
brackish water ; and they comparatively seldom present themselves
in a fossilized state. The genns Stauroneis, which belongs to the
same group, differs from all the preceding forms in having the
central nodule of each valve dilated laterally into a band free from
striae, which forms a cross with the longitudinal band : of this very
beautiful form, some species are fresh-water, others marine ; and
the former present themselves frequently in certain Infusorial
earths.*
258. Of the members of the sub-family Schizonemece, consisting
of those Naviculece in which the frustules are united by a gela-
Fig. 154.
Schizonema Grevillii: — A, natural size; B, portion magnified five
diameters ; c, filament magnified 100 diameters ; D, single frus-
tule.
tinous envelope, some are remarkable for the great external resem-
blance they bear to acknowledged Algae. This is especially the
* For some very curious examples of the extent to which variation in form,
size, and distance in striae, may take-place in this group, among individuals
which must be accounted as of the same species, see the Memoirs of Profs.
W. Smith and W. Gregory already referred to (p. 318, note).
Z
MICROSCOPIC FORMS OF VEGETABLE LIFE.
case with the Genus Schizonema ; of which the gelatinous enve-
lope forms a regular tubular frond, more or less branched, and
of nearly equal diameter throughout, within which the frustules
lie either in single file or without any definite arrangement
(Fig. 154) ; all these frustules having arisen from the self -division
of one individual. In the genus Mastogloia, which is specially
distinguished by having the annulus furnished with internal
costae projecting into the cavity of the frustule, each frustule
is separately supported on a gelatinous cushion (Fig. 155, b),
which may itself be either borne on a branching stipes (a), or may
be aggregated with others into an indefinite mass (Fig. 156). The
Fig. 155.
Fig. 156.
Fig. 155. Mastogloia Smithii .—A, entire stipes; B, frustule _ in its
gelatinous envelope ; c— F, different forms of frustule as seen in side
view ; G, front view ; h, frustule undergoing subdivision.
Fig. 156. Mastogloia lanceolata.
careful study of these composite forms is a matter of great im-
portance ; since it enables us to bring into comparison with each
other great numbers of frustules which have unquestionably a
common descent, and which must therefore be accounted as of
the same Species ; and thus to obtain an idea of the range of
DIATOMACE^E: — VARIABILITY; HABITS. 339
variation prevailing in this group, without a knowledge of which
specific definition is altogether unsafe. Of the very strongly
marked varieties which may occur within the limits of a single
species, we have an example in the valves c, d, e, f (Fig. 155),
which would scarcely have been supposed to belong to the same
specific type, did they not occur upon the same stipes. The careful
study of these varieties in every instance in which any disposition
to variation shows itself, so as to reduce the enormous number of
species with which our systematic treatises are loaded, is a pursuit
of far greater real value than the multiplication of species by the
detection of such minute differences as may be presented by forms
discovered in newly-explored localities ; such differences as already
pointed out, being, probably, in a large proportion of cases, the
result of the multiplication of some one form, which, under modify-
ing influences that we do not yet understand, has departed from
the ordinary type. The more faithfully and comprehensively this
study is carried out in any department of Natural History, the
more does it prove that the range of variation is far more extensive
than had been previously imagined ; and this is especially likely to
be the case with such humble organisms as those we have been
considering, since they are obviously more influenced than
those of higher types by the conditions under which they are de-
veloped, whilst, from the very wide Geographical range through
which the same forms are diffused, they are subject to very great
diversities of such conditions.
259. The general habits of this most interesting group cannot
be better stated than in the words of Prof. W. Smith. " The
Diatomaceas inhabit the sea, or fresh water ; but the species
peculiar to the one are never found in a living state in any other
locality ; though there are some which prefer a medium of a mixed
nature, and are only to be met with in water more or less brackish.
The latter are often found in great abundance and variety in dis-
tricts occasionally subject to marine influences, such as marshes
in the neighbourhood of the sea, or the deltas of rivers, where, on
the occurrence of high tides, the freshness of the water is affected
by percolation from the adjoining stream, or more directly by the
occasional overflow of its banks. Other favourite habitats of the
Diatomaceae are stones of mountain streams or waterfalls, and the
shallow pools left by the retiring tide at the mouths of our larger
rivers. They are not, however, confined to the localities I have
mentioned, — they are, in fact, most ubiquitous, and there is hardJy
a roadside-ditch, water-trough, or cistern, which will not reward a
search, and furnish specimens of the tribe." Such is their abun-
dance in some rivers and estuaries, that their multiplication is
affirmed by Prof. Ehrenberg to have exercised an important influ-
ence in blocking-up harbours and diminishing the depth of channels !
Of their extraordinary abundance in certain parts of the Ocean,
the best evidence is afforded by the observations of Dr. J. D.
Hooker upon the Diatomaceas of the southern seas ; for within the
z2
340 MIICROSCOPIC FOEMS. OF VEGETABLE LIFE.
Antarctic Circle they are rendered peculiarly conspicuous by be-
coming enclosed in the newly-formed ice, and by being washed-up
in myriads by the sea on to the ' pack' and ' bergs,' everywhere
staining the white ice and snow of a pale ochreous brown. A
deposit of mud, chiefly consisting of the siliceous loricae of Diato-
niaceas, not less than 400 miles long and 120 miles broad, was found
at a depth of between 200 and 400 feet, on the flanks of Yictoria
Land in 70° South latitude. Of the thickness of this deposit no
conjecture could be formed; but that it must be continually in-
creasing is evident, the silex of which it is in a great measure
composed being indestructible. A fact of peculiar interest in con-
nection with this deposit is its extension over the submarine flanks
of Mount Erebus, an active Volcano of 12,400 feet elevation ; since
a communication between the ocean-waters and the bowels of a
volcano, such as there are other reasons for believing to be occa-
sionally formed, would account for the presence of Diatomacese in
volcanic ashes and pumice, which was discovered by Prof. Ehren-
berg. It is remarked by Dr. Hooker, that the universal presence
of this invisible vegetation throughout the South Polar Ocean is
a most important feature, since there is a marked deficiency in
this region of higher forms of vegetation ; and were it not for them,
there would neither be food for aquatic Animals, nor (if it were pos-
sible for these to maintain themselves by preying on one another)
could the ocean-waters be purified of the carbonic acid which
animal respiration and decomposition would be continually impart-
ing to them. It is interesting to observe that some species of marine
Diatomaceas are found through every degree of latitude between
Spitzbergen and "Victoria Land, whilst others seem limited to
particular regions. One of the most singular instances of the
preservation of Diatomaceous forms, is their existence in Guano ;
into which they must have passed from the intestinal canals of the
Birds of whose accumulated excrement that substance is composed,
those birds having received them, it is probable, from Shell-fish,
to which these minute organisms serve as ordinary food (§ 261).
260. The indestructible nature of the Loricee of Diatomacecn
has also served to perpetuate their presence in numerous localities
from which their living forms have long since disappeared ; for the
accumulation of sediment formed by their successive production
and death, even on the bed of the Ocean, or on the bottoms of
fresh-water Lakes, gives-rise to deposits which may attain consi-
derable thickness, and which, by subsequent changes of level, may
come to form part of the dry land. Thus very extensive Siliceous
strata, consisting almost entirely of marine Diatomacece, are found
to alternate, in the neighbourhood of the Mediterranean, with
Calcareous strata chiefly formed of Foraminifera (Chap, x.) ; the
whole series being the representative of the Chalk formation of
Northern Europe, in which the silex that was probably deposited at
first in this form has undergone conversion into flint, by agencies
hereafter to be considered (Chaps, x., xix.). Of the Diatomaceous
DIATOMACEJE: — FOSSIL DEPOSITS. 341
composition of these strata we have a characteristic example in
Fig. 157, which represents the Fossil Diatomaceae of Oran in
Algeria. The so-called ' Infusorial Earth ' of Eichmond in Vir-
ginia, and that of Bermuda, also Marine deposits, are very celebrated
Fig. 157.
Fossil Diatomace<B, &c, from Oran : — a, o, a, Coscinodiscus ; 6, b, b,
Actinoeylus ; c, Dictyochya fibula; d, Lithasteriscus radiatus: e, Spon-
golithis acicularis -, /, /, Grammatophora parallela (side view) ; g, g,
Grammatophora angulosa (front view.i.
among Microscopists for the nnmber and beauty of the forms they
have yielded ; the former constitutes a stratum of 18 feet in thick-
ness, underlying the whole city, and extending over an area whose
limits are not known. Several deposits of more limited extent,
and apparently of fresh- water origin, have been found in our own
islands ; as for instance at Dolgelly in North Wales, at South
Mourne in Ireland (Fig. 158), and in the island of Mull in Scotland.
Similar deposits in Sweden and Norway are known under the
name of berg-meld or mountain-flour ; and in times of scarcity the
inhabitants of those countries are accustomed to mix these sub-
stances with their dough in making bread. This has been supposed
merely to have the effect of giving increased bulk to their loaves,
so as to render the really nutritive portion more satisfying ; but as
342
MICROSCOPIC FORMS OF VEGETABLE LIFE.
the berg-mehl lias been found to lose from a quarter to a third of
its weight by exposure to a red-heat, there seems a strong proba-
bility that it contains Organic matter enough to render it nutritious
Fig. 15S.
Fossil Diatomacece, &c, from Mourne mountain, Ireland : — a, a, a,
Gaillonella (Melosira) procera, and G. granulata ; d, d, d, G. biseriata
(side view); b, b, Surirella plieata ; c, S. craticula ; k, S. caledonica;
e, Gomphonema gracile ; /, Cocconema fnsidium ; g, Tabellaria vul-
garis; h, Pinriularia dactylus ; ?", P. nobilsi; I, Synedra ulna.
in itself. When thus occurring in strata of a fossil or sub-fossil
character, the Diatomaceous deposits are generally distinguishable
as white or cream-coloured powders of extreme fineness.
261. For collecting fresh Diatomacece those general methods
are to be had recourse to which have been already described
(§ 227). "Their living masses," says Prof. W. Smith, "present
themselves as coloured fringes attached to larger plants, or forming
a covering to stones or rocks in cushion-like tufts — or spread over
their surface as delicate velvet — or depositing themselves as a filmy
stratum on the mud, or intermixed with the scum of living or
decayed vegetation floating on the surface of the water. Their
colour is usually a yellowish-brown of a greater or less intensity,
varying from a light chestnut, in individual specimens, to a shade
COLLECTION OF DIATOMACE.E. 343
almost approaching black in the aggregated masses. Their presence
may often be detected without the aid of a microscope, bv the
absence, in many species, of the fibrous tenacity which, distin-
guishes other plants : when removed from their natural position
they become distributed through the water, and are held in sus-
pension by it, only subsiding after some little time has elapsed."
Notwithstanding every care, the collected specimens are liable to
be mixed with much foreign matter : this may be partly got rid of
by repeated washings in pure water, and by taking advantage, at
the same time, of the different specific gravities of the Diatoms and
of the intermixed substances, to secure their separation. Sand,
being the heaviest, will subside first ; fine particles of mud on
the other hand, will float after the Diatoms have subsided. The
tendency of the Diatomaceae to make their way towards the light
will afford much assistance in procuring the free forms in a
tolerably clean state ; for if the gathering which contains them be
leftundisdurbed for a sufficient length of time in a shallow vessel
exposed to the sunlight, they may be skimmed from the surface.
Marine forms must be looked for upon Sea-weeds, and in the fine
mud or sand of soundings or dredgings ; they are frequently found
also in considerable numbers, in the stomachs of Holothurife,
Ascidians, and Salpas, in those of the oyster, scallop, whelk, and
other testaceous Mollusks, in those of the crab and lobster, and
other Crustacea, and even in those of the sole, turbot, and other
' flat-fish.' In fact the Diatom-collector will do well to examine
the digestive cavity of any small aquatic animals that may fall
in his way : rare and beautiful forms have been obtained from
the interior of Kodilnca (Fig. 306). The separation of the Diatoms
from the other contents of these stomachs must be accomplished
by the same process as that by which they are obtained from
Guano or the calcareous Infusorial Earths ; of this, the following
are the most essential particulars. The Guano or earth is first to
be washed several times in pure water, which should be well stirred,
and the sediment then allowed to subside for some hours before
the water is poured off, since, if it be decanted too soon, it may
carry the lighter forms away with it. Some kinds of earth have
so little impurity that one washing suffices ; but in any case it is
to be continued so long as the water remains coloured. The
deposit is then to be treated, in a flask or test-tube, with Hydro-
chloric (muriatic) acid ; and after the first effervescence is over, a
gentle heat may be applied. As soon as the action has ceased, and
time has been given for the sediment to subside, the acid should
be poured off, and another portion added ; and this should be
repeated as often as any effect is produced. When hydrochloric
acid ceases to act, strong Nitric acid should be substituted ; and
after the first effervescence is over, a continued heat of about 200°
should be applied for some hours. "When sufficient time has been
given for subsidence, the acid may be poured off and the sediment
treated with another portion ; and this is to be repeated until no
344 MICEOSCOPIC FORMS OF VEGETABLE LIFE.
further action takes place. The sediment is then to be washed
until all trace of the acid is removed ; and, if there have been no
admixture of siliceous sand in the earth or guano, this sediment
will consist almost entirely of Diatoniacege, with the addition,
perhaps, of Sponge-spieules. The separation of siliceous sand,
and the subdivision of the entire aggregate of Diatoms into the
larger and the finer kinds, may be accomplished by stirring the
sediment in a tall jar of water, and then, while it is still in motion,
pouring off the supernatant fluid as soon as the coarser particles
have subsided ; this fluid should be set aside, and, as soon as a
finer sediment has subsided, it should again be poured off ; and
this process may be repeated three or four times at increasing
intervals, until no further sediment subsides after the lapse of half
an hour. The first sediment will probably contain all the sandy
particles, with, perhaps, some of the largest Diatoms, which may
be- picked out from among them ; and the subsequent sediments
will consist almost exclusively of Diatoms, the sizes of which will
be so graduated, that the earliest sediments maybe examined with
the lower powers, the next with medium powers, while the latest
will require the higher powers — a separation which is attended
with great convenience.* It sometimes happens that fossilized
Diatoms are so strongly united to each other by Siliceous cement,
as not to be separable by ordinary methods ; in this case, small
lumps of the deposit should be boiled for a short time in a weak
Alkaline solution, which will act upon this cement more readily
than on the siliceous frustules ; and as soon as they are softened
so as to crumble to mud, this must be immediately washed in a
large quantity of water, and then treated in the usual way. If a
very weak alkaline solution does not answer the purpose, a
stronger one may then be tried. This method, devised by Prof.
Bailey, has been practised by him with much success in various
cases.f
262. The mode of mounting specimens of Diatomaceee will de-
pend upon the purpose which they are intended to serve. If they
can be obtained quite fresh, and it be desired that they should
exhibit, as closely as possible the appearance presented by the
living plants, they should be put up in Distilled Water within
Cement-cells (§ 184) ; but if they are not thus mounted within a
short time after they have been gathered, about a sixth-part of
Alcohol should be added to the water. If it be desired to exhibit
the stipitate forms in their natural parasitism upon other aquatic
plants, the entire mass may be mounted in Deane's Medium (§ 181)
* A somewhat more complicated method of applying the same principle is
described by Mr. Okeden in the " Quart. Journ. of Microsc. Science," Vol. iii.
(1855), p. 158. The Author believes, however, that the method above
described will answer every purpose.
f For other mothods of cleaning and preparing Diatoms, see " Quart. Journ.
of Microsc. Science," Vol. vii. (1859). p. 167, and Vol. i. N.S. (1861), p. 143 ; and
" Trans, of Microsc, Soc," Vol. xi. N.S. (1868), p. 4.
MOUNTING OF DIATOMACEJE. 345
or in Glycerine Jelly in a deeper cell ; and snch a preparation is a
very beautiful object for the black-ground illumination. If, on
the other hand, the minute structure of the siliceous envelopes is
the feature to be brought into view, the fresh Diatoms must be
boiled in nitric or hydrochloric acid, which must then be poured
off (sufficient time being allowed for the deposit of the residue) ;
and the sediment, after repeated washings, is to be either mounted
in Balsam in the ordinary manner (§ 174), or, if the species have
markings that are peculiarly difficult of resolution, is to be set up
dry between two pieces of thin-glass (§ 165). In order to obtain
a satisfactory view of these markings, Objectives of very wide
angular aperture are required, and all the refinements which have
recently been introduced into the methods of Illumination need to
be put in practice. (Chaps, in. iv.) — It will often be convenient
to mount certain particular forms of Diatomaceae separately from
the general aggregate ; but on account of their minuteness, they
cannot be selected and removed by the usual means. The larger
forms, which may be readily distinguished under a simple Micro-
scope, may be taken up by a camel-hair pencil which has been so
trimmed as to leave two or three hairs projecting beyond the rest.
But the smaller can only be dealt with by a single fine Bristle or
stout Sable-hair, which may be inserted into the cleft-end of a
slender wooden handle ; and if the bristle or hair should be split at
its extremity in a brush-like manner, it will be particularly useful.
(Such split-hairs may always be found in a Shaving-brush which
has been for some time in use ; those should be selected which
have their split portions so closely in contact, that they appear
single until touched at their ends.) When the split extremity of
such a hair touches the glass slide, its parts separate from each
other to an amount proportionate to the pressure ; and, on being
brought up to the object, first pushed to the edge of the fluid on
the slide, may generally be made to seize it. — Supposing that we
wish to select certain particular forms from a Diatomaceous sedi-
ment which has been obtained by the preceding processes, either
of the two following modes may be put in practice. A small
portion of the sediment being taken up in the Syringe or Dipping
tube, and allowed to escape upon the slide, so as to form a long
narrow line upon it ; this is to be examined with the lowest power
with which the object we are in search of can be distinguished
(the Erector and Draw-Tube, §§ 68, 69, will here be very useful) ;
and when one of the specimens has been found, it may be taken
up, if possible, on the point of the hair, and transferred to a new
slide, to which it may be made to adhere by first breathing on the
surface. But if it be found impracticable thus to remove the
specimens, on account of their minuteness, they may be pushed
on one side of the slide on which they are lying ; all the remainder,
of the sediment which it is not desired to preserve may be washed
off ; and the objects may then be pushed back into the middle of
the slide, and mounted in any way that may be desired.
346
MICROSCOPIC FORMS OF VEGETABLE LIFE.
263. Palmellacece. — To the family thus designated belong those
two Genera which have been already cited as illustrations of the
humblest types of Vegetation (§§ 204, 207) ; and the other forms
which are associated with those are scarcely less simple in their
essential characters, though sometimes attaining considerable di-
mensions. They all grow either on damp surfaces, or in fresh or
salt water ; and they may either form (1) a mere powdery layer, of
which the component particles have little or no adhesion to each
other, or they may present themselves (2) in the condition of an
indefinite slimy film, or (3) in that of a tolerably firm and definitely
bounded membranous ' frond.' The first of these states we have
seen to be characteristic of Pahnoglcea and Protococcus ; the new
cells, which are originated by the process of binary subdivision,
usually separating from each other after a short time ; and even
where they remain in cohesion, nothing like a frond or membra-
nous expansion being formed. The ' Eed Snow,' which sometimes
colours extensive tracts in Arctic or Alpine regions, penetrating
even to the depth of several feet, and vegetating actively at a
temperature which reduces most plants to a state of torpor, is
Fig. 159.
Hcematococcus sanguineus, in vai'ious stages of development: — a, single
cells, enclosed in their mucous envelope ; 6, c, clusters formed by sub-
division of parent -cell ; d, more numerous cluster, its component cells
in various stages of division ; e, large mass of young cells, formed by
the subdivision of tbe parent-endochrome, and enclosed within a com-
mon mucous envelope.
generally considered to be a species of Protococcus ; but as its cells
are connected by a tolerably firm gelatinous investment, it would
PALMELLACE.E :— fLEMATOCOCCUS ; PALMODICTYON. 347
rather seem to be a Palmella. The second is the condition of the
Genus Palmella ; of which one species, the P. cruenta, usually
known under the name of ' Gory Dew,' is common on damp walls
and in shady places, sometimes extending itself over a considerable
area as a tough gelatinous mass, of the colour and general appear-
ance of coagulated blood. A characteristic illustration of it is also
afforclel by the Hcematococcus sanguineus (Fig. 159), which chiefly
differs from Palmella in the partial persistence of the walls of the
parent-cells, so that the whole mass is subdivided by partitions,
which enclose a larger or smaller number of cells originating in the
subdivision of their contents. Besides increasing in the ordinary
mode of binary multiplication, the Palmella-cells seem occasionally
to rupture and diffuse their granular contents through the gela-
tinous stratum, and thus to give origin to a whole cluster at
once, as seen at e, after the manner of other simple Plants to
be presently described (§ 265), save that these minute segments
of the endochrome, having no power of spontaneous motion, cannot
be ranked as ' zoospores.' The gelatinous masses of the Palmellae
are frequently found to contain parasitic growths formed by the
extension of other plants through their substance ; but numerous
branched filaments sometimes present themselves, which, being
traceable into absolute continuity with the cells, must be con-
sidered as properly appertaining to them. Sometimes these fila-
ments radiate in various directions from a single central cell, and
must at first be considered as mere extensions of this ; their extre-
mities dilate, however, into new cells ; and when these are fully
formed, the tubular connections close-up, and the cells become
detached from each other.* Of the third condition, we have
an example in the curious Palmodidyon described by Kiitzing ;
the frond of which appears to the naked eye like a delicate
network consisting of anastomosing branches, each composed
of a single or double row of large vesicles, within every one of
which is produced a pair of elliptical cellules that ultimately escape
as 'zoospores.' The alternation between the 'motile' form
and the ' still ' or resting form, which has been described as
occurring in Protococcus (§ 208), has been observed in several
other forms of this group ; and it seems obviously intended,
like the production of ' zoospores,' to secure the dispersion of
the plant, and to prevent it from choking itself by overgrowth in
any one locality. From the close resemblance which many re-
puted PaJmellaceo3 bear to the early stages of higher Plants
(Fig. 160, a, b, c), considerable doubt has been felt by many Natu-
ralists whether they ought to be regarded in the light of distinct
and complete organisms, or whether they are anything else than
embryonic forms of more elevated types. The observations of Dr.
Hicks seem to indicate that a large proportion (to say the least)
* This fact, first made public by Mr. Th-waites (" Ann. of Nat, Hist.," 2nd
Series, Vol. ii., 1848, p. 313). is one of fundamental importance in the determina-
tion of the real characters of this group.
348
MICROSCOPIC FORMS OF VEGETABLE LIFE.
Fig. 160.
of these so-called Unicellular Algae are really the gonidia of'
Lichens.* On the other hand, there are Botanists who maintain
that Lichens are really Algse consolidated by want of moisture.
264. Notwithstanding the very definite form and large size
attained by the fronds or leafy expansions of the Ulvacece, to which
group belong the grass-green Sea-weeds (or 'Lavers ') found on every
coast, yet their essential structure differs bnt very little from that
of the preceding group ; and the principal advance is shown in this,
that the cells, when
multiplied by binary
subdivision, not only
remain in firm connec-
tion with each other,
but possess a very
regular arrangement (in
virtue of the determinate
plan on which the sub-
division takes place), and
form a definite mem-
branous expansion. The
mode in which this frond
is produced may be best
understood by studying
the history of its develop-
ment, some of the princi-
pal phases of which are
seen in Fig. 160 ; for the
isolated cells ( \), in which
it originates, resembling
m all points those of a
Protococcus, give rise, by
their successive subdivi-
sions in determinate
directions, to such regular
clusters as those seen at
b and c, or to such Con-
verfoid filaments as that
shown at d. A continua-
tion of the same regular
mode of subdivision,
taking place alternately
in two directions, may at
once extend the clusters b and c into leaf -like expansions ; or, if the
filamentous stage be passed through (different species presenting
variations in the history of their development), the filament increases
in breadth as well as in length (as seen at e), and finally becomes
such a frond as is shown at f, g. In the simple membranous expan-
* See his admirable " Memoirs in Quart. Journ. of Microsc. Science," Vol.
viii. (1860), p. 239, and Vol. i. N.S. (1861), pp. 15, 90, 157.
,(!« BtB (J* JjJWJ ai« mm «*<
'„, ,■;!': u .'i,.i el ' a;. . .in Jin
VtfftSttW"*1™1 Will Will MM;
!'■!•'. ,|l JlVS'"1^ 5'.:iUI.! >■■»':
JmSSIS eifflwudifigiiiiiiiy
ipoia© MHHS fluiaiiroW
Successive stages of development of TJlva.
ULVACE.E : — PRODUCTION OF ZOOSPOEES.
349
sions thus formed, there is no approach to a differentiation of
parts by even the semblance of a formation of Eoot, Stem, and
Leaf, such as the higher Algas present ; every portion is the exact
counterpart of every other ; and every portion seems to take an
equal share in the operations of growth and reproduction. Each
cell is very commonly found to exhibit an imperfect partitioning
into four parts, preparatory to multiplication by double subdivision ;
and the entire frond usually shows the groups of cells arranged
in clusters containing some multiple of four.
265. Besides this continuous increase of the individual frond,
however, we find in most species of Viva a provision for extending
the plant by the dispersion of ' zoospores ;' for the endochrome
(Fig. 161, a) subdivides into numerous segments (as at b and c),
nr/o/Ccft
mm mp$%
Formation of Zoospores in Phycoseris gigantea (Ulva latissima) :—
o, portion of the ordinary frond ; 6, cells in which the endochrome is
beginning to break up into segments ; c, cells from the boundary be-
tween the coloured and colourless portion, some of them containing
zoospores, others being empty; d, ciliated zoospores, as in active
motion ; e, subsequent development of the zoospores.
which at first are seen to lie in close contact within the cell that
contains them, then begin to exhibit a kind of restless motion, and
at last pass-forth through an aperture in the cell-wall, acquire four
or more cilia (d), and swim freely through the water for some time.
At last, however, they come to rest, attach themselves to some fixed
point, and begin to grow into clusters or filaments (e), in the manner
350 MICEOSCOPIC FORMS OF VEGETABLE LIFE.
already described. The walls of the cells which have thus discharged
their Endochrome remain as colourless spots on the frond ; some-
times these are intermingled with the portions still vegetating in
the usual mode ; but sometimes the whole endochrome of one por-
tion of the frond may thus escape in the form of zoospores, thus
leaving behind it nothing but a white flaccid membrane. If the
Microscopist who meets with a frond of an Ulva in this condition
should examine the line of separation between its green and its
coloured portion, he may not improbably meet with cells in the
very act of discharging their zoospores, which ' swarm ' around
their points of exit very much in the manner that Animalcules
are often seen to do around particular spots of the field of view,
and which might easily be taken for true Infusoria ; but on carry-
ing his observations further, he would see that similar bodies are
moving within cells a little more remote from the dividing line,
and that, a little further still, they are obviously but masses of
Endochrome in the act of subdivision.*
266. Of the true Generative process in the Ulvacece, nothing
whatever is known ; and it is consequently altogether uncertain
whether it takes-place by simple Conjugation, or according to that
more truly Sexual method which will be presently described. Here,
again, therefore, is an unsolved problem of the greatest Physiological
interest, which probably requires nothing more for its solution than
patient and intelligent study. And the Author would point out,
that it is by no means unlikely that the Generative process may not
be performed in the complete plant; but, as in the Ferns (§ 316),
in the early product of the development of the zoospore. — Although
the typical Ulvacece are marine, yet there are several fresh-water
species ; and there are some which can even vegetate on damp sur-
faces, such as those of rocks or garden-walks kept moist by the
percolation of water.
267. The OscillatoriacecB constitute another tribe of simple
Plants of great interest to the Microscopist, on account both of the
extreme simplicity of their structure, and of the peculiar Animal-
like movements which they exhibit. They are continuous tubular
filaments, formed by the elongation of their primordial cells, usually
lying together in bundles or in strata, sometimes quite free, and
sometimes invested by gelatinous sheaths. The Cellulose coat
(Fig. 162, a, a, a) usually exhibits some degree of transverse stria-
tion, as if the tube were undergoing division into cells ; but this
division is never perfected by the formation of complete partitions,
though the endochrome shows a disposition to separate into regular
segments (b, c), especially when treated with re-agents ; and the
filaments ultimately break up into distinct joints, the fragments of
endochrome, which are to be regarded as goniclia, usually escaping
* Such an observation the Author bad the good fortune to make in the year
1842, when the emission of zoospores from the Ulvacese, although it had
been described by the Swedish Algologist Agardh, had not been seen (he
believes) by any British naturalist.
MOVEMENTS OF OSCILLATOKACILE.
351
from their sheaths, and giving origin to new filaments.* These
Plants are commonly of some shade of green, often mingled, how-
ever, with bine ; but not unfrequently they are of a purplish hue,
and are sometimes so dark as when in
mass to seem nearly black. They occur
not only in fresh, stagnant, brackish, and
salt waters (certain species being peculiar
to each), but also in mud, on wet stones,
or on damp ground. Their very curious
movements constitute the most remark-
able feature in their history. These are
described by Dr. Harveyf as of three
kinds; first, a pendulum-like movement
from side to side, performed by one end,
whilst the other remains fixed so as to
form a sort of pivot ; second, a movement
of flexure of the filament itself, the oscil-
lating extremity bending over first from
one side then to the other, like the head
of a worm or caterpillar seeking some-
thing on its line of march ; and third, a
simple onward movement of progression.
" The whole phenomenon," continues Dr.
H., " may perhaps be resolved into a spiral
onward movement of the filament. If a
Structure of Oscillaioria
contexta ; — A, portion of a
filament, showing the stria-
tums on the cellulose-coat,
o, a, where the endochrome
is wanting; b, portion of
filament treated with weak
piece of the stratum of an Oscillatoria be syrup, showing a dieposi-
placed in a vessel of water, and allowed tion to a regular breaking-
to remain there for some hours, its edge UP of the endochrome into
.-,1 n , i p. -, ., , A-. P masses ; c, portion of fila-
will first become fringed with filaments, ment treJe^ witll strong
radiating as from a central point, with solution of chloride of cal-
their tips outwards. These filaments, by cium, showing a more ad-
their constant oscillatory movements, are vanced stage of the same
continually loosened from their hold on separation,
the stratum, cast into the water, and at
the same time propelled forward ; and as the oscillation con-
tinues after the filament has left its nest, the little swimmer gra-
dually moves along, till it not only reaches the edge of the vessel,
but often — as if in the attempt to escape confinement — continues
its voyage up the sides, till it is stopped by dryness. Thus in a very
short time a small piece of Oscillatoria will spread itself over a large
vessel of water." This rhythmical movement, impelling the filaments
* According to Dr. F. d'Alquen (" Quart. Journ. Microsc. Science," Vol. iv.
p. 245), each filament — at least in certain species — has an axis of different
composition from the surrounding endochrome ; being solid, highly refractive,
but slightly affected by iodine, and nearly colourless when moist, though
slightly greenish when dry. And reasons are given by this observer for the
belief that, the peculiar motor power of the filament resides specially, if not
exclusively, in this axis.
t " Manual of British Marine Algae," p. 220.
352
MICROSCOPIC FORMS OF VEGETABLE LIFE.
Fig. 163.
in an undeviating onward course, is evidently of a nature alto-
gether different from the truly spontaneous motions of Animals ;
and must be considered simply as the expression of certain vital
changes taking place in the interior of the cells. It is greatly
influenced by temperature and light, being much more active in
warmth and sunshine than in cold and shade ; and it is checked by
any strong chemical agents. — The true Generation of Oscillatoriacece
is as yet completely unknown ; and it does not seem at all unlikely
that these plants may (like the Nostochaceaz, § 268), be the 'motile'
forms of some others, probably Lichens, which in their ' still ' con-
dition present an aspect altogether different.
268. Nearly allied to the preceding is the little tribe of Nos-
tocliacece ; which consists of distinctly -beaded filaments, lying in
firmly-gelatinous fronds of definite outline (Fig. 163). The fila-
ments are usually simple, though some-
times branched ; and are almost always
curved or twisted, often taking a spiral
direction. The masses of jelly in which
they are imbedded are sometimes glo-
bular or nearly so, and sometimes ex-
tend in more or less regular branches :
they frequently attain a very consi-
derable size ; and as they occasionally
present themselves quite suddenly (es-
pecially in the latter part of autumn,
on damp garden- walks), 'they have re-
ceived the name of ' fallen stars.' They
are not always so suddenly produced,
however, as they appear to be ; for
they shrink up into mere films in dry
weather, and expand again with the
first shower. There is strong evidence
that Nostocs are really the ' gonidia '
of Gollema and other Lichens, which,
I " ^v__^cJ^sL^ Ja \ when set free from the plants that
^ xfeecffi" x^v>^ \ produced them, enter upon an entirely
» new phase of existence.* They then
multiply themselves, like the Oscilla-
toriaceae, by the subdivision of their
filaments, the portions of which escape
from the gelatinous mass wherein they were imbedded, and move
slowly through the water in the direction of their length : after a
time they cease to move, and a new gelatinous envelope is formed
around each piece, which then begins not only to increase in length
by the transverse subdivision of its segments, but also to double
itself by longitudinal fission, so that each filament splits length-
Portion of gelatinous frond
of Nostoc.
* See Hicks in "Quart. Journ. of Microsc. Science," Vol. i. N.S. (1861),
p. 90.
FAMILY SIPHON ACE^E:— VAUCHEEIA. 353
ways (as it were) into two new ones. By the repetition of this
process a mass of new filaments is produced, the parts of which are
at first confused, bnt afterwards become more distinctly separated
by the interposition of the gelatinous substance developed between
them. Besides the ordinary cells of the beaded filaments, two other
kinds are occasionally observable : namely, ' vesicular cells ' of
larger size than the rest (sometimes occurring at one end of the
filaments, sometimes in the centre, and sometimes at intervals
along their whole length), which are destitute of endochrome, and
are sometimes furnished with cilia ; and ' sporangial cells,' which
seem like enlarged forms of the ordinary cells, and which are
usually found in the neighbourhood of the preceding. It has been
supposed that the - vesicular cells' are • antheridia ' or sperm- cells,
producing 'antherozoids,' and that the ' sporangial cells' contain
germs, which, when fertilized by the antherozoids, and set free, be-
come ' resting-spores,' as in certain members of the family to be
next noticed.
269. Although many of the plants belonging to the Family
Sipho/iacece attain a considerable size, and resemble the higher Sea-
weeds in their general mode of growth, yet they retain a simplicity
of structure so extreme that it apparently requires them to be
ranked among the Protophytes. They are inhabitants both of
Fresh-water and of the Sea ; and consist of very large tubular cells,
which commonly extend' themselves into branches, so as to form
an arborescent frond. These branches, however, are seldom sepa-
rated from the stem by any intervening partition ; but the whole
frond is composed of a simple continuous tube, the entire contents
of which may be readily pressed-out through an orifice made by
wounding any part of the wall. The Vaucheria, named after the
Genevese botanist by whom the Fresh-water Confervas were first
carefully studied, may be selected as a jDarticularly good illustra-
tion of this family ; its history having been pretty completely made
out. Most of its species are inhabitants of Fresh water ; but some
are Marine ; and they commonly present themselves in the form of
cushion-like masses, composed of irregularly branching filaments,
which, although they remain distinct, are densely tufted together
and variously interwoven. — The formation of motile gonidia or
' zoospores ' may be readily observed in these plants, the whole
process usually occupying but a very short time. The extremity
of one of the filaments usually swells up in the form of a club, and
the endochrome accumulates in it so as to give it a darker hue
than the rest ; a separation of this part from the remainder of the
filament, by the interposition of a transparent space, is next seen ;
a new envelope is then formed around the mass thus cut off ; and
at last the membranous wall of the investing tube gives way, and
the zoospore escapes, not, however, until it has undergone marked
changes of form, and exhibited curious movements. Its motions
continue for some time after its escape, and are then plainly seen
to be due to the action of the cilia with which its whole surface is
354
MICROSCOPIC FORMS OF VEGETABLE LIFE.
clothed. If it be placed in water in which some carmine or_ indigo
has been rnbbed, the colonred grannies are seen to be driven in
snch a manner as to show
Fig. 164. that a powerful current is
produced by their propul-
sive action, and a long track
is left behind it. When it
meets with an obstacle, the
ciliary action not being ar-
rested, the zoospore is flat-
tened against the object ; and
it may thus be compressed,
even to the extent of causing
its endochrome to be dis-
charged. The cilia are best
seen when their movements
have been retarded or entirely
arrested by means of opium,
iodine, or other chemical re-
agents. The motion of the
spore continues for abouttwo
hours ; but after the lapse of
that time it soon comes to an
end, and the spore begins to
develope itself into a new
plant. It has been observed
by Unger, that the escape
of the zoospores generally
takes place towards 8 a.m. ;
to watch this phenomenon,
therefore, the plant should
be gathered the day before,
and its tufts examined early
in the morning. It is stated
by Dr. Hassall, that he has
seen the same filament give
off two or three zoospores
successively : their emission
is obviously to be regarded
Successive phases of Generative process ag a method of increase by
in Vaucheria sessilis .—at A are seen one of gemmation, rather than as a
the & ,. ' ,
the ' horns' or Antheridia (a) and one of
Capsules (6), as yet unopened ; at B the an-
theridium is seen in the act of emitting the
antherozoids (c), of which many enter the
opening at the apex of the capsule, whilst
others (d) which do not enter it, display
their cilia when they become motionless ;
at c the orifice of the capsule is closed again
by the formation of a proper coat around'the ago suspected by Vaucher,
endochrome-mass. though upon no sufficient
generative act.
270. Eecent discoveries
have shown that there exists
in this humble plant a true
process of Sexual Genera-
tion, as was, indeed, lono;
GENERATION OF VAUCHERIA :— ACHLYA. 355
grounds. The branching filaments are often seei to bear at their
sides peculiar globular or oval capsular protuberances, sometimes
separated bj the interposition of a stalk, which are filled with dark
endochrome ; and these have been observed to give exit to large
bodies covered with a firm envelope, from which, after a time, new
plants arise. In the immediate neighbourhood of these ' capsules '
are always found certain other projections, which, from being
usually pointed and somewhat curved, have been named ' horns '
(Fig. 164, a, a) ; and these have been shown by Pringsheim to be ' An-
theridia,' which, like those of the Characece (§ 280), produce anthe-
rozoids in their interior ; whilst the capsules (a, b) are ' Germ-cells,'
who^e aggregate mass of endochrome is destined to become, when
fertilized, the primordial cell of a new generation. The antherozoids
(b, c, d) when set free from the antheridium a, swarm over the ex-
terior of the capsule b, and have actually been seen to penetrate its
cavity through an aperture which opportunely forms in its wall,
and to come into contact with the surface of its endochrome-mass,
over which they diffuse themselves : there they seem to undergo
dissolution, their contents mingling themselves with those of the
germ-cell; and the endochrome-mass, which had previously no
proper investment of its own, soon begins to form an envelope (c, b),
which increases in thickness and strength, until it has acquired
such a density as enables it to afford a firm protection to its contents.
This body, possessing no power of spontaneous movement, is known
as a ' resting-spore,' in contradistinction to the zoospores already
described ; or it may be termed an ' oo-spore,' since it answers the
purpose of a seed in laying the foundation for a new generation,
whilst the zoospores merely multiply the individual by a process
analogous to budding.
271. The Microscopist who wishes to study the development of
Zoospores, as well as several other phenomena of this low type of
vegetation, may advantageously have recourse to the little plant
termed Achlya prolifera, which grows parasitically upon the bodies
of dead Flies lying in the water, but also not unfrequently attaches
itself to the gills of Fish, and is occasionally found on "the bodies
of Frogs. Its tufts are distinguishable by the naked eye as
clusters of minute colourless filaments ; and these are found, when
examined by the microscope, to be long tubes devoid of all parti-
tions, extending themselves in various directions. The tubes con-
tain a colourless slightly-granular protoplasm, the particles of
which are seen to move slowly in streams along the walls, as in
Chara, the currents occasionally anastomosing with each other
(Fig. 165, c). "Within about thirty-six hours after the first appear-
ance of the parasite on any body, the protoplasm begins to accu-
mulate in the dilated ends of the filaments, each of which is cut off
from the remainder by the formation of a partition ; and within
this dilated cell the movement of the protoplasm continues for a
time to be distinguishable. Yery speedily, however, its endo-
chrome shows the appearance of being broken up into a large
aa2
356
MICROSCOPIC FORMS OF VEGETABLE LIFE.
number of distinct masses, which are at first in close contact with
each other and with the walls of the cell
Fig. 165.
(Fig. 165, a), but which
gradually become more
isolated, each seeming
to acquire a proper cell-
wall; they then begin
to move about within
the parent-cell ; and,
when quite mature,
they are set free by the
rupture of its wall (b),
to go forth and form
new attachments, and
to develope themselves
into tubiform cells re-
sembling those from
which they sprang.
Each of these ' motile
gonidia' is possessed of
only two cilia ; their
movements are not so
powerful as those of
the zoospores of Yau-
cheria ; and they come
to an end sooner. This
plant forms ' resting-
. spores' also, like those
of Yaucheria ; and
there is every proba-
Development of Achlya prolifpra .—a, dilated ex- bnrtv that they are ge-
treniity of a filament &, separated from the rest by nerated by a like Sex-
a partition a, and containing gonidia in progress of rial process. They may
formation; — b, conoeptacle discharging itself, and remain unchanged for
setting-free gonidia, a, 6, c,— C, portion of fila- { f- • S f
ment, showing the course of the circulation of a iong time m ™er
granular protoplasm.. when no appropriate
nidus exists for them;
but will quickly germinate if a dead Insect or other suitable
object be thrown in.
272. One of the most curious forms of this group is the Hydro-
dictyon utriculatum, which is found in fresh-water pools in the
midland and southern counties of England. Its frond consists of
a green open network of filaments, acquiring, when full grown, a
length of from four to six inches, and composed of a vast number
of cylindrical tubular cells, which attain the length of four lines or
more, and adhere to each other by their rounded extremities, the
points of junction corresponding to the knots or intersections of
the network. Each of these cells may form within itself an
enormous multitude (from 7000 to 20,000) of gonidia ; which, at
a certain stage of their development, are observed in active motion
MULTIPLICATION OF HYDRODICTYON. 357
in its interior ; but of which, groups are afterwards formed by
their mutual adhesion, that are set-free by the dissolution of their
envelopes, each group, or ' macro -goniclium,' giving origin to a
new plant-net. Besides these bodies, however, certain cells pro-
duce from 30,000 to 100,000 more minute bodies of longer shape,
each furnished with four long cilia and a red spot, which are
termed ' micro-gonidia :' these escape from the cell in a swarm,
move freely in the water for some time, and then come to rest and
sink to the bottom, where they remain heaped in green masses.
It appears from the observations of Pringsheim (" Quart. Journ.
of Microsc. Science," ]ST.S., Yol. ii. 1862, p. 51), that they become
surrounded with a firm cellulose envelope, and may remain in a
dormant condition for a considerable length of time, like the
' statospores' of Yolvox (§ 216) ; and that in this condition they
are able to endure being completely dried-up without the loss of
their vitality, provided that they are secluded from the action of
Light, which causes them to wither and die. In this state they
bear a strong resemblance to the cells of Protococcus. The first
change that manifests itself in them is a simple enlargement ;
next the endochrome divides itself successively into distinct
masses, usually from two to five in number ; and these, when set
free by the giving-way of the enveloping membrane, present the
characters of ordinary Zoospores, each of them possessing one or
two vibratile filaments at its anterior semi-transparent extremity.
Their motile condition, however, does not last long, often giving
place to the motionless stage before they have quite freed them-
selves from the parent-cell ; they then project long angular pro-
cesses, so as to assume the form of irregular polyhedra, at the
same time augmenting in size ; and the endochrome contained
within each of these breaks-up into a multitude of gonidia, which
are at first quite independent and move actively within the cell-
cavity, but soon unite into a network that becomes invested with a
gelatinous envelope, and speedily increases so much in size as to
rupture the containing cell-wall, on escaping from which it presents
all the essential characters of a young Hydrodictyon. Thus,
whilst this plant multiplies itself by Macro-gonidia during the
period of its most active vegetation, this method of multiplication
by Micro-gonidia appears destined to secure its perpetuation under
conditions that would be fatal to it in its perfect form. The
rapidity of the growth of this curious organism is not one of the
least remarkable parts of its history. The individual cells of
which the net is composed, at the time of their emersion as Gonidia,
measure no more than l-2500th of an inch in length ; but in the
course of a few weeks, they grow to a length of from l-12th to
l-3rd of an inch. — Nothing has been as yet ascertained respecting
the Sexual Generation of this type ; and the search for this is an
object worthy of the pursuit of any Microscopist who may possess
the requisite opportunities.
273. Almost every pond and ditch contains some members of
358
MICROSCOPIC FORMS OF VEGETABLE LIFE.
Fig. 166.
the Family Confervacece ; but they are especially abundant in
moving water ; and they constitute the greater part of those green
threads which are to be seen attached to stones, with their free ends
floating in the direction of the current, in every running stream,
and upon almost every part of the sea-shore, and which are com-
monly known under the name of s silk -weeds,' or ' crow-silk.' Their
form is usually very regular, each thread being a long cylinder
made-up by the union of a single file of short cylindrical cells
united to each other by their flattened extremities : sometimes
these threads give-off lateral branches, which have the same struc-
ture. The endochrome, though usually green, is occasionally
of a brown or purple hue ; it is sometimes distributed uniformly
throughout the cell (as in Fig. 166), whilst in other instances it is
arranged in a pattern of some
kind, as a network or spiral ; but
this may be only a transitory
stage in its development. The
Plants of this order are extremely
favourable subjects for the study
of the method of Cell-multiplica-
tion by binary subdivision. This
process usually takes-place only
in the terminal cell ; and it may
be almost always observed there
in some one of its stages. The
first step is seen to be the subdi-
vision of the endochrome, and the
inflexion of the primordial utricle
around it (Fig. 166, a, a) ; and
thus there is gradually formed
a sort of hour-glass contraction
across the cavity of the parent-
cell, by which it is divided into
two equal halves (b). The two
surfaces of the infolded utricle pro-
duce a double layer of Cellulose-
Process of cell-multiplication in membrane between them ; this is
Conferva glomerata:-A, portion of fila- not confined however, to the con-
ment with incomplete separation at a. >• r. -cat, n
and complete partition at b; B, the se^ tiguous surfaces of the young cell,
paration completed, a new cellulose but extends oyer the whole ex-
partition being formed at a ; c, forma- terior of the primordial utricle, so
tion of additional layers of cellulose that the new septum becomes con-
wall c, beneath the mucous investment tinuous with a new layer that is
*£&•££%£%££££ " Wd throughout tWWerior of
the cellulose wall of the original
cell (c). Sometimes, however, as in Conferva glomerata (a com-
mon species), new cells may originate as branches from any part
of the surface, by a process of budding ; which, notwithstanding
its difference of mode, agrees with that just described in its essential
C0NFEKVA0EJ3 ;— (EDOGONIUM ; SPEJBROPLEA. 359
character, being the result of the subdivision of the original cell.
A certain portion of the primordial utricle seems to undergo in-
creased nutrition, for it is seen to project, carrying the cellulose en-
velope before it, so as to form a little protuberance ; and this
sometimes attains a considerable length, before any separation of
its cavity from that of the cell which gave origin to it begins to
take place. This separation is gradually effected, however, by the
infolding of the primordial utricle, just as in the preceding case :
and thus the endochrome of the branch-cell becomes completely
severed from that of the stock. The branch then begins to elon-
gate itself by the subdivision of its first-formed cell ; and this
process may be repeated for a time in all the cells of the filament,
though it usually comes to be restricted at last to the terminal cell.
The Confervacece multiply themselves by Zoospores, which are
produced within their cells, and are then set-free, just as in the
Ulvacege (§ 265) ; in most of the genera the endochrome of each cell
divides into numerous zoospores, which are of course very minute ;
but in CEdogonium — a fresh-water genus distinguished by the cir-
cular markings which form rings round the extremities of many of
the cells, and by many interesting peculiarities of growth and re-
production*— only a single large zoospore is set free from each
cell; andits liberation is accomplished by the almost complete fission
of the wall of the cell through one of these rings, a small part only
remaining uncleft, which serves as a kind of hinge whereby the two
parts of the filament are prevented from being altogether separated.
Sometimes the zoospore does not completely extricate itself from
the parent-cell ; and it may begin to grow in this situation, the
root-like processes which it puts-forth being extended into the
cavity. Professor A. M. Edwards (U.S.) states that he has seen the
so-called 'motile spores' of the (Edogonium develope into objects
exactly resembling Euglenos, and finally reproducing " a filament
exactly like that from which the original green spore was projected."
He further asserts he has seen the cell-contents of (Edogonium
develope into forms identical with several genera of Ehrenberg's
Polygastric Animalcules.f Observations of an analogous character
were previously made by Cohn and Itzigsohn.
274. A true Sexual Generation has been observed in several
Confervaceas, and is probably universal throughout the group. It
is presented under a very interesting form in a plant termed
Sphcero2Jlea ammdina, the development and generation of which
have been specially studied by Dr. F. Cohn.j The 'oo-spore,'
which is the product of the sexual process to be presently described,
is filled when mature with a red oil, and is enveloped by two mem-
branes, of which the outer one is furnished with stellate pro-
longations (Plate XL, Fig. 1). When it begins to vegetate, its
* See the account of these processes in the " Microarraphic Dictionary',"
2nd Edit. p. 501.
f " Monthly Microsc. Journal," Vol. viii. (1&72), p. 28.
t "Ann. des iSci. Nat.," -iieme Ser., Botan., Tom. v. p. 187.
360 MICROSCOPIC FOEMS OF VEGETABLE LIFE.
Endochrome breaks up — first into two halves (Fig. 2), and then by
successive subdivisions into numerous segments (Figs. 3, 4), at the
same time becoming green towards its margin. These segments,
set free by the rapture of their containing envelope, escape as
Micro-gonidia, which are at first rounded or oval, each having a
semi-transparent beak whence proceed two vibratile filaments,
but which gradually elongate so as to become fusiform (Fig. 5), at
the same time changing their colour from red to green. These
move actively for a time like the zoospores of other Protophytes,
and then, losing their motile power, begin to develope them-
selves into filaments. The first stage in this development con-
sists in the elongation of the cell, and the separation of the endo-
chrome of its two halves by the interposition of a vacuole (Fig. 6) ;
and in more advanced stages (Figs. 7, 8) a repetition of the like
interrjosition gives to the endochrome that annular arrangement
from which the plant derives its specific name. This is seen at a,
Fig. 9, as it presents itself in the filaments of the adult plant ; whilst
at b, in the same figure, we see a sort of frothy appearance which
the endochrome comes to possess through the multiplication of the
vacuoles. The next stage in the development of the filaments that
are to produce the spores, consists in the aggregation of the endo-
chrome into definite masses (as seen at Fig. 10, a), which soon
become star- shaped (as seen at b), each one being contained within
a distinct compartment of the cell. In a somewhat more advanced
stage (Fig. 11, a) the masses of endochrome begin to draw them-
selves together again ; and they soon assume a globular or ovoidal
shape (b), whilst at the same time definite openings (c) are formed
in their containing cell-wall. Through these openings the Anthe-
rozoids developed within other filaments gain admission, as shown
at cl, Fig. 12 ; and they seem to dissolve away (as it were) upon
the surface of the before-mentioned masses, which soon afterwards
become invested with a firm membranous envelope, as shown in
the lower part of Fig. 12, thenceforward constituting true Spores.
These undergo further changes whilst still contained within their
tubular parent-cells ; their colour changing from green to red, and
a second investment being formed within the first, which extends
itself into stellate prolongations, as seen in Fig. 13 ; so that, when
set free, they precisely resemble the mature oo- spores which we
have taken as the starting-point in this curious history. Certain
of the filaments (Fig. 14), instead of giving origin to spores, have
their annular collections of endochrome converted into Antherozoids,
which, as soon as they have disengaged themselves from the
mucilaginous sheath that envelopes them, move about rapidly in
the cavity of their containing cell (a, b) around the large vacuoles
which occupy its interior ; and then make their escape through
apertures (c, d) which form themselves in its wall, to find their way
through similar apertures into the interior of the spore-bearing
cells, as already described. These Antherozoids are shown in
Fig. 15, as they appear when swimming actively through the water
PLATE XL
Development asd Repkodtjctioit of Sph;eroplea.
{To face p.
CONFERVACILE : — GENERATION OF (EDOGONIUM. 361
Fig. 167.
by means of the two motile filaments which each possesses. — The
peculiar interest of this history consists in the entire absence of
any special organs for the Generative process, the ordinary fila-
mentous cells developing Spores on the one hand, and Antherozoids
on the other ; and in the simplicity
of the means by which the fecun-
dating process is accomplished.
275. A curious variation of this
process is seen in (Edogonium; for
whilst the Oo-sphores are formed
within certain dilated cells of the
ordinary filament (Fig. 167, i), and
are fertilized by the penetration of
Antherozoids (2), these anthero-
zoids are not the immediate product
of the sperm-cells of the same or of
another filament, but are developed
within a body termed an ' Andro-
spore' (5), which is to be set free
from within a germ-cell (4), and
which, being furnished with a cir-
cular fringe of cilia, and having
motile powers, very strongly re-
sembles an ordinary zoospore. This
Andro-spore, after its period of
activity has come to an end, attaches
itself to the outer surface of a germ-
cell, as shown at 1 , h ; it then under-
goes a change of shape, and a sort of
lid drops off from its free extremity,
as seen in the upper part of 1, by
which its contained antherozoids
(2) are set free ; and at the same
time an aperture is formed in the
wall of the cell containing the Oo-
spore, by which the antherozoid en-
ters its cavity, and fertilizes its
contained mass by dissolving upon
it and blending with it. This mass interior of itfl andr0„s . % free
then becomes invested with a thick Antherozoids ; 3. mature Oo-spore,
wall of its own ; but even when still invested with tbe cell-mem-
mature (3) it retains more or less brane of the parent filament ; 4,
of the envelope derived from the
cell within which it was developed.*
It is probable that the same thing Andro-spc
happens inmany other Confervaceae,
and that some of the bodies which have been termed Micro-gonidia
* See Fringsheim in "Ann. des Sci. Nat.," 4ieme Sen, Botan., Tom.
v. p. 187.
Sexual production of (Edogonimn
ciliatum : — 1, filament with two Oo-
spores in process of formation, the
lower one having two Andro-spores
attached to its exterior, the con-
tents of the upper one in the act of
being fertilized by the entrance of
an antherozoid set free from the
portions of a filament bearing sperm-
cells, from one of which an Andro-
spore is being set free ; 5, liberated
362
MICKOSCOPIC FOBMS OF VEGETABLE LIFE.
are really Andro-spores. The offices of these different classes of
reproductive bodies are only now begining to be understood ; and
the inquiry is one so fraught with Physiological interest, and
from the facility of growing these plants in artificial Aquaria, may
be so easily pursued, that it may be hoped that Microscopists
will apply themselves to it so zealously as not long to leave any
part of it in obscurity.
276. The Family Conjugatece agrees with that of Confervacece in
its mode of growth, but differs from it in the plan on which its
Generative process is performed ; this being accomplished by an
act of Conjugation resembling that which has been described in the
simplest Protophytes. These plants are not found so much in
running streams, as in waters that are perfectly still, such as those
of ponds, reservoirs, ditches, or marshy grounds ; and they are for
the most part unattached, floating freely at or near the surface,
especially when buoyed-up by the bubbles of gas which are liberated
from the midst of them under the influence of solar light and heat.
In an early stage of their growth, whilst as yet the cells are under-
going multiplication by subdivision, the Endochrome is commonly
diffused pretty uniformly through their cavities (Fig. 168, a) ; but
Fig. 168.
Various stages of the history of Zygnema quininum : — A, three cells
a, b, c, of a a oung filament, of which b is undergoing subdivision ; B,
two filamen s in the first stage of conjugation, showing the spiral
disposition of their endochromes, and the protuberances from the
conjugating cells; c, completion of the act of conjugation, the
endochromes of the cells of the filament a having entirely passed
over to those of filament b, in which the Oo-spores are formed.
as they advance towards the stage of conjugation, the endochrome
ordinarily arranges itself into regular spirals (b), but occasionally
CONJUGATED ;— CHJETOPHORACM.
363
in some other forms. The act of Conjugation usually occurs
between the cells of two distinct filaments that happen to lie in
proximity to each other ; and all the cells of each filament generally
take part in it at once. The adjacent cells put forth little pro-
tuberances, which come into contact with each other, and then
coalesce by the breaking down of the intervening partitions, so as
to establish a free passage between the cavities of the conjugating
cells. In some genera of this family (such as Mesoccuyus), the
conjugating cells pour their endochromes into a dilatation of the
passage that has been
established between them, Fig. 169.
and it is there that they
commingle so as to form
the Oo-spore. But in the
Zygnema (Fig. 168),which
is amongst the commonest
and best-known forms of
Conjugates, the endo-
chrome of one cell passes
over entirely into the
cavity of the other ; and
it is within the latter that
the Spore is formed (c),
the two endochromes coa-
lescing into a simple mass,
around which a firm en-
velope gradually makes
its appearance. Further,
it may be generally ob-
served that all the cells
of one filament thus empty
themselves, whilst all the
cells of the other filament
become the recipients :
here, therefore, we seem
to have a foreshadowing
of the Sexual distinction
of the Generative cells
into ' Sperm- cells ' and
1 Germ-cells,' which we
have just seen to exist in the Confervacese. And this transition will
be still more complete, if (as Itzigsohn has affirmed) the endochrome
of certain filaments of 8pirogyra breaks up before conjugation into
little spherical aggregations, which are gradually converted into
nearly colourless spiral filaments, having an active spontaneous
motion, and therefore corresponding precisely to the Antherozoids
of the truly sexual Protophytes.
277. The Chcetophoracece constitute another beautiful and in-
teresting little group of Confervoid plants, of which some species
Branches of Chcetophora elegans, in the act of
discharging ciliated zoospores, which are seen,
as in motion, on the right.
364
MICROSCOPIC FORMS OF VEGETABLE LIFE.
inhabit the Sea, whilst others are found in Fresh and pure water, —
rather in that of gently moving streams, however, than in strongly
flowing currents. Generally speaking, their filaments put forth
lateral branches, and extend themselves into arborescent fronds ;
and one of the distinctive characters of the group is afforded by the
fact, that the extremities of these branches are usually prolonged
into bristle-shaped processes (Fig. 169). As in many preceding cases,
these plants multiply themselves by the conversion of the endo-
chrome of certain of their cells into zoospores ; and these, when set
free, are seen to be furnished with four large cilia. ' Eesting-spores '
have also been seen in many species ; and it is probable that these,
as in Confervacese, are really Oo-spores, that is, are generative
products of the fertilization of the contents of Germ-cells by an-
therozoids developed within Sperm-cells (§274).
278. Nearly allied to the preceding are the Batraclwspermece,
whose name is indicative of the strong resemblance which their
beaded filaments bear to
frog- spawn ; these exhibit
a somewhat greater com-
plexity of structure, and
afford objects of extreme
beauty to the Microscopist
(Fig. 170). The plants of
this family are all in-
habitants of Fresh water,
and they are chiefly found
in that which is pure and
gently-flowing. " They are
so extremely flexible," says
Dr. Hassall, "that they
obey the slightest motion of
the fluid which surrounds
them ; and nothing can
surpass the ease and grace
of their movements. When
removed from the water
they lose all form, and
appear like pieces of jelly,
without trace of organiza-
tion ; on immersion, however, the branches quickly resume their
former disposition." Their colour is for the most part of a
brownish-green ; but sometimes they are of a reddish or bluish
purple. The central axis of each plant is originally composed
of a single file of large cylindrical cells laid end to end; but
this is subsequently invested by other cells, in the manner to be
presently described. It bears, at pretty regular intervals, whorls
of short radiating branches, each of them composed of rounded
cells, arranged in a bead-like row, and sometimes subdividing again
into two, or themselves giving off lateral branches. Each of the
Batracliospermum moniliforme.
CHAKACEJS ;— CYCLOSIS. 365
primary branches originates in a little protuberance from the
primitive cell of the central axis, precisely after the manner of the
lateral cells of Conferva gomerata (§ 273) ; as this protuberance
increases in size, its cavity is cut off by a septum, so as to render
it an independent cell ; and by the continual repetition of the pro-
cess of binary subdivision, this single cell becomes converted into
a beaded filament. Certain of these branches, however, instead of
radiating from the main axis, grow downwards upon it, so as to
form a closely-fitting investment that seems properly to belong to
it. Some of the radiating branches grow out into long transparent
points, like those of Chsetophoraceae ; and it does not seem by any
means improbable that these, like the 'horns' of Vaucheria
(§ 270), are really Antheridia. For within certain cells of other
branches ' resting-spores ' are found, by the agglomeration of
which are produced the large dark bodies that are seen in the
midst of the whorls of branches (Fig. 170).
279. This seems the most appropriate place to consider a group
of humble Plants having a peculiar interest for Microscopists —
that, namely, of Characece; in which we have a Vegetative ap-
paratus as simple as that of the Protophytes already described,
whilst their Generative apparatus is even more highly developed
than that of the proper Alga?. They are for the most part
inhabitants of Fresh waters, and are found rather in such as are
still, than in those which" are in motion ; one species, however, may
be met with in ditches whose waters are rendered salt by com-
munication with the Sea. They may be easily grown for the
purposes of observation in large glass jars exposed to the light ;
all that is necessary being to pour off the water occasionally from
the upper part of the vessel (thus carrying away a filni that is apt
to form on its surface), and to replace this by i'resh water. Each
plant is composed of an assemblage of long tubiform cells, placed
end to end ; with a distinct central axis, around which the branches
are disposed at intervals with great regularity (Fig. 171, a). In
the genus Nitella, the stem and branches are simple cells,
which sometimes attain the length of several inches ; whilst in the
true Chara each central tube is surrounded by an envelope of
smaller ones, which is formed as in Batrachospernieas, save that
the investing cells grow upwards as well as downwards from each
joint, and meet each other on the stem half-way between the
joints. Some species have the power of secreting carbonate of
lime from the water in which they grow, if this be at all impreg-
nated with calcareous matter ; and by the deposition of it beneath
their teguments they have gained their popular name of ' stone-
worts.' These humble Plants have attracted much attention, in
consequence of the facility with which the cyclosis, or movement
of fluid in the interior of the individual Cells, may be seen in
them. Each cell, in the healthy state, is lined by a layer of green
oval granules, which cover every part, except two longitudinal
lines that remain nearly colourless (Fig. 171, b) ; and a constant
MICROSCOPIC POEMS OP VEGETABLE LIFE.
stream of semi-fluid matter containing numerous jelly-like globules
is seen to flow over the green layer, the current passing up one
side, changing its direction at the extremity, and flowing down the
other side, the ascending and descending spaces being bounded by
the transparent lines just mentioned. That the currents are in
some way directed by the layer of granules, appears from the fact
noticed by Mr. Yarley,* that if accident damages or removes them
near the boundary between the ascending and descending cur-
Fig. 171.
f§ «
Nitella fleocillis : — A, stem and branches of the natural size ; a, b, c, d,
four verticils of branches issuing from the stem ; e, f, subdivision
of the branches; — B, portion of the stem and branches enlarged ; a, b,
joints of stem ; c, d, verticils ; e,/, new cells sprouting from the sides
of the branches ; g, /«, new cells sprouting at the extremities of the
branches.
rents, a portion of the fluid of the two currents will intermingle
by passing the boundary ; whilst, if the injury be repaired by the
development of new granules on the part from which they had
been detached, the circulation resumes its regularity, no part of
either current passing the boundary. In the young cells, however,
* " Transactions of the Microscopical Society," (First Series), Vol. ii.p. 99.
CHAKACEJE :— CYCLOSIS ; GENERATIVE ORGANS. 367
the rotation may be seen before the granular lining is formed. The
rate of the movement is affected by anything that influences the
vital activity of the Plant ; thus, it is accelerated by moderate
warmth, whilst it is retarded by cold ; and it may be at once
checked by a slight electric discharge through the plant. The
moving globules, which consist of starchy matter, are of various
sizes ; being sometimes very small and of definite figure, whilst in
other instances they are seen as large irregular masses, which
appear to be formed by the aggregation of the smaller particles.*
The production of new Cells for the extension of the stem or
branches, or for the origination of new whorls, is not here accom-
plished by the subdivision of the parent-cell, but takes place by
the method of out-growth (Fig. 171, b, e,f, g, li), which, as already
shown (§ 273), is nothing but a modification of the usual process
of cell-multipiication : in this manner, the extension of the indi-
vidual plant is effected with considerable rapidity. When these
plants are well supplied with nutriment, and are actively vegetating
under the influence of light, warmth, &c, they not unfrequently
develope ' bulbels,' or Gonidia of a peculiar kind, which serve the
same purpose in multiplying the individual, as is answered by the
Zoospores of the simpler Protophytes ; these are little clusters of
cells, filled with starch, which sprout from the sides of the central
axis, and then, falling off, evolve the long tubiform cells character-
istic of the plant from which they were produced.f The Characece
may also be multiplied by artificial subdivision ; the separated
parts continuing to grow under favourable circumstances, and
developing themselves into the typical form.
280. The Generative apparatus of Characece consists of two sets
of bodies, both of which grow at the bases of the branches (Fig.
172, a, b) ; one set is known by the designation of ' globules,' the
other by that of ' nucules.' The former are really Antheridia,
whilst the latter contain the Germ-cells. The ' globules,' which
are nearly spherical, have an envelope made up of eight triangular
valves (b, c), often curiously marked, which encloses a nucleus of a
light reddish colour : this nucleus is principally composed of a mass
of filaments rolled up compactly together ; and each of these fila-
ments (c) consists, like a Conferva, of a linear succession of cells.
In every one of these cells there is formed, by a gradual change in
its contents (the successive stages of which are seen at d, e, f), a
spiral thread of two or three coils, which, at first motionless, after
* This interesting phenomenon may be readily observed, by taking a small
portion of the Plant out of the water in which it is growing, and either placing
it in a large Aquatic Box (§ 108) or in the Zoophyte-Trongh (§ 110), or laying
it on the glass Stage-plate (§ 107) and covering it with thin glass. The
modification of the stage-plate termed the ' Growing Slide ' (§ 107) will enable
the Microscopist to keep a portion of Chara under observation for many days
together.
t This multiplication by bulbels was described by Amici in 1827 ; but his
observations seem to have been forgotten by Botanists, until the re-discovery
of the fact by M. Montagne.
368
MICROSCOPIC FORMS OF VEGETABLE LIFE.
a time begins to move and revolve within the cell ; and at last the
cell-wall gives way, and the spiral thread makes its way out (g),
partially straightens itself, and moves actively through the water
Fig. 17
Antheridia of Chara fragilis : — A, antheridiurn or ' globule ' de-
veloped at the base of pistillidium or 'nucule ' ; — B, nucule enlarged,
and globule laid open by the separation of its Valves ; — c, one of
the valves, with its group of antheridial filaments, each composed
of a linear series of ceils, within every one of which an antherozoid
is formed ; — in D, E, and r, the successive stages of this formation
are seen; — and at g is shown the escape of the mature anthero-
zoids, h.
for some time (h) in a tolerably determinate direction, by the lash-
ing action of two long and very delicate filaments with which they
GENERATIVE APPARATUS OF CHARA. 369
are furnished. The exterior of the ' nucule ' (a, b) is formed by five
spirally -twisted tubes, that give it a very peculiar aspect; and
these enclose a central sac containing protoplasm, oil, and starch-
globules. At a certain period the spirally-twisted tubes, which
form a kind of crown around the summit, separate from each other,
leaving a canal that leads down to the central cell ; and it is pro-
bable that through this canal the antherozoids make their way
down, to perform the act of fertilization. Ultimately the nucule
falls off like a seed, and gives origin to a single new plant by a
kind of germination. — The complete specialization of the Genera-
tive apparatus which we here observe (the organs of which it is
composed being distinctly separated from the ordinary Vegetative
portion of the fabric), as well as the complex structure of the organs
themselves, mark out this group, in spite of the simplicity of the
rest of its structure, as belonging to a grade very much above that
of the other Families that have been treated of in this chapter ;
but as scarcely any two Botanists agree upon the exact place
which ought to be assigned to it, the convenience of associating it
with other forms of vegetation of which the Microscopist especially
takes cognizance, is a sufficient reason for so arranging it in a work
like the present.*
* It was affirmed by Dr. Hartig (see "Quart. Journ. of Microsc. Science,'*
Vol. iv., 1856, p. 51) that the antherozoids of Chara and Kitella, as of
Marchantia and Mosses, may undergo a kind of metamorphosis into Spirilla, Vibri-
ones, and Monads ; and that, by the coalescence of these last, Am&bce are pro-
duced. And further, it ~ as asserted by Mr. H. Carter, of Bombay, that the
protoplasm of the ordinary cells of the CharaccB and other aquatic plants might
become transformed into an Actinophrys (see "Ann. of Nat. Hist.," 2nd Ser.,
Vol. xix., p. 287). More recently, however, this doctrine has been retracted
by Mr. Carter (" A.N.H.," 3rd Ser., Vol. viii.. p. 289), who accounts for the
phenomena which he observed on the hypothesis of parasitism. Yet the
original statements of Dr. Hartig and Mr. Carter have received independent
support from the observations of Dr. Hicks on Tolvox (§ 217) and on the root-
fibres of Mosses (§ 309), and from those of De Bary on the so-called Mycetozoa
(§ 300). And the Author is informed by a most excellent and trustworthy
observer, Mr. "W". Archer, of Dublin, that he has in several instances witnessed
the conversion of Vegetable protoplasm into Amoeboid and other Bhizopodal
forms, having all the attributes of Animal organisms.
B B
CHAPTER VII.
MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA.
281. From those simple Protophytes, whose minuteness causes
their entire fabrics to be fitting objects for Microscopic examina-
tion, we pass to those higher forms of Vegetable life, whose larger
dimensions require that they should be analyzed (so to speak) by
the examination of their separate parts. And in the present Chap-
ter we shall bring under notice some of the principal points of
interest to the Microscopist which are presented by the Cri/pto-
gamic series ; commencing with those simpler Algas which scarcely
rank higher than some of the Protophytes already described, and
ending with the Ferns and their allies, which closely abut upon
the Phanerogamia or Flowering Plants. In ascending this series,
we shall have to notice a gradual differentiation of organs ; those
set apart for Reproduction being in the first place separated from
those appropriated to Nutrition (as we have already seen them to
be in the Characece) ; while the principal parts of the Nutritive ap-
paratus, which are at first so blended into a uniform exjDansion or
thallus that no real distinction exists between Root, Stem, and
Leaf, are progressively evolved on types
more and more peculiar to each respec-
tively, and have their functions more and
more limited to themselves alone. Hence
we find a -differentiation, not merely in
the external form, but also in the inti-
mate structure of organs ; its degree
bearing a close correspondence to the
degree in which their functions are
respectively specialized or limited to
particular actions. Thus in the simple
TJlvce (Fig. 160), whatever may be the
extent of the Thallus, every part has ex-
actly the same structure, and performs
the same actions, as every other part,
living for and bij itself alone. In
Batrachospermum (Fig. 170) we have
seen a definite arrangement of branches
; and while the branches are formed of
simple necklace-like rows of rounded cells, the cells of the stem are
elongated and adhere to one another by flattened ends. This
Mesogloia vermicularis.
upon an axis of growth
STRUCTURE OF HIGHER ALG.E.
371
kind of differentiation is seen to be carried to a still greater
extent in Mesogloia (Fig. 173), a plant that may be considered as
one of the connecting links between snch Protophytes as Batracho-
spermeee, which it resembles in general plan of structure, and the
Fucoid Algge, which it resembles in fructification.
282. When we pass to the higher Sea-weeds, such as the common
Fucus and Laminaria, we observe a certain foreshadowing of the
distinction between Eoot, Stem, and Leaf ; but this distinction is
Fig. 174
A, Terminal portion of branch of Sphacelaria cirrhosa ; B,
lateral branchlet of S. tribuloides, the terminal cell of which is
emitting- antherozoids.
very imperfectly carried out, the root-like and stem-like portions
serving for little else than the mechanical attachment of the leaf-
like part of the plant, and each still absorbing and assimilating its
own nutriment, so that no transmission of fluid takes place from
one portion of the fabric to another. Hence we find that there is
not yet any departure from the simple cellular type of structure ;
the only modification being that the several layers of cells, where
many exist, are of different sizes and shapes, the texture being
372 MICEOSCOPIC STRUCTURE OF HICtHER CRYPTOGAMIA.
usually closer on the exterior and looser within; and that the
texture of the stem and roots is denser than that of the leaf -like
expansions or fronds. The group of Melanospermous or olive-
green sea-weeds, which in the family Fucacece exhibits the highest
type of Algal structure, presents us with the lowest in the family
Fctocarpacece ; which, notwithstanding, contains some of the most
elegant and delicate structures that are anywhere to be found in
the group, the full beauty of which can only be discerned by the
Microscope. Such is the case, for example, with the Sphacelaria, a
small and delicate sea- weed, which is very commonly found para-
sitic upon larger Algse, either near low-water-mark, or altogether
submerged ; its general form being remarkably characterized by a
symmetry that extends also to the individual branches (Fig. 174, a),
the ends of which, however, have a decayed look that seems to have
suggested the name of the genus (from the Greek <r0a/ce\os, gangrene).
From the recent observations of Pringsheim, it appears that this
apparent decay really consists in the resolution of the endochrome
of the terminal cells into antherozoids, which, when mature, escape
by an opening with a long tubular neck, which forms itself in the
wall of the sphacela. The same happens with the terminal cells of
the peculiar lateral branchlets, which are known as propagative
buds ; as is shown at b. The germ-cells have not been certainly
recognised ; but they are believed to be produced in what have
been considered as propagative buds in other individuals.
283. The study of the higher and larger members of this group
has recently come to present a new and very attractive source of
interest to the Microscopist, in consequence of the discovery of the
truly Sexual nature of their fructification ; and we shall take that
of a common species of Fucus as the type of that of the order gene-
rally. The ' receptacles,' which are borne at the extremities of
the fronds, here contain both ' sperm-cells ' and ' germ-cells ;' in
some other species, however, these are disposed in different re-
ceptacles on the same plant ; whilst in the commonest of all
F. vesiculosus (bladder- wrack), they are limited to different indi-
viduals.* When a section is made through one of the flattened
receptacles of F. platycarpus, its interior is seen to be a nearly
globular cavity (Fig. 175), lined with filamentous cells, some of
which are greatly elongated, so as to project through the pore by
which the cavity opens on the surface. Among these are to be
distinguished, towards the period of their maturity, certain fila-
ments (Fig. 176, a), whose granular contents acquire an orange
hue, and gradually shape themselves into oval bodies (b), each
with an orange-coloured spot, and two long thread-like appendages,
which, when discharged by the rupture of the containing cell, have
for a time a rapid undulatory motion, whereby those antherozoids
* It was at first stated by MM. Thuret and Decaisne that this species was
sometimes dioecious, sometimes hermaphrodite ; but they now consider the
hermaphrodite form to be a distinct species, the F. platycarpus described
above.
GENERATIVE APPARATUS OF FUCACEJE.
373
are diffused through the surrounding liquid. Lying amidst the
filamentous mass, near the walls of the cavity, are seen (Fig. 175)
numerous dark pear-shaped bodies, which are the sporangia, or
parent-cells of the ' germ-cells.' Each of these sporangia gives
Fig. 175.
fcr
liiP
mm
l^r
~SP
mm3
-^I||1B
—
I
1
-ST- fC^ -
l£c^
w?
Vertical section of receptacle of Fucus platycarpus, lined
with filaments, among which lie the antheridial cells, and
the sporangia containing octospores.
origin, by binary subdivision, to a cluster of eight cells, which is
thence known as an ' octospore ;' and these are liberated from
their envelopes before the act of fertilization takes place. This
act consists in the swarming of the antherozoids over the surface
of the germ-cells, to which they communicate a rotatory motion
by the vibration of their own filaments : it takes place within the
receptacles in the hermaphrodite Fuci, so that the spores do not
make their exit from the cavity until after they have been fecun-
374 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA.
dated ; but in the monoecious and dioecious species, each kind of
receptacle separately discharges its contents, which come into
mutual contact on their exterior. The antheridial cells are
usually ejected entire, but soon rupture so as to give exit to their
filaments ; the sporangia of the female receptacles discharge their
globular octospores within the receptacle ; and these, soon after
Fig. 176.
Antheridia and antherozoids of Fucus platycarjms : — A.
branching articulated hairs, detached from the walls of the
receptacle, bearing antheridia in different stages of develop-
ment ; B, antherozoids, some of them free, others still included
in their antheridial cells.
passing-forth, liberate their separate spores, which speedily meet
with antherozoids, and are fecundated by them. The Spores,
when fertilized, soon acquire a new and firmer envelope ; and
under favourable circumstances they speedily begin to develope
themselves into new plants. The first change seen in them is the
projection and narrowing of one end into a kind of footstalk, by
which the spore attaches itself, its form passing from the globular
to the pear-shaped ; a partition is speedily observable in its interior,
its single cell being subdivided into two ; and by a continuation of
a like process of duplication, first a filament and then a frondose
expansion is produced, which gradually evolves itself into the
likeness of the parent plant.*
284. The whole of this process may be watched without diffi-
culty, by obtaining specimens of F. vesiculosus at the period at
which the fructification is shown to be mature by the recent dis-
charge of the contents of the conceptacles in little gelatinous
masses on their orifices ; for if some of the spores which have
REPRODUCTION OF FLOEIDE^. 375
been set free from the olive-green (female) receptacles be placed in
a drop of sea-water in a very shallow cell, and a small quantity of
the mass of antherozoids, set free from the orange-yellow (male)
receptacles, be mingled with the fluid, they will speedily be observed,
with the aid of a magnifying power of 200 or 250 diameters, to go
through the actions just described ; and the subsequent processes
of germination may be watched by means of the ' growing-slide.'*
The winter months, from December to March, are the most favour-
able for the observation of these phenomena ; but where Fuci
abound, some individuals will usually be found in fructification at
almost any period of the year. Even in the Fucacece, according
to recent observations, a multiplication by Zoospores, like that of
the Ulvaceae (§ 265), still takes place ; these bodies being produced
within certain of the cells that form the superficial layer of the
frond, and swimming about freely for a time after their emission,
until they fix themselves and begin to grow. That they are to be
considered as gemmce (or buds), and not as generative products,
appears certain from the fact that they will vegetate without the
assistance of any other bodies : whereas the antherozoids of them-
selves never come to anything ; while the octospores undergo no
further changes, but decay away (as M. Thuret has experimentally
ascertained) if not fecundated by the antherozoids.
285. Among the Rhodospermece, or red Sea-weeds, also, we
find various simple but most beautiful forms, which connect this
group with the more elevated Protophytes, especially with the family
Chcetoplwracece (§277); such delicate feathery or leaf-like fronds
belong for the most part to the Family Ceramiacece, some members
of which are found upon every part of our coasts, attached either
to rocks or stones or to larger Algae, and often themselves afford-
ing an attachment to Zoophytes and Polyzoa. They chiefly live in
deeper water than the other sea-weeds ; and their richest tints
are only exhibited when they grow under the shade of projecting
rocks or of larger dark-coloured Algae. Hence in growing them
artificially in Aquaria, it is requisite to protect them from an
excess of light ; since otherwise they become unhealthy. — The
nature of the fructification of the Rhodospermece (or Floridece) is
less perfectly understood than that of the Fucoid Algae. It is
certain, however, that antheridia exist among them ; these being
developed in individuals that do not produce spores, and in pretty
much the same situations. The products of these antheridia,
however, do not exhibit the spontaneous motion of ordinary
antherozoids. Of the Spores there are two kinds, of which one
set are probably gemmce, whilst the other are germ-cells ; but it
is not yet determined to which of the two these characters respec-
tively belong. The ' tetraspores ' — which are peculiarly charac-
teristic of this group, being found in every one of its subdivisions —
* If the drop be covered, a shallow cell should be used, so as to keep the
pressure of the thin glass from the minute bodies beneath, whose movements it
will otherwise impede.
376 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA.
are usually imbedded in the general substance of the frond,
though they sometimes congregate in particular parts, or are
restricted to a special branch. Each group (Fig. 177, b) seems to
be evolved within one of the ordinary cells of the frond, which
undergoes binary subdivision ; the four secondary cells, however,
remain enclosed within their primary cell until the period of
maturity, a new envelope, the ' perispore,' being formed around
them. — In the true Corallines, which are Sea-weeds whose tissue
Fig. 177.
I
Arrangement of Tetraspores in Carpocaulon mediterraneum :
— A, entire plant ; B, longitudinal section of branch. (N.B.
Where only three tetraspores are seen, it is merely because
the f ourth did not happen to be so placed as to be seen at the
same view.)
is consolidated by calcareous deposit, the tetraspores are included
within hollow conceptacles ; but, generally speaking, it is the
simple spores only which are thus specially protected. These are
never scattered through the frond, like the tetraspores ; and are
commonly developed within a ceramidium, which is an urn-
shaped case, furnished with a pore at its summit, and containing a
tuft of pear-shaped spores arising from the base of its cavity. The
resemblance of these bodies in position to the octospores of Fuci
would seem to justify the conclusion that they are the true gene-
STEUCTUEE AND DEVELOPMENT OF LICHENS. 377
rative spores, whilst the tetraspores are gemmce, as Harvey
and Thwaites consider them ; but a different view is taken by
Decaisne, Agardh, and other eminent Algologists, who regard the
tetraspores as the true generative spores, and consider the simple
spores to be gemmae. It is, therefore, a point of much interest to
determine by careful observation and experiment which is the right
view ; and Microscopists who have the opportunity of studying
these plants, either in their native haunts or in artificial
Aquaria, can scarcely apply themselves to a better subject of
investigation. .
286. The Class of Lichens, which consists of Plants that closely
correspond with Algae in simplicity of organization, but differ from
them widely in habit, does not present so many objects of attractive
interest to the Microscopist ; and the peculiar density which
usually characterizes their structure, renders a minute examina-
tion of it more than ordinarily difficult. Lichens are commonly
found growing upon the trunks or branches of trees, upon rocks
or stones, upon hard earth, or in other situations in which they are
sparingly supplied with moisture, but are freely exposed to light
and air. In the simpler forms of this group, the ' primordial cell'
gives origin, by the ordinary process of subdivision, to a single
layer of cells, which may spread itself over the surface to which it
is attached, in a more or less circular form ; and one or more
additional layers being afterwards developed upon its free surface,
a thallus is formed, which has no very defined limit, and which,
in consequence of the very slight adhesion of its component cells,
is said to be 'pulverulent.' Sometimes, however, the cells of the
thallus are rather arranged in the form of filaments, which pene-
trate the superficial layers of the bark whereon such Lichens grow,
and which are sometimes also so interwoven at the outer surface
as to form a sort of cuticle. Interposed among the ordinary cells
of the thallus, we very commonly find certain green globular cells,
arranged in single bead-like filaments ; these, which are termed
gonidia, being found to be capable of reproducing the plant when
detached, must be considered as gemmce. From the recent obser-
vations of various Botanists, and especially from those of Dr.
Hicks (p. 348), it appears that many of the forms which have been
ranked among unicellular Algae, are in reality transitory conditions
of these gonidia, which may multiply themselves by binary sub-
division to a vast extent, without any essential change in their
condition. It was long since observed by Mr. Thwaites (p. 347,
note), that interlacing filaments are sometimes found in the midst
of the intercellular substance which holds together the cells of
masses of Palmella ; and this seems to constitute a very definite
approach to the Lichenoid condition. For in the higher tribes of
Lichens, we find the interlacing filaments forming a tough cortical
envelope to both surfaces ; whilst in the interior of the firm
' crustaceous ' thallus the gonidial cells are found in regular layers.
Sometimes these increase in particular spots, and make their way
378 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA.
through, the upper cortical layer, so as to appear on the surface as
little masses of dust, which are called soredia.
287. Besides these, Lichens are believed to contain proper
generative organs, by which a true Sexual re}Droduction is effected.
In addition to the ' fructification,' which is commonly recognised
by its projection from the surface of the thallus, the researches of
M. Tulasne have detected a set of peculiar organs of much smaller
size, not unlike the male receptacles of Fuci (§ 283), to which he
has given the appellation of spermogonia. From the exterior of
the cellular filaments which line these cavities, a vast number of
minute oval bodies termed spermatid are budded off, which, when
mature, escape in great numbers from the orifices of the spermo-
gonia. They differ from ordinary antherozoids in being destitute
of any power of spontaneous movement, and we cannot yet indubi-
tably assign to them the Male character, although various con-
siderations concur to render their perfornance of this function
highly probable. The Female portion of the generative apparatus,
though sometimes dispersed through the thallus, is usually col-
lected into special aggregations, which form projections of various
shapes ; these, although they have received a variety of designations
according to their particular conformation, may all be included
under the general term a/pothecia. When divided by a vertical
section, these bodies at their maturity are found to contain a
number of asci or spore-cases, arranged vertically in the midst of
straight elongated cells or filaments, which are termed paraphyses.
Each of the asci contains a definite number of spores (usually
eight, but always a multiple of two), which are projected from the
apothecia with some force, the emission being kept up continuously
for some time : this discharge seems to be due to the different
effect of moisture upon the different layers of the apothecium.
When and how the act of Fecundation is accomplished, is a matter
still hidden in obscurity ; and the problem is one which will only
be resolved by a combination of sagacity, manipulative skill, and
perseverance on the part of Microscopic observers who may devote
themselves to the study.*
288. In the simplest forms of Fungi we again return to the
lowest type of Vegetable existence, namely, the single Cell ; and
such, if perfect Plants, would probably take rank among the lowest
Protophytes. But there is good reason for regarding many —
perhaps all — of those which seem most simple, as the imperfectly
developed states of other plants, which, if they attained their full
evolution, would present a much more complex structure. This is
the case, for example, with the Torula cerevisice or ' yeast-plant,'
which so abounds in Yeast that this substance may be said to be
almost entirely made up of it. When a small quantity of yeast is
* For the latest information on this subject, see Dr. Lauder Lindsay's
Memoir on Polymorphism in the Fructification of Lichens (" Quart. Journ. of
Microsc. Science," Vol. viii., N.S., 1860, p. 1), and the authors therein
referred to.
TORULA CEREVISLE OR YEAST-PLANT. 379
placed Tinder the Microscope, and is magnified 300 or 400 diameters,
it is fonnd to be full of globules, which are clearly cells ; and
these cells vegetate, when placed in a fermentible fluid containing
some form of albuminous matter in addition to sugar, in the
manner represented in Fig. 178. Each cell puts forth one or two
projections, which seem to be young cells developed as buds or
offsets from their predecessors ; these, in the course of a short time,
become complete cells, and again perform the same process ; and
in this manner the single cells of yeast develope themselves, in the
course of a few hours, into rows of four, five, or six, which re-
main in continuity with each other whilst the plant is still grow-
ing, but which separate if the fermenting process be checked,
Fig. 178.
C r> Co £
Torula cerevisice, or Yeast-plant, as developed during the
process of fermentation: — a, b, c, d, successive stages of cell-
multiplication.
and return to the isolated condition of those which originally con-
stituted the yeast. Thus it is that the quantity of yeast first
introduced into the fermentible fluid, is multiplied six times or
more during the changes in which it takes part. The full develop-
ment of the plant, and the evolution of its apparatus of Fructifi-
cation, however, only occur when the fermenting process is allowed
to go on without check ; and it seems capable of producing a con-
siderable variety of forms, whose precise relationship to each other
has not yet been made clear. It would appear that Yeast may be
£>roduced by sowing in a liquid favourable to its development (such
as an aqueous solution of cane-sugar, with a little fruit -juice) the
sporules of any one of the ordinary ' moulds,' such as Penicillmm
glaucum, Mucor, or Asjjergillus, provided the temperature be kept-
up to blood-heat ; aud this even though the solution has been pre-
viously heated to 284° Fahrv, a temperature which must kill any
germs it may itself contain* And if this prove to be the case, we
must either regard the yeast-plant as the early common form of
several different Fungi, or regard the mature forms as only
different developments (under varying conditions of temperature,
nutriment, &c.) of one and the same type. The extraordinary
polymorphism which this group is known to exhibit (§ 299) seems to
render the latter interpretation the more probable.
* See the observations of Mad. Luders. in Schulze's " Archiv fur Mikro-
scopische Anatomie," Band, in., abstracted in "Quart. Joum. Micros. Sci.,"
N.S.,vol. viii. (1868), p. 35.
38a MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA.
289. This is, perhaps., the most appropriate place to notice the
minute bodies termed Bacteria and Vibriones, to which great im-
portance has of late come to "be attached ; on account, on the one
hand, of the relation they bear to the processes of fermentation
and putrefaction, and, on the other, of the assertions which have
been made* as to their production altogether de novo, under cir-
cumstances which are supposed to preclude the introduction of
germs from without. Bacteria are extremely minute, colourless,
transparent, rod-like bodies, usually from two to five times as long
as they are broad, sometimes showing a sort of jointing from
imperfect transverse divisions ; but not exhibiting, even under the
highest amplification, any other trace of structure. They have
usually a slight vacillating movement, which differs from that of Os-
cillatoricB (§ 267), in not being undulatory, but agrees with it in its
general uniformity. By Vibriones are designated minute monili-
form filaments, each formed of a series of colourless granules,
having an occasional wriggling eel-like motion, which propels them
rapidly across the field, whilst at other times they remain sta-
tionary or nearly so, — this alternation of activity and tranquillity
being very different from the rhythmical regularity of the Oscilla-
torice. There is strong reason for regarding the Vibriones as more
advanced forms of the Bacteria : for they appear under precisely
the same circumstances, and the jointing of the Bacteria ap-
pears to lead up to the necklace-like beading of the Yibriones.
Originally ranked by Ehrenberg and Dujardin as Animalcules,
their Vegetable affinities were first indicated by Cohn,* who, how-
ever, regarded them as allied to the Algce, considering Bacterium
termo to be the motile swarming form of a genus (Zooglcea)
closely allied to Palmella. It is clear, however, that they agree
with Fungi, rather than with Alga3, in this fundamental par-
ticular ; — that they cannot live in pure water, or develope them-
selves at the sole expense of inorganic elements, but that they
require as their nutritive material decomposing or decomposable
organic matter ; whilst (as in the case of Yeast) the chemical
change which takes place in such matter when exposed to the
atmosphere, is the result of their vegetative action. Further,
there is strong reason to believe that they are producible (like the
yeast-plant) from germs supplied by various forms of higher
Fungi, which develope themselves into vibriones when sown
in water in which animal flesh has been boiled, just as they
develope themselves into yeast in a saccharine solution. Two sets
of tubes, previously exposed to a strong dry heat, having been
filled with boiled flesh-water, sporules of various 'moulds ' were
introduced into one set, and both sets were then carefully closed
up and kept in a warm bath ; in the course of twenty-four hours a
cloudiness was often observable in the contents of the tubes in
which the Fungus-spores had been sown, and which were then
* See especially the work entitled, " The Beginnings of Life," by Dr. H.
Chorlton Bastian.
BACTERIA AND VIBEIONES. 381
found swarming with vibriones, whilst the contents of the other
set of tubes, containing the same fluid, and prepared in precisely
the same manner save as regards the introduction of the spores,
remained quite unchanged.*
290. Knowing, as we do, the universality of the diffusion
of the sporules of Fungi through the atmosphere (§ 298), we
can readily understand how they come to sow themselves in
any liquid exposed to it, and to increase and multiply — decompos-
ing the liquid in the act of doing so — if that liquid should supply
the nutriment they require. It was formerly supposed that
it was by the privation of oxygen, that the complete seclusion of
organic substances (as in the case of the preservation of meat, &c,
in air-tight tins) prevented their decomposition. But it is now
known that air may be freely admitted without giving rise to
decomposition, if it be effectually filter ed of its floating germs.
Thus it has been shown by Pasteur, that if milk be boiled in a
flask, of which the mouth is plugged with cotton- wool before the
boiling has ceased, the milk remains sweet for any length of time ;
whilst milk boiled in a similar flask left unplugged first turns sour
and then becomes putrescent within a few days, with abundant
development of Bacteria and Vibriones. And it has been further
shown by the same admirable experimenter, that if gun-cotton be
used as the plug, and after having been left for some time in the
flask be dissolved in ether, the sporules of Fungi which have been
filtered-out by the plug are found in the etherial solution, and will
then, if introduced into the flask, give rise to decomposition of its
contained liquid. Pasteur further varied the experiment by insert-
ing a tube of small bore, instead of a cotton-wool plug, into
the neck of the flask, and either drawing it to a fine point, or
simply turning it with its orifice downwards ; and though in each
case air had ready access to the liquid in the flask, yet no decom-
position took place, although it speedily ensued when free access
was opened, by "cutting short the tube near its insertion, for
its floating germs also.f
291. The intimate relation of Yibriones to Yeast-cells further ap-
* See the experiments of Mad. Liiders, loc. cit.
t The results of experiments of this class, which have been repeated over
and over again with the same results, appear to the Author far more conclusive
than those which depend on conditions which it is more difficult to secure. And
in regard to the latter he must express his unhesitating conviction that greater
confidence is to be placed in the researches of M. Pasteur, who has established
a reputation of the very highest character, by a life devoted to experimental
researches requiring the greatest skill and accuracy, than to those of Dr. Bastian
and other advocates of the origination of Organic Germs without progenitors,
in whose experiments it is by no means difficult to discover flaws that lead to
doubts of their trustworthiness. (See, for example, the criticism of certain of
Dr. Bastian's experiments by Messrs. Pode and Lankester, in " Proceed, of
Pioyal Society," June 19th, 1873 ; and the important observations of Messrs.
Dallinger and Drysdale on the development of Infusoria, and on the survival of
their germs after exposure to a dry heat much above that of boiling water, of
which a notice will be given hereafter, §§ 396, 397.)
382 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA.
pears from the experiments of Mad. Liiders (loc. cit) ; who found
that if the vibriones of a putrescent fluid were introduced into a
saccharine solution kept at blood-heat, Torula would appear in the
course of forty-eight hours ; whilst vice versa the introduction of
Yeast-cells into a putrescible animal fluid would speedily give rise
to a plentiful development of vibriones. It is further pointed out
by Professor Hensen, in his commentary on these experiments, that
all recorded observations on the subject indicate that the production
of Vibriones, the formation of Yeast-cells, and the germination of
Fungi, never proceed at one and the same time in the same liquid,
but are always successive ; one form disappearing, while another
takes its place, as if the phase of development were determined by
the condition of the medium. — The subject is one not only of the
greatest scientific interest, but of the highest practical importance ;
and as many competent observers are now at work upon it, much
additional information may be looked-for ere long.
292. In connexion with the foregoing, it may be here appropriate
to notice the researches which have been recently made upon the
communicability of various special forms of Disease by minute
molecules to which the name of microzymes has been given ; though
there is at present no proof of their derivation from any form of
Fungoid Vegetation. It has been ascertained by careful micro-
scopical examination of the fluid of the Vaccine vesicle, that it is
charged with a multitude of minute granules not above 20o0q of
an inch in diameter ; and it has been further determined that these,
rather than the fluid in which they are suspended, are the active
agents in the production of a similar vesicle in the skin into which
they are inserted. This vesicle must contain hundreds or thousands
of ' microzymes' for every one originally introduced; and it is
obvious that their multiplication has so strong an analogy to that
of the yeast-cells, as to suggest the idea that they have a like power
of reproducing themselves. Similar observations have been made
upon glanders, sheep-pox, and cattle plague ; so that an animal
suffering under either of these terrible diseases is a focus of in-
fection to others, for precisely the same reason that a tub of fer-
menting beer is capable of propagating its fermentation to fresh
wort. A most notable instance of such propagation is afforded by
the spread of the disease termed ' pebrine' among the Silkworms
of the south of France ; the mortality caused by it being estimated
to produce a money-loss of from three to four millions sterling
annually, for several years following 1853, when it first broke out
with violence. It has been shown by microscopic investigation,
that in silkworms strongly affected with this disease, every tissue
and organ in the body is swarming with minute cylindrical cor-
puscles about K-fioo of an inch long ; and that these even pass into
the undeveloped eggs of the female moth, so that the disease is
hereditarily transmitted. And it has been further ascertained by
the researches of Pasteur, that these corpuscles are the active agents
in the production of the disease, which is engendered in healthy
FUNGI INHABITING BODIES OF ANIMALS.
383
silkworms by their reception into their bodies, whilst, if due pre-
cautions be taken against their transmission, the malady may be
completely exterminated.
293. iSTot only are many of the simpler forms of Fungi inhabi-
tants of the interior of the bodies of Animals, but some are
only known as living in these situations. Among these may first
be mentioned the So
Fig. 179.
#3^3
BnSsS
3
Sarcina ventriculi.
)Circina
ventriculi (Fig. 179), which
is most frequently found
in the matters vomited by
persons suffering under
disorder of the Stomach,
but has also been met with
in other diseased parts of
the body. The Plant has
been detected in the con-
tents of the stomach, how-
ever, under circumstances
which seem to indicate
that it is not an uncommon
tenant of that organ even
in health, and that it may
accumulate there to a con-
siderable amount without
producing any inconveni-
ence ; it seems probable,
therefore, that its presence in disease is rather to be considered as
favoured by the changed state of the fluids which the disease
induces (either an acid or a f ermentible state of the contents of the
stomach having been generally found to exist in the cases in which
the plant has been most abundant), than to be itself the occasion
of the disease, as some have supposed. The Sarcina presents
itself in the form of clusters of adherent cells arranged in squares,
each square containing from 4 to 64, and the number of cells being
obviously multiplied by duplicative subdivision in directions trans-
verse to each other. In fact, its general mode of growth would
indicate a near relation to Gonium, one of the Yolvocineas, which
presents itself in similar quadripartite aggregations ; and many
Botanists, looking to this circumstance, and to the residence of the
plant in liquid, regard it as belonging to the group of Algas. It
agrees with the Fungi, however, in not living elsewhere than in
liquids containing organic matter ; and there can be little doubt
that as no fructification has yet been seen in it, only its earlier and
simpler condition is yet known to us. Its true place cannot be de-
termined until its whole life-history shall have been followed out.
294. A form of Fungous vegetation is prone to develope itself
within the living body, which is of great economic importance as
well as of scientific interest; this is the Botrytis bassiana (Fig.
180), a kind of ' mould,' the growth of which is the real source of
384 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA.
tlie disease termed muscardine, "which formerly carried off Silk-
worms in large numbers, just when they were about to enter
the chrysalis state, to the great injury of their breeders. The
plant presents itself under a considerable variety of forms (a-f),
Fig. 180.
Botrytis bassiana: — A, the fungus as it first appears at the
orifices of the stigmata ; B, tubular filaments bearing short
branches, as seen two days afterwards ; E, magnified view of
the same ; c, D, appearance of filaments on the fourth and
sixth days ; F, masses of mature spores falling off the branches,
with filaments proceeding from them.
all of which, however, are of extremely simple structure, consisting
of elongated or rounded cells, connected in necklace-like filaments.
FUNGI INHABITING BODIES OF ANIMALS. 385
very nearly as in the ordinary ' bead-moulds.' The spornles of
this Fungus, floating in the air, enter the breathing-pores (Fig. 372)
which open into the tracheal system of the Silkworm : they first
develope themselves within the air-tubes, which are soon blocked
up by their growth ; and they then extend themselves through the
fatty mass beneath the skin, occasioning the destruction of this
tissue, which is very important as a reservoir of nutriment to the
animal when it is about to pass into a state of complete inactivity.
The disease invariably occasions the death of the worm which it
attacks ; but it seldom shows itself externally until afterwards,
when it rapidly shoots forth from beneath the skin, especially at
the junction of the rings of the body. Although it spontaneously
attacks only the larva, yet it may be communicated by inoculation
to the chrysalis and the moth, as well as to the worm ; and it has
also been observed to attack other Lepidopterous Insects. A care-
ful investigation of the circumstances which favour the develop-
ment of this disease was made by Audouin, who first discovered
its real nature ; and he showed that its spread was favoured by
the overcrowding of the worms in the breeding establishments,
and particularly by the practice of throwing the bodies of such as
died into a heap in the immediate neighbourhood of the living
worms : this heap speedily became covered with this kind of
'mould,' which found upon it a most congenial soil; and it kept
up a continual supply of sporules, which, being diffused through
the atmosphere of the neighbourhood, were drawn into the breath-
ing pores of individuals previously healthy. The precautions
obviously suggested by the knowledge of the nature of the disease,
thus afforded by the Microscope, having been duly put in force, its
extension was kept within comparatively limited bounds.
295. An example of the like kind is frequently presented in the
destruction of the common house-fly by a minute Fungus termed
Empusa musci. In its fully developed condition, the spore-
bearing filaments of this plant stand out from the body of the fly
like the " pile" of velvet ; and the spores thrown off from these in
all directions form a white circle round it as it rests motionless on
a window-pane. The filaments which show themselves externally
are the fructification of the fungus which occupies the interior of
the Fly's body ; and this originates in minute corpuscles which
find their way into the circulating fluid from without. A health}-
fly shut up with a diseased one takes the disease from it by the de-
posit of a sporule on some part of its surface ; for this, beginning
to germinate, sends out a process which finds its way into the
interior, either through the breathing-pores, or between the rings
of the body ; and having reached the interior cavities, it gives off
the minute corpuscles which constitute the earliest stage of the
Empusa. Again, it is not at all uncommon in the West Indies, to
see individuals of a species of Polistes (the representative of the
"Wasp of our own country) flying about with plants of their own
length projecting from some part of their surface, the germs
c c
386 MICKOSCOPIC STEUCTUEE OF HIGHER CEYPTOGAMIA.
of which have been probably introduced (as in the preceding case)
through the breathing-pores at their sides, and have taken root in
their substance, so as to produce a luxuriant vegetation. In time,
however, this fungous growth spreads through the body, and
destroys the life of the insect ; it then seems to grow more
rapidly, the decomposing tissue of the dead body being still more
adapted than the living structure to afford it nutriment. — A
similar growth of different species of the genus Sjphceria takes place
in the bodies of certain Caterpillars in New Zealand, Australia,
and China ; and being thus completely pervaded by a dense sub-
stance, which, when dried, has almost the solidity of wood, these
caterpillars come to present the appearance of twigs, with long
F 1R1 slender stalks that are formed
by the projection of the fungus
itself. The Chinese species is
valued as a medicinal drug.
296. The stomachs and in-
testines of many Worms and
Insects are infested with para-
sitic Fungi, which grow there
with great luxuriance. In the
accompanying two illustrations
(Figs. 181, 182) are shown some
of the forms of the Enterobryus,*
which has been found by Dr.
Leidyf to be so constantly pre-
sent in the stomach of certain
species of lulus (gaily- worm),
Growth of Enterobryus spiralis from that it is extremely rare to meet
mucous membrane of stomach of lulus .— ^^ individuals whose sto-
a, epithelial cells of mucous membrane ;
machs do not contain it. The En-
6, spiral thallus of Enterobryus: c, J
primary cell ; d, secondary cell. terobryus originally consists _ ot
a single long tubular cell, which
sometimes grows in a spiral mode (Fig. 181), sometimes straight and
tapering (Fig. 182, a). In its young state the cell. contains a trans-
parent protoplasm, with granules and globules of various sizes ;
but in its more advanced condition the tube of the filament is
occupied by cells in various stages of development ; these distend
the terminal part of the cell (Fig. 182, b), and press so much
against each other that their walls become flattened; whilst
nearer the middle of the same filament (c) we find them retaining
their rounded form, and merely lying in contact with each other ;
and at the base (d), they lie detached in the midst of the granular
* This plant, also, has much affinity to Algse in its general type of structure,
and is referred to that group by many botanists ; but the conditions of its
growth, as. in the case of &ra'wa, seem rather to indicate its affinity to the
Eungi ; and until its proper fructification shall Lave been made out, its true
place in the scale must be considered as undetermined.
t "Smithsonian Contributions to Knowledge," Vol. v.
FUNGI INHABITING BODIES OF ANIMALS.
387
protoplasm. In E. spiralis the primary cells (Fig. 181, b, c) very
commonly have secondary and even ternary cells (d) developed
at their extremities ; but this is rarely seen in E. attenuatus
(Fig. 182). It may be considered as next to certain that the
tubular filaments rupture, when the contained cells have arrived at
maturity, and give them exit ; and that these cells are developed,
under favourable circumstances, into tubular filaments like those
from which they sprang; but the process has not yet been thoroughly
made out. This is obviously not the true Generation of the plant,
but is analogous to the development of zoospores in. Aehly a (§271).
Fig. 182.
Structure of Enterobryus : — A, growth of E. attenuatus, from
mucous membrane of stomach of Passulus ; B. dilated extre-
mity of primary cell of E. elegans, filled with secondary cells,
which, near its termination, become mutually flattened by
pressure ; c, lower portion of the same filament, containing
cells mingled with granules ; D, base of the same filament,
containing globules interspersed among granules.
It is not a little curious, moreover, that the Entozoa or parasitic
"Worms infesting the alimentary canal of these animals should be
frequently clothed externally with an abundant growth of such
plants: in one instance Dr. Leidy found an Ascaris bearing
twenty-three filaments of Enterobnjus, " which appeared to cause
no inconvenience to the animal, as it moved and wriggled about
with all the ordinary activity of the species." The presence of
this kind of Yegetation seems to be related to the peculiar food of
the animals in whose stomachs it is found ; for Dr. Leidy could
not discover a trace of these or of any other parasitic plants in the
alimentary canal of the carnivorous Myriapods which he examined ;
whilst he met with a constant and most extraordinary profusion
c c 2
388 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA.
of vegetation (Fig. 183) in the stomach of a herbivorous Beetle,
the Passulus comutus, which lives, like the Iuli, in stumps of old
trees, and feeds as they do on decaying wood. Of this vegetation
some parts present themselves in tolerably definite forms, which
Fig. 183.
Fungoid Vegetation, clothing membrane of Stomach of Passuhts, inter-
mingled with brush-like hairs.
have been described under various names ; whilst other portions
have the indefiniteness of imperfectly-developed organisms, and
can scarcely be characterized in the present state of our knowledge
of them. With regard to several forms, indeed, Dr. Leidy expresses
a doubt whether they are vegetable parasites, or outgrowths of
the membrane itself.
297. There are various diseased conditions of the Human Skin
and Mucous membranes, in which there is a combination of fun-
goid Yegetation and morbid growth of the Animal tissues : this is
the case, for example, with the Tinea favosa, a disease of the
scalp, in which yellow crusts are formed that consist almost entirely
of the mycelium, receptacles, and sporules of a fungus ; and the like
is true also of those white patches (Aphthce) on the lining membrane
of the mouth of infants, which are known as Thrush, and of the
exudations of 'false membrane' in the disease termed Diphtheria*
* Nearly allied to these is the form of Vegetation observed on many
FUNGI INHABITING BODIES OF ANIMALS.
389
In these and similar cases, two opinions are entertained as to the
relation of the Fungi to the Diseases in which they present them-
selves ; some maintaining that their presence is the essential con-
dition of these diseases, which originate in the introduction of the
vegetable germs ; and others considering their presence to be
secondary to some morbid alteration of the parts wherein the
fungi appear, which alteration favours their development. The
first of these doctrines derives a strong support from the fact, that
the diseases in question may be communicated to healthy indi-
viduals, through the introduction of the germs of the Fungi by
inoculation ; whilst the second is rather consistent with general
analogy, and especially with what is known of the conditions under
which the various kinds of fungoid ' blights' develope themselves
in or upon growing Plants (§ 300). — It is not a little remarkable
that even Shells, Fish-scales, and other hard tissues of Animals,
are not unfrequently penetrated by fungous Vegetation, which
Fig. 185.
Fig. 184.
BEE
Shell of Anomia penetrated by
Parasitic Fungus.
Stysanus caput-meduscB.
usually presents itself in the form of simple tubes more or less
regularly disposed (Fig. 184), and closely resembling those of an
ordinary mijcelium (compare Fig. 188, a), but occasionally exhibits
a distinct fructification that enables its true character to be
recognised*
specimens of imported Hair, which has been wrongly described as a Gregarini-
form parasite. See Dr. Tilbury Fox in " Science Gossip," May, 1867.
* See Professor Kolliker ' On the frequent Occurrence of Vegetable Para-
sites in the Hard Tissues of Animals,' in " Quart. Journ. of Microsc. Science"
Vol. viii., 1860, p. 171. — Previously to the publication of his friend Professor
K.'s paper, the Author had himself arrived at a similar conclusion in regard to
the parasitic nature of many of the Tubular structures which had been origi-
nally regarded not merely by himself, but by Prof. Kolliker, as proper to the
Shells in which they occur.
390 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA.
298. There are scarcely any Microscopic objects more beautiful
than some of those forms of ' mould ' or ' mildew,' which are
commonly found growing upon the surface of jams and other pre-
serves ; especially when they are viewed with a low magnifying
power, by reflected light. For they present themselves as a forest
of stems and branches, of extremely varied and elegant forms
(Fig. 185), loaded with fruit of a singular delicacy of conformation,
all glistening brightly on a dark ground. In removing a portion
of the ' mould ' from the surface whereon it grows, for the purpose
of microscopic examination, it is desirable to disturb it no more than
can be helped, in order that it may be seen as nearly as possible in
its natural condition ; and it is therefore preferable to take up a
portion of the membrane-like substance whereon it usually rests,
which is, in fact, a mycelium composed of interlacing filaments of
the vegetative part of the plant, the stems and branches being its
reproductive portion (§ 303). The universality of the appearance
of these simple forms of Fungi upon all spots favourable to their
development, has given rise to the belief that they are spontaneously
produced by decaying substances ; but there is no occasion for this
mode of accounting for it ; since the extraordinary means adopted
by Nature for the production and diffusion of the germs of these
plants adequately suffices to explain the facts of the case. The
number of sporules which any one Fungus may develope is almost
incalculable ; a single individual of the puff-ball tribe has been
computed to send forth no fewer than ten millions. And their
minuteness is such that they are scattered through the air in the
condition of the finest possible dust; so that it is difficult to conceive
of a place from which they should be excluded. This universal dif-
fusion was clearly proved several years ago by an experiment made
by Dr. Brittan of Bristol ; who caused air to be pumped for several
hours together through an inverted siphon, the bend of which was
immersed in a freezing mixture, so as to condense the aqueous
vapour of the atmosphere. This water at last came to be tinged of
a deep brown hue ; and was found, when microscopically examined,
to be charged with multitudes of sporules of Fungi. More recently,
Prof. Tyndall has shown, by a peculiar application of electric light,
that all ordinary air has suspended in it a multitude of excessively
minute solid particles ; that these, being for the most part destructi-
ble by heat, are chiefly organic ; and that they may be strained
off, so as to render the filtered air "optically pure" by passing it
through cotton wool, thus according with the experiments of
Pasteur (§ 290).
299. This mode of explanation has received further confirmation
from the facts recently ascertained, in regard to the great number
of forms under which a single germ may develope itself. For it has
been ascertained with regard to the Fungi generally, that different
individuals of the very same species may not only develope them-
selves according to a great number of very dissimilar modes of
MYXOGASTEIC FUNGI. 391
growth, but that they may even bear very dissimilar types of
Fructification ; and further, that even the same individual may put
forth, at different periods of its life, those two kinds of fructifica-
tion— the Basidio-sporous, in which the spores are developed by
outgrowth from free points (basidia), and the Thecasporous, in
which they are developed in the interior of cases (thecce or asci,
Fig. 186) — which had been previously considered as separately
characterizing the two principal groups into which the Class is
primarily divided.
300. A very curious set of phenomena to which attention was
first called by Prof, de Bary, is presented by certain members of
the group of Myxogastric Fungi, which are parasitic upon decaying
wood, bark, heaps of decaying leaves, tan beds, &c. ; the JEthaliwm
septicum, to which his observations specially relate, being very
common in the last-named situation. When the spores of this plant
are placed in water, and are protected from evaporation, their ex-
ternal envelopes rupture, and their contents escape in the condition
of cells invested only by a very thin primordial utricle ; each of
which comes to possess, after several changes of form, one or two
cilia, by which it executes movements of progression and rotation,
and two or three vacuoles, of which one at least always pulsates.
After a few days these lose their cilia, acquire a larger size with
more numerous and less regular vacuoles, and move in a creeping
manner by the protrusion of parts of the body, which continually
changes its form ; thus resembling an Amoeba (Fig. 252) in all
essential particulars. The next stage consists in the enormous
extension of contractile protoplasmic threads, which form a sort of
mycelium that eventually gives origin to the fructification; whether
each of these groups of threads — which bears a strong resemblance,
except in its far larger size, to the sarcodic network put forth by
Rhizopods (Fig. 250) — originates in a single amcebiform body,
or is formed by the coalescence of several, is not yet certainly
ascertained. Now this protoplasmic substance is found to contain
foreign particles, such as cells of Alga3, sporules of Fungi, &c, in
its interior ; and it was originally urged by De Bary that the
particles thus taken-in serve, as in the case of the Rhizopods, for
food, and that the Myxogastres, in this stage of their existence, are
to be accounted Animals, and may claim the designation Mycetozoa.
There is no sufficient evidence, however, that such is their true
character ; and taking for granted the general truthfulness of the
account just given, all that it can be fairly considered to prove is,
that the actively-moving Animalcule -like " zoospore " which is the
first production of the spore, undergoes a change in its condition
similar to that already described in the cells of Volvox (§ 217), and
that the protoplasmic substance of the amoeboid body thus formed
extends itself into diverging threads in a manner that strongly
reminds us of the sarcodic network of the Rhizopods. That such
a resemblance should exist can scarcely be considered surprising,
when it is borne in mind that the Vegetable protoplasm and the
392 MICKOSCOPIC STEUCTUEE OF HIGHEE CEYPTOGAMIA.
Fig. 1SG.
Animal sarcode are essentially identical substances ; and that not
merely the network of inosculating threads of Gromia (Fig. 250),
bnt the circulation of particles constantly kept np in it, has its
parallel in the network of viscid protoplasm which may be traced
on the internal wall of many Yegetable cells (§§ 279,322), and which
exhibits the like continual movement of its constituent particles.
Thus, then, it may be considered that the observations of De Bary
tend to confirm those of Drs. Hartig and Hicks (p. 369, note) in
regard to the amoeboid form which may be assumed by certain un-
doubtedly Yegetable products ; whilst if themselves interpreted by
the light of those phenomena, and by the undoubtedly Fungous
nature of the fructification of the Myxogastres, they indicate nothing
more than that the tribe in question affords a most remarkable ex-
ample of the same metamorphosis.*
301. The Entophytic Fungi which infest some of the Vegetables
most important to Man as furnishing his staple articles of food,
constitute a group of special interest to the Microscopist ; of which
a few of the chief examples may
here be noticed. The Mildew which
is often found attacking the straw
of Wheat, shows itself externally in
the form of circular clusters of
pear-shaped spore-cases (Fig. 186),
each containing two compartments
filled with sporules ; these (known
as the Puccinia graminis) arise
from a filamentous tissue consti-
tuting its mycelium, the threads of
which interweave themselves with
the tissue of the straw; and they
generally make their way to the
surface through the ' stomata '
or breathing-pores of its epidermis.
The Bust, which makes its appear-
ance on the leaves and chaff -scales
of Wheat, has a fructification that
seems essentially distinct from that
just described, consisting of oval
spore-cases, which grow without
any regularity of arrangement from
the threads of the mycelium; and
hence it has been considered to belong to a different genus and
species, Uredo rubigo. But from the observations of Prof. Henslow,
it seems certain that 'rust' is only an earlier form of 'mildew;'
the one form being capable of development into the other, and the
fructification characteristic of the two supposed genera having been
* Dr. De Bary's latest views on this subject, which are in accordance with
what is stated above, will be found in his contribution to Prof. Hofmeister's
"Handbuch der Physiologischen Botanik," Band ii. p. 295.
Puccinia graminis, or Mildew.
WHEAT-BLIGHTS. — POTATO -DISEASE. — VINE-DISEASE. 393
evolved on one and the same individual. Another repnted species of
Uredo (the JJ. segetum) it is which, when it attacks the flower of the
Wheat, reducing the ears to black masses of sooty powder, is known
as Smut or Dust-brand. The Corn-grains are entirely replaced by
aggregations of spores ; and these, being of extreme minuteness,
are very easily and very extensively diffused. The Bunt or Stinking
Rtist is another species of Uredo (the U. fcetida), which is chiefly
distinguished by its disgusting odour.
302. The prevalence of these Blights to any considerable extent
seems generally traceable to some seasonal influences unfavourable
to the healthy development of the Wheat-plant ; but they often
make their appearance in particular localities through careless
cultivation, or want of due precaution in the selection of seed. It
may be considered as certain that an admixture of the spores of
any of these Fungi with the grains will endanger the plant raised
from them ; but it is equally certain that the fungi have little
tendency to develope themselves in plants that are vegetating with
perfect healthfulness. The wide prevalence of such blights in bad
seasons is not difficult to account for, if it be true (as the observa-
tions of Mr. John Marshall several years since rendered probable)
that there are really very few wheat-grains near the points of which
one or two sporules of Fungi may not be found, entangled among
their minute hairs ; and it may be fairly surmised that these germs
remain dormant, unless an unfavourable season should favour
their development by inducing an unhealthy condition of the wheat-
plant. The same general doctrine probably applies to the Botrytis,
which, from 1847 to the present time, has had a large share in the
production of the " Potato-disease;" and to the Oidium, which has
a like relation to the " Vine-disease " that was prevalent for some
years through the south of Europe. There seems no doubt that,
m the fully developed-disease, the Fungus is always present ; and
that its growth and multiplication have a large share in the increase
and extension of the disorder, just as the growth of the Yeast-plant
excites and accelerates fermentation, while its reproduction enables
this action to be indefinitely extended through its instrumentality.
But j ust as the Yeast-plant will not vegetate save in a f ermentible
fluid — that is, in a solution which, in addition to Sugar, contains some
decomposable Albuminous matter, — so does it seem probable, on
consideration of all the phenomena of the Potato- and Vine-diseases,
that neither the Botrytis of the one nor the Oidium of the other
will vegetate in perfect healthy plants ; but that a disordered con-
dition, induced either by forcing and therefore unnatural systems of
cultivation, or by unfavourable seasons, or by a combination of both,
is necessary as a ' predisposing ' condition. This condition, in the
case of the Potato-disease, is said by Prof. De Bary to consist in an
undue thinness of the cuticle, accompanied by excessive humidity ;
whereby the sporules of the fungus will germinate on the surface
of the plant, sending out proc*esses which penetrate to its interior,
though otherwise germinating only on cut surfaces.
394 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA.
303. In those lower forms of this Class to which our notice of it
has hitherto been chiefly restricted, there is not any very complete
separation between the Nutritive or vegetative and the Reproductive
portions ; every cell, as in the simplest Protophytes, being equally
concerned in both. But such a separation makes itself apparent
in the higher ; and this in a very curious mode. For the ostensible
a Fig. 187. b
• jEcidium tussilaginis : — A, portion of the plant magnified ;• B, section of one
of the conceptacles with its spores.
Fungi of almost every description (Fig. 187) consist, in fact, of
nothing else than the organs of fructification ; the nutritive ap-
paratus of these plants being composed of an indefinite mycelium,
which is a filamentous expansion (Fig. 188) composed of elongated
branching cells (a), interlacing amongst each other, but having no
Fig. 188.
Clavaria crispula : — a, portion of the mycelium magnified.
intimate connexion ; and this has such an indefiniteness of form,
and varies so little in the different tribes of Fungi, that no deter-
mination of species, genus, or even family, could be certainly made
from it alone. From the researches of Prof. Oersted upon Agaricus
HEPATIC^ OR LIVERWORTS*
395
variabilis, it appears that the true Generative process in the
Agarics and their allies is carried on in this mycelium; and that
what has hitherto been considered as their Fructification is really
a mass of gemma?, like the 'urns' of Mosses and the ' thecas' of
Ferns, which, as will be shown hereafter (§§ 310, 316), are products
of the sexual union which takes place in the earlier stages of the
existence of those plants. This, if confirmed, will prove a most
important discovery.*
30-i. The whole history of the development of the Fungi, and the
question of the relationship of its different forms to each other, is
one that most urgently calls for re-examination at the present time,
under theguidance of our recently-acquired knowledge, and with the
assistance of improved instruments of Microscopic investigation ;
and whilst there is a wide field for the labours of those who possess
instruments of but moderate capacity, there are several questions
which can only be worked out by means of the highest powers and
the most careful appliances which the practised Microscopist can
bring to bear upon them.f
305. The little group of Hejmticce or ' Liverworts,' which is inter-
mediate between Lichens and ordinary Mosses — rather agreeing
with the former in its general mode of growth, whilst approaching
the latter in its fructification — presents numerous objects of great
interest to the Microscopist ; and no species is richer in these than
the very common Marchantia polymoiyha, which may often be
found growing between the paving-stones of damp court-yards, but
which particularly luxuriates in the neighbourhood of springs or
waterfalls, where its lobed fronds are found covering extensive
surfaces of moist rock or soil,
adhering by the radical (root)
filaments which arise from their
lower surface. At the period
of fructification these fronds
send up stalks, which carry
at their summits either round
shield-like disks, or radiating
bodies that bear some resem-
blance to a wheel without its
tire (Fig. 189) : the former carry
the male organs, or antheridia ;
while the latter in the first in-
stance bear the female organs,
or arclieqoiiia, which after- -p, 1 * ,, , ±. 7 , .,-.
, .y -.' , ,, i ronrl of Marchantia polymorplia, with
wards give place to the spo- genimiparous Conceptacles, and lobed Ke-
rangia or spore-cases.J ceptacles bearing pistillidia.
* See "Quart. Journ. of Microsc. Science," Yol. viii., N.S. (1868), p. 18.
+ For an example of what has to be done in this direction, see the mag-
nificent work of MM. Tulasne, entitled "Selecta Fuugorum Carpologia,"
Paris, 1861.
% In some species, the same shields bear both sets of organs ; and in
MIOEOSOOPIC STEUOTUEE OF HIGHEE CEYPTOGAMIA.
306. The green surface of the frond of this Liverwort is seen
under a low magnifying power to be divided into minute diamond
shaped spaces (Fig. 190, a, a, a) bounded by raised bands (c, c) ;
every one of these spaces has in its centre a curious brownish-
coloured body (b, b), with an
Tig. 190. opening in its middle, which
allows a few small green
cells to be seen through it.
"When a thin vertical section
is made of the frond (b), it is
seen that each of the lozenge-
shaped divisions of its sur-
face corresponds with an air-
chamber in its interior, which
is bounded below by a floor
(a, a) of closely-set cells (from
whose under surface the ra-
dical filaments arise) ; at the
sides by walls (c, c) of similar
solid parenchyma, the pro-
jection of whose summits
forms the raised bands on
the surface ; and above by a
cuticle (b, b) formed of a
single layer of cells ; whilst
its interior is occupied by a
very loosely arranged pa-
renchyma, composed of
branching rows of cells (/, f)
MarcliantiapolymorpTia .—a, portion of frond that seem to spring from the
seen from above ; «, o, lozenge-shaped divi-
sions ; b. b, stomata seen in the centre of the
lozenges ; c, c, greenish bands separating the
lozenges : — B, vertical section of the frond,
floor, — these cells being what
are seen from above, when
the observer looks down
showing a, a, the dense layer of cellular through the central aperture
tissue forming the floor of the cavity, d, d
the cuticular layer, b, &, forming its roof;
c, c, its walls ; /, /, loose cells in its interior ;
g, stoma divided perpendicularly ; h, rings of
cells forming its wall ; i, cells forming the
obturator-ring.
just mentioned. If the verti-
cal section should happen to
traverse one of the peculiar
bodies which occupies the
centres of the divisions, it
will bring into view a struc-
ture of remarkable complexity. Each of these stomata (as they are
termed, from the Greek a-Top.a, mouth) forms a sort of shaft (g), com-
posed of four or five rings (like the ' courses ' of bricks in a chimney)
placed one upon the other (h), every ring being made up of four or
five cells ; and the lowest of these rings (i) appears to regulate the
aperture, by the contraction or expansion of the cells which compose
Marchantia androgyna we find the upper surface of one half of the pelta
developing antheridia, whilst the under surface of the other half bears arche-
gonia.
GEMMIPAEOUS CONCEPTAOLES OF MAECHANTIA. 397
Fig. 191.
it, and it is hence termed the ' obturator-ring.' In this manner
each of the air chambers of the frond is brought into communica-
tion with the external atmosphere, the degree of that communica-
tion being regulated by the limitation of the aperture. We shall
hereafter find (§ 353) that the leaves of the higher Plants contain
intercellular spaces, which also communicate with the exterior by
stomata ; but that the structure of these organs is far less complex
in them than it is in this humble Liverwort.
307. The frond of Mcurchantia usually bears upon its surface, as
shown in Fig. 189, a number of little open basket-shaped con-
ceptacles (Fig. 191), which may
often be found in all stages of
development, and are structures
of singular beauty. They con-
tain, when mature, a number of
little green round or oblong
disks, each composed of two
or more layers of cells ; and
their wall is surmounted by a
glistening fringe of ' teeth,'
whose edges are themselves
regularly fringed with minute
out-growths. This fringe is at
first formed by the splitting-up
of the epidermis, as seen at b,
at the time when the concep-
tacle and its contents are first
making their way above the
surface. The little disks (some-
times termed ' bulbels,' from
their analogy to the bulbels or
detached buds of Flowering
Plants) are at first evolved as
single globular cells, supported
upon other cells which form
their footstalks; these single cells
gradually undergo multiplica-
tion by duplicative subdivision,
until they evolve themselves into
the disks ; and these disks, when Gemmiparous Concepta c les of Marchan-
mature, spontaneously detach tia polymorpha .— a, cone eptacle fully ex-
themselves from their footstalks, panded, rising from the surface of the
and lie free within the cavity of frond a> a' and containin S disks already
trip ronppntarlp Mod- porn detached :—B, first appearance of concep-
conceptacie. Most com- tacleonthe surfaceof the frond, showing
monly they are at last washed the formation of its fringe by the splitting
out by rain, and are thus of the cuticle,
carried to different parts of
the neighbouring soil, on which they grow very rapidly when well
supplied with moisture ; sometimes, however, they may be found
^^sSMmM^^
MICROSCOPIC STEUCTUEE OF HIGHER CEYPTOGAMIA.
growing whilst still contained within the conceptacles, forming
natural grafts (so to speak) npon the stock from which they
have been developed and detached; and many of the irregular
lobes which the frond of the Marchantia puts forth, seem to have
this origin. The very cnrions observation was long ago made
by Mirbel, who carefully watched the development of these
gemmce, that stomata are formed on the side which happens to
be exposed to the light, and that root-fibres are put forth from
the lower side ; it being apparently a matter of indifference which
side of the little disk is at first turned upwards, since each has the
power of developing either stomata or root-fibres according to the
influence it receives. After the tendency to the formation of these
organs has once been given, however, by the -sufficiently prolonged
influence of light upon one side and of darkness and moisture on
the other, any attempt to alter it is found to be vain ; for if the
surfaces of the young fronds be then inverted, a twisting growth
soon restores them to their original aspect.
308. When this Plant vegetates in damp shady situations which
are favourable to the nutritive processes, it does not readily pro-
duce the true Fructification, which is to be looked for rather in
plants growing in more exposed places. Each of the stalked peltate
(shield-like) disks contains a number of flask-shaped cavities open-
ing upon its upper surface, which are brought into view by a
vertical section ; and in each of these cavities is lodged an
antheridium composed of a mass of ' sperm-cells,' within which are
developed antherozoids like those of Chara (§ 280), and surmounted
by a long neck that projects through the mouth of the flask- shaped
cavity. The wheel-like receptacles (Fig. 189), on the other hand,
bear on their under surface, at an early stage, concealed between
membranes that connect the origins
of the lobes with one another, a set
of archegonia, shaped like flasks
with elongated necks (Fig. 192) ;
each of these has in its interior a
' geim-cell,' to which a canal leads
down from the extremity of the
neck ; and there is every reason to
believe that, as in Ferns, the germ-
cell is fertilized by the penetration
of the antherozoids through this
canal until they reach it. Instead,
however, of at once evolving itself
into a new plant resembling its
parent, the fertilized germ-cell or
' embryo-cell' developes itself into a
mass of cells enclosed within a cap-
sule, which is termed a sporangium ; and thus the mature receptacle,
in place of archegonia, bears capsules or sporangia, which finally
burst open and discharge their contents. These contents consist of
Fig. 192.
Archegonia of Marchantia poly-
morpha, in successive stages of de-
velopment.
ELATEES OF LIVEEWOETS. — MOSSES. 399
spores, which are isolated cells enclosed in firm yellow envelopes ;
and of elaters, which are ovoidal cells, each containing a double
spiral fibre coiled up in its interior. This fibre is so elastic that,
when the surrounding pressure is withdrawn by
the bursting of the- sporangium, the spires extend
themselves (Fig. 193), tearing apart the cell mem- |
brane ; and they do this so suddenly as to jerk ft
forth the spores which may be adherent to their
coils, and thus to assist in their dispersion. The
spores, when subjected to moisture, with a moderate
amount of light and warmth, develope themselves
into little collections of cells, which gradually assume
the form of a flattened frond ; and thus the species
is very extensively multiplied, every one of the
mass of spores, which is the product of a single
germ-cell, being capable of giving origin to an inde-
pendent individual.
309. The tribe of Mosses is as remarkable for
the delicacy and minuteness of all the plants com-
posing it, as other orders of the Vegetable King-
dom are for the majesty of their forms, the richness
of their foliage, or the splendour of their blossoms.
There is not one of thia little tribe whose external
organs do not serve as beautiful objects when viewed
with low powers of the Microscope ; while their
more concealed wonders are admirably fitted for the
detailed scrutiny of the practised observer. The
Mosses always possess a distinct axis of growth,
commonly more or less erect, on which the minute
and delicately-formed leaves are arranged with
great regularity. The stem shows some indication
of the separation of a cortical or bark-like portion $
from the medullary or pith-like, by the intervention tt
of a circle of bundles of elongated cells, which
seem to prefigure the woody portion of the stem jj
of higher plants, and from which prolongations
pass into the leaves, so as to afford them a sort of Eliter and Spores
midrib. The leaf usually consists of either a single 0f Marchantia.
or a double layer of cells, having flattened sides
by which they adhere one to another : they rarely present any distinct
epidermic layer ; but such a layer, perforated by stomata of
simple structure, is commonly found on the setce or bristle-like
footstalks bearing the fructification, and sometimes on the midribs
of the leaves. The root-fibres of Mosses, like those of Marchantia,
consist of long tubular cells of extreme transparence, within which
the protoplasm may frequently be seen to circulate, as in the
elongated cells of Chara ; and according to Dr. Hicks (" Quart.
Journ. Microsc. Science," 1ST.S., Yol. ii., 1862, p. 96), it is not un-
common for portions of the protoplasmic substance to pass into an
400 MICROSCOPIC STRUCTURE OF HIGHER CEYPTOGAMIA.
amoeboid condition resembling that of the gonidia of Yolvox
(§ 217). The protoplasm first detaches itself from contact with
the cell-wall, and collects itself into ovoid masses of various sizes ;
these gradually change their colour to red or reddish-brown, subse-
quently, however, becoming almost colourless ; and they protrude
and retract processes, exactly after the manner of Amcebce, occa-
sionally elongating themselves into an almost linear form, and
Fig. 191
Structure of Mosses: — A, Plant of Funaria hygrometrica,
showing/ the leaves, u the urns supported upon the setas or
footstalks s, closed by the operculum o, and covered by the
calyptra c : — B, Urns of Encalyptra vulgaris, one of them
closed and covered with the calyptra, the other open ; u, v,
the urns ; o, o, the op-reula ; c, calyptra ; p, peristome ; s, s,
setas : — c, longitudinal section of very young urn of Splach-
num; a, solid tissue forming the lower part of the capsule;
c, columella ; Z, loculus or space around it for the development
of the spores; e, epidermic layer of cells, thickened at the
top to form the operculum o ,• p, two intermediate layers, from
which the peristome will be formed ; s, inner layer of cells
forming the wall of the loculus.
travelling up and down in the interior of the tubular cells. This
kind of movement was observed by Dr. Hicks to subside gradually,
the masses of protoplasm then returning to their ovoid form ; but
their exterior subsequently became invested with minute cilia, by
which they were kept in constant agitation within their containing
cells. As to their subsequent history, we are at present entirely in
the dark ; and the verification and extension of Dr. Hicks's obser-
SEXUAL APPARATUS OF MOSSES.
401
Antheridia and Antherozoids of Polytriclium commune: — A,
group of antheridia, mingled with hairs and sterile filaments
(paraphyses) : of the three antheridia, the central one is in
the act of discharging its contents ; that on the left is not yet
mature ; while that on the right has already emptied itself, so
that the cellular structure of its walls becomes apparent; — B,
cellular contents of an antheridium, previously to the de-
velopment of the antherozoids ; — c, the same, showing the
first appearance of the antherozoids ; — D, the same, mature
and discharging the antherozoids.
D L
402 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA.
vations constitute an object well worthy of the attention of Micro-
scopists.
310. The chief interest of the Mosses to the Microscopist,
however, lies in their Fructification, which recent discoveries have
invested with a new character. What has commonly been regarded
in that light — namely, the Urn or Capsule filled with sporules,
which is borne at the top of a long footstalk that springs from the
centre of a cluster of leaves (Fig. 194, a) — is not the real fructifica-
tion, but its product ; for Mosses, like Liverworts, possess both
antheridia and pistilliclia, although these are by no means con-
spicuous. These organs are sometimes found in the same envelope
(or perigone), sometimes on different parts of the same plant,
sometimes only on different individuals ; but in either case they
are usually situated close to the axis, among the bases of the leaves.
The Antheridia are globular, oval, or elongated bodies (Fig. 195, a),
composed of aggregations of cells, of which the exterior form a sort
of capsule, whilst the interior are sperm-cells, each of which, as it
comes to maturity, developes within itself an antherozoid (b, c, d) ;
and the antherozoids, set free by the rupture of the cells within
which they are formed, make their escape by a passage that opens
for them at the summit of the antheridium. The antheridia are
generally surrounded by a cluster of hair-like filaments, composed
of cells joined together (Fig. 195, a), and called paraphyses ;
these seem to be ' sterile' or undeveloped antheridia. The Arche-
gonia bear a general resemblance to those of Marchantia (Fig. 189) ;
and there is every reason to believe that the fertilization of their
contained germ-cells is accomplished in the manner already
described. The fertilized ' embryo-cell' becomes gradually developed
by cell-division into a conical body elevated upon a stalk ; and this
at length tears across the walls of the flask-shaped archegonium by
a circular fissure, carrying the higher part upwards as a calyptra
or ' hood' (Fig. 194, b, c) upon its summit, while the lower part
remains to form a kind of collar round the base of the stalk.
311. The Urn or spore-capsule, which is thus the immediate
product of the generative act, and which must really be considered
as the offspring of the plant that bears it (although grafted-on to
it, and drawing its nourishment from it), is closed at its summit by
an operculum or lid (Fig. 194, b, o, o), which falls off when the
contents of the capsule are mature, so as to give them free exit ;
and the mouth thus laid open is surrounded by a beautiful toothed
fringe, which is termed the peristome. This fringe, as seen in its
original undisturbed position, is shown in Fig. 196, and is a beau-
ful object for the Binocular Microscope ; it is very hygrometric,
executing when breathed-on a curious movement, which is probably
concerned in the dispersion of the spores. In Figs. 197-199 are
shown three different forms of Peristome, spread out and detached,
illustrating the varieties which it exhibits in different genera of
Mosses, — varieties whose existence and readiness of recognition
render them characters of extreme value to the systematic Botanist,
PERISTOME ■ OF MOSSES.
403
whilst they furnish objects of great interest and beauty for the
Microscopist. The peristome seems always to be originally double,
one layer springing from the outer, and the other from the inner, of
two layers of cells which may be always distinguished in the imma-
Fig. 196.
Fig. 197.
Mouth of capsule of Funaria, showing the
Peristome in situ.
Double Peristome of
Fontinalis amtipyretica.
ture capsule (Fig. 194, c, p) ; but one or other of these is frequently
wanting at the time of maturity, and sometimes both are obliterated,
so that there is no peristome at all. The number of the ' teeth' is
always a multiple of 4, varying from 4 to o'4 : sometimes they are
prolonged into straight or twisted hairs. — The spores are contained
Fig. 199.
_ -Sffl
Double Peristome of Bryum
intermedium.
'V^MfWP^
Double Peristome of Cincliclium
arcticum.
404 MICROSCOPIC STRUCTURE .OF HIGHER CRYPTOGAMIA.
in the upper part of the capsule, where they are clustered round
a central pillar, which is termed the columella. In the young
capsule the whole mass is nearly solid (Fig. 194, c), the space (I)
in which the spores are developed being very small ; but this
gradually augments, the walls becoming more condensed ; and at
the time of maturity the interior of the capsule is almost entirely
occupied by the spores, in the dispersion of which the peristome
seems in some degree to answer the same purpose as the elaters of
Hepaticaa.
312. The development of the Spores into new plants commences
with the rupture of their outer walls and protrusion of their inner
coats ; and from the projecting extremity new cells are put forth
by a process of out-growth, which form a sort of Confervoid fila-
ment (as in Fig. 206, c). At certain points of this filament its com-
ponent cells multiply by subdivision, so as to form rounded clusters,
from every one of which an independent plant may arise ; so that
several individuals may be evolved from a single spore. A nume-
rous aggregation of spores may be developed, as we have seen, from
a single germ-cell : so that the immediate product of each act of
fertilization does not consist (as in the higher Plants) of a single
seed, that afterwards developes itself into a composite fabric,
whence are put forth a multitude of leaf-buds, every one of which
is capable (under favourable circumstances) of evolving itself into a
complete Plant ; but divides itself at once into a mass of isolated
cells (spores), of which every one may be considered in the light of
a bud or gemma of the simplest possible kind, and one of the first
acts of which is to put forth other buds, whereby the rapid exten-
sion of the Mosses is secured, although no separate individual ever
attains more than a very limited size.
313. The tribe of Sphagnacece, or Bog-Mosses, is now separated
by Muscologists from true Mosses, on account of the marked diffe-
rences by which they are distinguished ; the three groups, Hepa-
ticoe, BryaceoB (or ordinary Mosses), and Sphagnacece, being ranked
as together forming the Muscal Alliance. The stem of the Sphag-
nacece is more distinctly differentiated than that of the Bryaceoe
into the central or medullary, the outer or cortical, and the inter-
mediate or woody portions ; and a very rapid passage of fluid takes
place through its elongated cells, especially in the medullary and
cortical layers, so that if one of the plants be placed dry in a flask
of water, with its capitulum of leaves bent downwards, the water
will speedily drop from this until the flask is emptied. The leaf-
cells of the Sphagnacece exhibit a very curious departure from the
ordinary type ; for instead of being small and polygonal, they are
large and elongated (Fig. 200) ; they contain no chlorophyll, but
have spiral fibres loosely coiled in their interior ; and their mem-
branous walls have large rounded apertures, by which their cavities
freely communicate with one another, as is sometimes curiously
evidenced by the passage of Wheel- Animalcules that make their
SPHAGNACE.E OK BOG-MOSSES.
405
habitation in these chambers. Between these coarsely-spiral cells
are some thick-walled narrow elongated cells, containing chloro-
phyll ; these, which give to the leaf its firmness, do not, in the very
young leaf (as Mr. Huxley has pointed out) differ much in appear-
ance "from the others, the peculiarities of both being evolved by
a gradual process of differentiation.*
The male organs of Sphagnacece re-
semble those of Hepaticce, rather than
those of Mosses, in the form and
arrangement of the antheridia ; they
are grouped in catkins at the tips of
lateral branches, each of the imbri-
cated perigonal leaves enclosing a
single globose antheridium on a
slender footstalk ; and they are sur-
rounded by very long branched para-
physes of cobweb-like tenuity. The
archegonia, which do not differ in
structure from those of Mosses, are
grouped together in a sheath of deep
green leaves at the end of one of the
short lateral branchlets at the side of
the capitulum or summit-crown of
leaves. The Capsule, which is formed
as the product of impregnation, is very
uniform in all the species, being al-
most spherical, having a slightly showing the large cells, a, a, o,
convex lid, without beak or point, ^ith sPiral fibres and communica-
n-nd ^hnwino- "no traw of a ™»ri ting apertures ; and the mterve-
and showing no trace oi a _ peri- ni band ^ composed of
stome. The Spores contained in the small elongated cells.
sporangium, or spore-sac, are (like
those of the Lycopodiacece) of two kinds —macrospores, produced by
fours in a mother-cell, and tetrahedral in form ; and microspores,
which are more spherical, and of not half the size. When germi-
nating, they do not produce the branched confervoid filament of
true Mosses ; but, if growing on wet peat, evolve themselves into
a lobed foliaceous prothallium, resembling the frond of Liverworts ;
whilst, if they develope in water, a single long filament is formed, of
which the lower end gives off root-fibres, while the upper enlarges
into a nodule from which the young plant is evolved. In either case
the prothallium and its temporary roots wither away as soon as
the young plant begins to branch. From their extraordinary
power of imbibing and holding water, the Sphagnacece are of great
importance in the economy of Nature ; clothing with vegetation
many areas which would otherwise be sterile, and serving as
* See Mr. Huxley's very important Article on ' The Cell-Theory ' in the
"British and Foreign Medico-Chirurgical Eeview," Vol. xii. (Oct. 1853),
pp. 306, 3U7.
Portion of the leaf of Sphagnum;
406 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA.
reservoirs for storing up moisture for the use of higher forms of
vegetation.*
314. In the Ferns we have in many respects a near approxi-
mation to Flowering plants ; but this approximation does not
extend to their Reproductive apparatus, which is formed upon a
type essentially the same as that of Mosses, though evolved at a
very different period of life. As the tissues of which their fabrics
are composed are essentially the same as those to be described in
the next chapter, it will not be requisite here to dwell upon them.
The Stem (where it exists) is for the most part made up of cellular
parenchyma, which is separated into a cortical and a medullary
portion by the interposition of a circular series of fibro-vascular
bundles containing true Woody tissue and Ducts. These bundles
form a kind of irregular network, from which prolongations are
given off that pass into the leaf- stalks, and thence into the midrib
and its lateral branches ; and it is their peculiar arrangement in
•FlG 201 ^ne leaf-stalks, which gives to the
transverse section of these the figured
marking commonly known as " King
Charles in the oak." A thin section,
especially if somewhat oblique (Fig.
201), displays extremely well the pe-
culiar character of the ducts of the
Fern j; which are termed ' scalari-
ty~^l form,' from the resemblance of the
regular markings on their walls to
the rungs of a ladder.
315. What is usually considered the
fructification of the Ferns affords a
most beautiful and readily -prepared
class of opaque objects for the lowest
powers of the Microscope; nothing
more being necessary than to lay a
Oblique section of footstalk of fragment 0f the frond that bears it
Fern-leaf, showing bundle of Sea- ° ih -. Q+ao-p.-nla+p nr +0
lariform Ducts. ^P™ me glass btage-piate, or io
hold it m the btage-iorceps, and
to throw an adequate light upon it by the Side-condenser. It
usually presents itself in the form of isolated spots on the under
surface of the frond, termed sori, as in the common Polypodium
(Fig. 202), and in the Aspidium (Fig. 204) ; but sometimes these
' sori ' are elongated into bands, as in the common Scolopendrum
(hart's-tongue) : and these bands may coalesce with each other, so
as almost to cover the surface of the frond with a network, as in
Hoemionitis (Fig. 203) ; or they may form merely a single band
along its borders, as in the common Pteris (brake-fern). The sori are
sometimes ' naked ' on the under surface of the fronds ; but they
* See Dr. Braitlrwaite's Papers on the Sphagnacece in the " Monthly Micro-
scopical Journal," Vol. vi., etseq.
FRUCTIFICATION OF FERNS.
4<>;
Fig. 2C2.
are frequently covered with a delicate membrane termed the Inclu-
sium, which may either form a sort of cap npon the summit of each
sorus, as in Aspidium (Fig. 204), or a long fold, as in Scolopendrvni
and Pteris ; or a sort of cup, as in Deparia (Fig. 205). Each of
these sori, when sufficiently magnified, is
found to be made up of a multitude of Cap-
sules or thecce (Figs. 204, 205), which are
sometimes closely attached to the surface of
the frond, but more commonly spring from it
by a pedicle or footstalk. The wall of the
theca is composed of flattened cells, applied
to each other by their edges ; but there is
generally one row of these thicker and larger
than the rest, which springs from the pe-
dicle, and is continued over the summit of
the capsule, so as to form a projecting ring,
which is known as the annulus. This ring
has an elasticity superior to that of all the
rest of the capsular wall, causing it to split
across when mature, so that the contained
spores may escape ; and in many instances
carrying the two halves of the capsule widely
apart from each other (Fig. 205), the fissure
extending to such a depth as to separate
them completely. It will frequently happen
that specimens of Fem-fructification ga-
thered for the Microscope will be found to
have all the capsules burst and the spores
dispersed, whilst in others less advanced the
capsules may all be closed ; others, however,
may often be met with in which some of the
capsules are closed and others are open ;
and if these be watched with sufficient
attention, the rupture of some of the thecas and the dispersion of
the spores may be observed to take place whilst the specimen is
under observation in the field of the Microscope. In sori whose
capsules have all burst, the annuli connecting their two halves are
the most conspicuous objects, looking, when a strong light is
thrown upon them, like strongly-banded worms of a bright brown
hue. This is particularly the case in Scolopendrum, whose elon-
gated sori are remarkably beautiful objects for the Microscope in
all their stages; until quite mature, however, they need to be
brought into view by turning back the two indusial folds that
cover them. The commonest Ferns, indeed, which are found in
almost every hedge, furnish objects of no less beauty than those
yielded by the rarest exotics; and it is in every respect a most
valuable training to the young, to teach them how much there may
be found to interest, when looked for with intelligent eyes, even in
Leaflet of Polypodium,
with Sori.
408 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA.
the most familiar, and therefore disregarded, specimens of Nature's
handiwork.
Fig. 203.
I
Portion of Frond of Hamionitis, -with Sori.
Fig. 204.
Fig. 205.
Sorus and Indusium of Aspidium. Sorus and cup-shaped Indusium of
Deparia prolifera.
316. The Spores (Fig. 206, a), set free by the bursting of the
thecee, usually have a somewhat angular form, and are invested by
DEVELOPMENT AND EEPEODUCTION OF FEENS. 409
a yellowish or brownish, outer coat, which is marked very much in
the manner of pollen-grains (Fig. 248) with points, streaks, ridges,
or reticulations. When placed upon a damp surface, and exposed
to a sufficiency of light and warmth, the spore begins to ' germi-
nate,' the first indication of its vegetative activity being a slight
enlargement, which is manifested in the rounding-off of its angles;
this is followed by the putting forth of a tubular prolongation (b, a)
of the internal cell- wall through an aperture in the outer spore-coat ;
and by the absorption of moisture through this root-fibre, the inner
Fig. 206.
Development of Prothallium of Pteris serrulata : — A, Spore
set free from the theca ; — B, Spore beginning to germinate,
putting forth the tubular prolongation a from the principal
cell b ; — c, first-formed linear series of cells ; — D, Prothallium
taking the form of a leaf-like expansion ; a first, and 6 second
radical fibre ; c, d, the two lobes, and e the indentation be-
tween them ; /, f, first-formed part of the prothallium ; g,
external coat of the original spore ; /*, 7j, antheridia.
cell is so distended that it bursts the external unyielding integu-
ment, and soon begins to elongate itself in a direction opposite to
that of the root-fibre. A production of new cells by subdivision
then takes place from its growing extremity : this at first proceeds
in a single series, so as to form a kind of confervoid filament (c) ;
but the multiplication of cells by subdivision soon takes place trans-
versely as well as longitudinally, so that a flattened leaf-like
expansion (d) is produced, so closely resembling that of a young
Marchantia as to be readily mistaken for it. This expansion,
which is termed the prothallium, varies in its configuration in diffe-
410 MICROSCOPIC STRUCTURE OF HIGHEE CEYPTOCAMIA.
rent species; but its essential structure always remains the same.
From its under surface are developed not merely the root-fibres
(a, b), which serve at the same time to fix it in the soil and to supply
Development of the Antlieridia and Antherozoids of Pteris
serrulata: — a, projection of one of the cells of the Prothalliuni,
showing the antheridial cell, b, with its sperm-cells, e, within
the cavity of the original cell, a; — B, Antheridium com-
pletely developed ; a, wall of antheridial cell ; e, sperm-cells,
each enclosing an antherozoid ; — c, one of the Antherozoids
more highly magnified, showing a, its large extremity, b, its
small extremity, d, d, its cilia.
Fig. 208.
Archegonium of Pteris serrulata : — A, as seen from above ;
a, a, a, cells surrounding the base of the cavity ; 6, c, d, suc-
cessive layers of cells, tae highest enclosing a quadrangular
orifice : — B, side view, showing A, A, cavity containing the
germ-cell, a ; B, B, walls of the archegonium, made up of the
four layers of cells, b, c, d, e, and having an opening,/, on the
summit ; c, c, antherozoids within the cavity ; g, large ex-
tremity ; h, thread-like portion ; ?', small extremity in contact
with the germ-celL and dilated.
it with moisture, but also the antheridia and archegonia which
constitute the true representatives of the essential parts of the
SEXUAL APPAEATUS OF FEEXS. 411
Flower of higher Plants. Some of the anthericlia may be dis-
tinguished at an early period of the development of the prothal-
lium (h, h) ; and at the time of its complete evolution these bodies
are seen in considerable numbers, especially about the origins of the
root-fibres. Each has its origin in a peculiar protrusion that takes
place from one of the cells of the prothallium (Fig. 207, a, a) : this
is at first entirely filled -with chlorophyll-granules ; but soon a
peculiar free cell (b) is seen in its interior, filled with mucilage and
colourless granules. This cell gradually becomes filled with another
brood of young cells (e), and increases considerably in its dimen-
sions, so as to fill the projection which encloses it : this part of the
original cavity is now cut off from that of the cell of which it was
an offshoot, and the antheridium henceforth ranks as a distinct
and independent organ. Each of the sperm-cells (b, e) included
within the antheridial cell, is seen, as it approaches maturity,
to contain a spirally-coiled filament ; and when they have been set
free by the bursting of the antheridium, the sperm-cells themselves
burst, and give exit to their antherozoids (c), which execute rapid
movements of rotation on their axes, partly dependent on the six
long cilia with which they are furnished. — The archegonia are fewer
in number, and are found upon a different part of the prothallium.
Each of them at its origin presents itself only as a slight elevation
of the cellular layer .of the prothallium, within which is a large
intercellular space containing a peculiar cell (the germ-cell), and
opening externally by an orifice at the summit of the projection ;
but when fully developed (Fig. 208), it is composed of from ten to
twelve cells, built up in layers of four cells each, one upon another,
so as to form a kind of chimney or shaft, having a central passage
that leads down to the cavity at its base, wherein the germ-cell
(B, a) is contained. Into this cavity the antherozoids penetrate, so
as to come into contact with the germ-cell ; and, by the softening
of the membrane at its apex, they are even enabled to enter its
cavity, within which a minute ' embryonal corpuscle ' was pre-
viously distinguishable. This corpuscle, when fertilized by the
antherozoids which move actively round it, becomes the 'primordial
cell ' of a new plant, the development of which speedily commences.*
* See Hofmeister, in "Ann. of Nat. Hist.," 2nd Ser., Vol. xiv., p. 272,
and his Treatise on the Higher Cryptogainia, published by the Kay Society.
The study of the development of the spores of Ferns, and of the act of fer-
tilization and of its products, may be conveniently prosecuted as follows : — Let
a frond of a Fern whose fructification is mature be laid upon a piece of fine
paper, with its spore-bearing surface downwards ; in the course of a day or
two this paper will be found to be covered with a very fine brownish dust,
which consists of the discharged spores. This must be carefully collected, and
should be spread upon the surface of a smoothed fragment of porous sandstone,
the stone being placed in a saucer, the bottom of which is covered with water;
and a glass tumbler being inverted over it, the requisite supply of moisture is
ensured, and the spores will germinate luxuriantly. Some of the prothallia
soon advance beyond the rest; and at the time when the advanced ones have
long ceased to produce antheridia, and bear abundance of archegonia, those
412 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA.
By the usual process of binary subdivision a globular homogeneous
mass of cells is at first formed ; but rudiments of special organs
soon begin to make their appearance ; the embryo grows at the
expense of the nutriment prepared for it by the prothallium ; and
it soon bursts forth from the cavity of the archegonium, which
organ in the meantime is becoming atrophied. In the very begin-
ning of its development, the tendency is seen in the cells of one
extremity to grow upward, so as to evolve the stem and leaves,
and in those of the other extremity to grow downward to form the
root ; and when these organs have been sufficiently developed to
absorb and prepare the nutriment which the young Fern requires,
the prothallium, whose function as a ' nurse ' is now discharged,
decays away.
317. The little group of Equisetacece (Horsetails) which seem
nearly allied to the Ferns in the type of their generative apparatus,
though that of their vegetative portion is very different, affords
certain objects of considerable interest to the Microscopist. The
whole of their structure is penetrated to such an extraordinary
degree by silex, that, even when its organic portion has been
destroyed by prolonged maceration in dilute nitric acid, a consistent
skeleton still remains. This mineral, in fact, constitutes in some
species not less than 13 per cent, of the whole solid matter, and 50
per cent, of the inorganic ash ; and it especially abounds in the
Cuticle, which is used by cabinet-makers for smoothing the surface
of wood. Some of the siliceous particles are distributed in two
lines, parallel to the axis ; others, however, are grouped into oval
forms, connected with each other, like the jewels of a necklace, by
a chain of particles forming a sort of curvilinear quadrangle ; and
these (which are, in fact, the particles occupying the cells of the
stomata) are arranged in pairs. Their form and arrangement are
peculiarly well seen under Polarized light, for which the prepared
cuticle is an extremely beautiful object ; and it is asserted by Sir D.
Brewster (whose authority upon tlris point has been generally
followed), that each siliceous particle has a regular axis of double
which have remained behind in their growth are beginning to be covered
with antheridia. If the crop be now kept with little moisture for several
weeks, and then suddenly watered, a large number of antheridia and arche-
gonia simultaneously open ; and in a few hours afterwards, the surface of the
larger prothallia will be found almost covered with moving antherozoids. Such
prothallia as exhibit freshly-opened archegonia are now to be held by one lobe
between the forefinger and thumb of the left hand, so that the upper surface of
the prothallium lies upon the thumb ; and the thinnest possible sections are
then to be made with a thin narrow-bladed knife, perpendicularly to its sur-
face. Of these sections, which, after much practice, may be made no more
than l-15th of a line in thickness, some will probably lay open the canals of
the archegonia ; and within these, when examined with a power of 200 or
300 diameters, antherozoids may be occasionally distinguished. The pro-
thallium of the common Osmunda regalis will be found to afford peculiar
facilities for observation of the development of the antheridia, which
are produced at its margin. (See Rev. F. Howlett in " Intellectual Observer,"
Vol. vii. p. 32.)
FRUCTIFICATION OF EQUISETACE^.
413
refraction. According to Prof. Bailey, However, the effect of this
and similar objects (such as the cuticle of grasses) upon Polarized
light is not produced by the siliceous particles, but by the organized
tissues ; since when the latter have been entirely got rid of, the
residual silex shows no doubly-refracting power.* — What is usually
designated as the Fructification of the Equisetacese forms a cone
or spike at the extremity of certain of the stem-like branches (the
real stem being a horizontal rhizoma) ; and consists of a cluster of
shield-like disks, each of which carries a circle of thecce or spore-
cases, that open by longitudinal slits to set free the spores. Each
of the spores has, attached to it, two pairs of elastic filaments
(Fig. 209), that are originally formed as spiral fibres on the interior
Fig. 209.
Spores of Equisetum, -with their Elastic Filaments.
of the wall of the primary cell within which it is generated, and
are set free by its rupture ; these are at first coiled up around
the spore, in the manner represented at a, though more closely
applied to the surface ; but, on the liberation of the spore, they
extend themselves in the manner shown at b, — the slightest applica-
tion of moisture, however, serving to make them close together (the
assistance which they afford in the dispersion of the spores being
no longer required) when the spores have alighted on a damp
surface. If a number of these spores be spread out on a slip of glass
under the field of view, and, whilst the observer watches them, a
bystander breathes gently upon the glass, all the filaments will be
instantaneously put in motion, thus presenting an extremely curious
spectacle ; and will almost as suddenly return to their previous
condition when the effect of the moisture has passed off. If one
of the thecce which has opened, but not discharged its spores, be
mounted in a slide with a moveable cover (§ 171), this curious action
may be exhibited over and over again. These spores are to be
regarded in the same light as those of Ferns, namely, as gemmce
or rudimentary buds, not as seeds. They evolve themselves after
the like method into a prothallium ; and this developes antheridia
and archegonia, by the conjoint action of which an embryo is
produced.
* See " Silliinan's American Journal of Science,'1 May, 1856.
414 MICKOSCOPIC STEUCTUEE OF HIGHEE CEYPTOGAMIA.
318. In ascending, as we have now done, from the lower to the
higher Cryptogamia, we have seen a gradual change in the general
plan of structure ; so that the superior types present a close ap-
proximation to the Flowering Plant, which is undoubtedly the
highest form of vegetation. But we have everywhere encountered
a mode of Generation, which, whilst essentially the same through-
out the series, is essentially distinct from that of the Phanerogamia :
the fertilizing material of the ' sperm-cell' being embodied as it
were, in self-moving filaments, which find their way to the ' germ-
cells' by their own independent movements ; and the ' embryo-cell'
being destitute of that store of prepared nutriment, which sur-
rounds it in the true Seed, and supplies the material for its early
development. In the lower Cryptogamia, we have seen that the
' embryo-cell,' after fertilization, is thrown at once upon the world
(so to speak) to get its own living ; but in Liverworts, Mosses,
and Ferns, the embryo-cell is nurtured by the parent plant, for a
period that varies in each case according to the nature of the fabric
into which it evolves itself. While the true reproduction of the
species is effected by the proper Generative act, the multiplication
of the individual is accomplished by the production and dispersion
of Spores ; and this production, as we have seen, takes place at very
different periods of existence in the several groups, dividing the life
of each into two separate epochs, in which it presents itself under
two very distinct phases that contrast remarkably with each other.
Thus, the frond of the Marchantia, bearing its antheridia and
archegonia, is that which seems naturally to constitute the Plant ;
but that which represents this phase in the Ferns is the minute
Marchantia-like prothallium. On the other hand, the product into
which the fertilized ' embryo-cell' evolves itself in the Ferns, is that
which is commonly regarded as the Plant ; and this is represented
in the Liverworts and Mosses by the spore-capsule alone.* — We shall
hereafter encounter a similar diversity (which has received the in-
appropriate designation of ' alternation of generations') between
the two phases in the lives of Hydrozoa, as well as in other Inver-
tebrate Animals. In some of the Hydrozoa it is the zoophytic
structure which constitutes what is commonly regarded as the
Animal, the free-swimming medusoid buds by which that structure
is reproduced being inconspicuous : whilst in others it is the Medusa
or generative segment which attracts notice by its large dimensions,
the earlier polypoid stage being only recognised when carefully
sought for. (See Chap, xi.)
* For more detailed information on the Structure and Classification of the
Cryptogamia generally, the reader is referred to the Bev. M. J. Berkeley's
" Introduction to Cryptogainic Botany ;" while the most recent information on
the Beproduction of the Higher Cryptogamia will be found in Prof. Hofmeister's
Treatise on that subject, published by the Bay Society, and in his " Handbuch
der Physiologischen Botanik."
CHAPTER YIII.
OF THE MICROSCOPIC STRUCTURE OE PHANEROGAMIC PLANTS.
319. Elementary Tissues. — In passing from the Cryptogamic
division of the Vegetable Kingdom to that larger and more
ostensibly important province which includes the Flowering Plants,
we do not meet with so wide a departure from those simple types
of structure we have already considered, as the great differences in
their general aspect and external conformation might naturally lead
us to expect. For a very large proportion of the fabric of even the
most elaborately formed Tree is made up of components of the
very same kind with those which constitute the entire organisms of
the simplest Cryptogamia ; and that proportion always includes the
parts most actively concerned in the performance or the Vegetative
functions. For although the Stems, Branches, and Roots of trees
and shrubs are principally composed of woody tissue, such as we
do not meet with in any but the highest Cryptogamia, yet the
special office of this is to afford mechanical support : when it is
once formed, it takes no further share in the vital economy, than
to serve for the conveyance of fluid from the roots upwards through
the stem and branches, to the leaves ; and even in these organs,
not only the pith and the bark, with the ' medullary rays,' which
serve to connect them, but that ' cambium-layer" intervening
between the bark and the wood (§ 342) in which the periodical
formation of the new layers both of bark and wood takes place, are
composed of cellular substance. This tissue is found, in fact,
wherever groivth is taking place ; as, for example, in the spongioles
or growing-points of the root-fibres, in the leaf-buds and leaves,
and in the flower-buds and sexual parts of the flower : it is only
when these organs attain an advanced stage of development, that
woody structure is found in them, — its function (as in the stem)
being merely to give support to their softer textures ; and the
small proportion of their substance which it forms, being at once
seen in those beautiful skeletons, which, by a little skill and per-
severance, may be made of leaves, flowers, and certain fruits. All
the softer and more pulpy tissue of these organs is composed of
cells, more or less compactly aggregated together, and having forms
that approximate more or less closely to the globular or ovoidal,
which may be considered as their original type.
320. As a general rule, the rounded shap. is preserved only when
416 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
the cells are but loosely aggregated, as in the parenchymatous (or
pulpy) substance of leaves (Fig. 210), and it is then only that the
distinctness of their walls becomes evident. When the tissue
becomes more solid, the sides of the vesicles are pressed against
each other, so as to flatten them and to bring them into close
apposition ; and they then adhere to one another in such a manner
that the partitions appear, except when carefully examined, to be
single instead of double, as they really are. Frequently it happens
Fig. 210.
Section of Leaf of Agave, treated with dilute nitric acid,
showing the primordial utricle contracted in the interior of
the cells : — a, Epidermic cells ; 6, boundary-cells of the stoma ;
c, cells of parenchyma ; d, their primordial utricles.
that the pressure is exerted more in one direction than in another,
so that the form presented by the outline of the cell varies accord-
ing to the direction in which the section is made. This is well
shown in the pith of the young shoots of Elder, Lilac, or other
rapidly-growing trees ; the cells of which, when cut transversely,
generally exhibit circular outlines, whilst, when the section is made
vertically, their borders are straight, so as to make them appear
like cubes or elongated prisms, as in Fig. 211. A very good
example of such a cellular parenchyma is to be found in the sub-
stance known as Rice-paper ; which is made by cutting the herba-
ceous stem of a Chinese plant termed Ar alia papyrif era* vertically
round and round with a long sharp knife, so that its tissue may be
(as it were) unrolled in a sheet. The shape of the cells, as seen in
the ' rice-paper' thus prepared, is irregularly prismatic, as shown in
Fig. 211, b ; but if the stem be cut transversely, their outlines are
* The sEschynomene, which is sometimes named as the source of this article,
is an Indian plant employed for a similar purpose.
VARIOUS FOEMS OF CELLULAE TISSUE. 417
seen to be circular or nearly so (a). When, as often happens, the
cells have a very elongated form, this elongation is in the direction
of their growth, which is that, of course, wherein there is least
resistance. Hence their greatest length is nearly always in the
direction of the axis ; but there is one remarkable exception, — that,
namely, which is afforded by the ' medullary rays' of Exogenous
stems_ (§ 340), whose cells are greatly elongated in the horizontal
direction (Fig. 234, a), their growth being from the centre of the
stem towards its circumference. It is obvious that fluids will be
more readily transmitted in the direction of greatest elongation,
being that in which they will have to pass through the least number
A Fig. 211.
Sections of Cellular Parenchyma of Aralia, or Rice-paper
plant : — A, transversely to the axis of the stem ; B, in the
direction of the axis.
of partitions ; and whilst their ordinary course is in the direction
of the length of the Eoots, Stems, or Branches, they will be enabled
by means of the medullary rays to find their way in the transverse
direction. — One of the most curious varieties of form which
Vegetable cells present, is that represented in Fig. 212, which con-
stitutes the stellate cell. This modification, to which we have
already seen an approximation in Volvox (§ 214), is found in the
spongy parenchymatous substance where lightness is an object ;
as in the stems of many aquatic plants, the Rush for example,
which are furnished with air-spaces. In other instances these air-
spaces are large cavities which are altogether left void of tissue :
such is the case in the Nuphar lutea (yellow water-lily), the foot-
stalks of whose leaves contain large air-chambers, the walls of which
are built up of very regular cubical cells, whilst some curiously
formed large stellate cells project into the cavity which they
bound (Fig. 213). The dimensions of the component vesicles of
£ E
418 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
Cellular tissue are extremely variable ; for although their diameter
is very commonly between l-300th and 1 -500th of an inch, they
occasionally measure as much
Fig. 212. ag ]-30th of an inch across,
whilst in other instances they
are not more than 1 -3000th.
321. The component cells of
Cellular tissue are usually held
together by an intercellular
substance, which may be con-
sidered analogous to the ' ge-
latinous ' layer that intervenes
between the cells of the Algae
(§ 204). This, in an early
stage of their development, is
often very abundant, occupying
more space than the cells
themselves, as is seen in Fig. 214, a ; and the cell-cavities are not
separated from it by the interposition of a distinct membrane. As
the cells enlarge and increase by duplicative subdivision (b), the
intervening substance diminishes in relative amount ; and as the
cells advance towards their ma-
Section of Cellular parenchyma of Hush.
Fig. 213.
ture condition (c), it merely
shows itself as a thin layer
between them. There are many
forms of fully-developed cellular
parenchyma, in which, in conse-
quence of the loose aggregation
of their component cells, these
may be readily isolated, so as to
be prepared for separate exa-
mination without the use of re-
agents which alter their con-
dition : this is the case with
the pulp of ripe fruits, such
as the Strawberry or Currant
(the Snowberry is a particularly
favourable subject for this kind
of examination), and with the
parenchyma of many fleshy
leaves, such as those of the
Carnation (Dianthus caryo-
Cubical parenchyma, with stellate cells, ptyllus) or the London Pride
from petiole of Nuphar lutea. (Saxifraga crassifolia). Such
cells usually contain evident
nuclei, which are turned brownish-yellow by iodine, whilst their
membrane is only turned pale-yellow ; and in this way the
nucleus may be brought into view, when, as often happens, it
DEVELOPMENT OF CELLS.— CYCLOSIS.
419
is not previously distinguishable. If a drop of the iodized solu-
tion of chloride of zinc be subsequently added, the cell- membrane
becomes of a beautiful blue colour, whilst the nucleus and the
granular protoplasm that surrounds it retain their brownish-
yellow tint. The use of dilute nitric or sulphuric acid, of alcohol,
of syrup, or of several other reagents, serves to bring into view the
primordial utricle (§ 201) ; its contents being made to coagulate
and shrink, so that it detaches itself from the cellulose wall with
which it is ordinarily in contact, and shrivels-up within its cavity,
as shown in Fig. 210. It would be a mistake, however, to regard
this as a distinct membrane ; for it is nothing else than the peri-
pheral layer of protoplasm, naturally somewhat more dense than
that which it includes, like the ectosarc of Ehizopods (§ 369), but
deriving its special consistence from the operations of reagents.
Successive stages of Cell-f ormation in the development of the Leaves
of Anacharis alsinastrum .-—A, growing point of the branch, consisting
of a protoplasmic mass with young cells, the projections at its base
being the rudiments of leaves ; B, portion of one of these incipient
leaves in a more advanced condition ; c, the same in a still later stage
of development.
322. It is probable that all Cells, at some stage or other of their
growth, exhibit, in a greater or less degree of intensity, that
curious movement of cijclosis, which has been already described as
occurring in the Characece (§ 279), and which consists in the steady
flow of one or of several currents of protoplasm over the inner
wall of the cell ; this being rendered apparent by the movement of
the particles which the current carries along with it. The best
examples of it are found among submerged Plants, in the cells of
which it continues for a much longer period than it usually does
E E 2
420 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS
elsewhere ; and among these are two, the Vallisneria spiralis and
the Anacharis alsinastrum, which are peculiarly fitted for the exhi-
bition of this interesting phenomenon. — The Vallisneria is an
aquatic plant that grows abundantly in the rivers of the south of
Europe, but is not a native of this country ; it may, however, be
readily grown in a tall glass jar having at the bottom a couple
of inches of mould, which, after the roots have been inserted into
it, should be closely pressed down, the jar being then filled with
water, of which a portion should be occasionally changed.* The
jar should be freely exposed to light, and should be kept in as
warm but equable a temperature as possible. The long grass-like
leaves of this plant are too thick to allow the transmission of
sufficient light through them for the purpose of this observation ;
and it is requisite to make a thin slice or shaving with a sharp
knife. If this be taken from the surface, so that the section
chiefly consists of the superficial layer of cells, these will be
found to be small, and the particles of chlorophyll, though in
great abundance, will rarely be seen in motion. This layer should
therefore be sliced off (or, perhaps still better, scraped away) so as
to bring into view the deeper layer, which consists of larger cells,
some of them greatly elongated, with particles of chlorophyll in
smaller number, but carried along in active rotation by the current
of protoplasm ; and it will often be noticed that the rotation takes
place, in contiguous cells, in opposite directions. If the movement
(as is generally the case) be checked by the shock of the operation,
it will be revived again by gentle warmth ; and it may continue
under favourable circumstances, in the separated fragment, for a
period of weeks, or even of months. Hence, when it is desired to
exhibit the phenomenon, the preferable method is to prepare the
sections a little time before they are likely to be wanted, and to
carry them in a small vial of water in the waistcoat pocket, so that
they may receive the gentle and continuous warmth of the body.
In summer, when the plant is in its most vigorous state of growth,
the section may be taken from any one of the leaves; but in winter,
it is preferable to select those which are a little yellow. An Objec-
tive of l-4th inch focus will serve for the observation of this in-
teresting phenomenon, and very little more can be seen with a l-8th
inch ; but the l-25th inch constructed by Messrs. Powell
and Lealand enables the borders of the protoplasmic current, which
carries along the particles of chlorophyll, to be distinctly defined ;
and this beautiful phenomenon may be most luxuriously watched
under their patent Binocular (§ 67).
323. The Anacharis alsinastrum is a water-weed, which, having
*.Mr. Quekett found it the most convenient method of changing the water
in the jars in which Chara, Vallisneria, &c, are growing, to place them occa-
sionally under a water-tap, and allow a very gentle stream to fall into them for
some hours ; for by the prolonged overflow thus occasioned, all the impure
water, with the Conferva that is apt to grow on the sides of the vessel, may be
readily got rid of.
CYCLOSIS IN ANACHAPJS. 421
been accidentally introduced into this country several years ago,
lias since spread itself with such rapidity through our canals and
rivers, as in many instances seriously to impede their navigation.
It does not require to root itself in the bottom, but floats in any
part of the water it inhabits ; and it is so tenacious of life, that
even small fragments are sufficient for the origination of new
plants. The leaves have no distinct cuticle, but are for the most
part composed of two layers of cells, and these are elongated and
colourless in the centre, forming a kind of midrib ; towards the
margins of the leaves, however, there is but a single layer. Hence
no preparation whatever is required for the exhibition of this
interesting phenomenon ; all that is necessary being to take a leaf
from the stem (one of the older yellowish leaves being preferable),
and to place it with a drop of water either in the Aquatic-box or
on a slip of glass beneath a thin-glass cover. A higher magnifying
power is required, however, than that which suffices for the exa-
mination of the cyclosis in Chara or in Yallisneria ; the l-8th inch
Object-glass being here preferable to the l-4th, and the assistance
of the Achromatic Condenser being desirable. "With this amplifi-
cation, the phenomenon may be best studied in the single layer of
marginal cells ; although, when a lower power is used, it is most
evident in the elongated cells forming the central portion of the
leaf. The number of chlorophyll-granules in each cell varies from
three or four to upwards of fifty ; they are somewhat irregular in
shape, some being nearly circular flattened disks, whilst others are
oval ; and they are usually from 1 -3000th to 1- 5000th of an inch
in diameter. When the rotation is active, the greater number of
these granules travel round the margin of the cells, a few, however,
remaining fixed in the centre ; their rate of movement, though only
l-40th of an inch per minute, being sufficient to carry them several
times round the cell within that period. As in the case of the
Yallisneria, the motion may frequently be observed to take place
in opposite directions in contiguous cells. The thickness of the
layer of protoplasm in which the granules are carried round, is
estimated by Mr. Wenham at no more than 1 -20,000th of an inch.
WTien high powers and careful illumination are employed, delicate
ripples may be seen in the protoplasmic currents. It was affirmed
by Dr. Branson* that the elongated cells along the margin of the
leaf and forming the midrib contain a large quantity of silex ; the
evidence of this being furnished by the effect of Polarized light,
especially after the leaf has been boiled for a few minutes in equal
parts of nitric acid and water, which removes part of the organic
substance, and thus renders the siliceous portion more distinct,
without destroying the form of the leaf. But the observations of
Prof. Bailey upon the parallel case of the Eqidsetum (§ 317) throw
a doubt on the validity of this conclusion.
* See Dr. Branson, in " Quart. Journ. of Microsc. Science," Vol. iii. (1855),
p. 274 ; and Mr. "Wenham, in the same, Vol. iii. p. 277.
422 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
Fig. 215.
324. The phenomenon of
Cyclosis, however, is by no
means restricted to sub-
merged Plants ; for, it has
been witnessed by numerous
observers in so great a variety
of other species, that it may
fairly be presumed to be uni-
versal. It is especially ob-
servable in the hairs of the
Epidermic surface ; and ac-
cording to Mr. Wenham,*
who has given much attention
to this subject, " the diffi-
culty is to find the exceptions,
for hairs taken alike from the
loftiest Elm of the forest to
the humblest weed that we
trample beneath our feet,
plainly exhibit this circula-
tion." Such hairs are furnished
by various parts of Plants;
and what is chiefly necessary
is, that the part from which
the hair is gathered should be
in a state of vigorous growth.
The hairs should be detached
by tearing off, with a pair of
fine-pointed forceps, the por-
tion of the cuticle from which
they spring ; care being taken
not to grasp the hair itself,
whereby such an injury would
be done to it as to check the
movement within it. The hair
should then be placed with a
drop of water under thin glass ;
and it will generally be found
advantageous to use a l-8th
inch Objective, with an Achromatic Condenser having a series of
diaphragms. The nature of the movement in the hairs of different
species is far from being uniform. In some instances, the currents
pass in single lines along the entire length of the cells, as in the
Rotation of fluid in Hairs of Tradescan-
tia Virginica ; — A, portion of cuticle with
hair attached ; a, 6, c, successive cells of
the hair; d, cells of the cuticle; e, Stoma:
— *B, joints of a beaded hair, showing seve-
ral currents ; a, Nucleus.
* ' On the Sap-Circulation in Plants,' in " Q.uart. Journ. of Microsc. Science,"
Vol. iv. (1856), p. 44. — It is unfortunate that Mr. Wenham should have used
the term ' circulation' to designate this phenomeuon, which has nothing in
common with that movement of nutritive fluid through tubes or channels,
to which the term is properly applicable ; whilst ttie term ' sap' cannot be ap-
propriately applied to the contents of the individual cell.
CYCLOSIS IN HAIRS AND CUTICLE. 423
hairs from the filaments of the Tradescantia virginica, or Yirgiman
Spiderwort (Fig. 215, a) ; in others there are several snch currents
which retain their distinctness, as in the jointed hairs of the calyx
of the same plant (b) ; in others, again, the streams coalesce into
a network, the reticulations of which change their position at
short intervals, as in the hairs of Glaucium luteum ; whilst there
are cases in which the current flows in a sluggish uniformly
moving sheet or layer. Where several distinct currents exist in
one cell, they are all found to have one common point of de-
parture and return, namely, the nucleus (b, a) ; from which it
seems fairly to be inferred that this body is the centre of the vital
activity of the cell* Mr. "Wenham states that in all cases in which
the cyclosis is seen in the Hairs of a plant, the cells of the Cuticle
also display it, provided that their walls are not so opaque or so
strongly marked as to prevent the movement from being dis-
tinguished. The cuticle may be most readily torn off from the
stalk or the midrib of the leaf ; and must then be examined as
speedily as possible, since it loses its vitality when thus detached
much sooner than do the hairs. Even where no obvious movement
of particles is to be seen, the existence of a Cyclosis may be con-
cluded from the peculiar arrangement of the molecules of the
protoplasm, which are remarkable for their high refractive power,
and which, when arranged in a ' moving-train,' appear as bright
lines across the cell ; and these lines, on being carefully watched,
are seen to alter their relative positions. The leaf of the common
Plantago (Plantain or Dock) furnishes an excellent example of
Cyclosis ; the movement being distinguishable at the same time
both in the cells and in the hairs of the cuticle torn from its stalk
or midrib. It is a curious circumstance that when a plant which ex-
hibits the Cyclosis is kept in a cold dark place for one or two days, not
only is the movement suspended, but the moving particles collect
together in little heaps, which are broken up again by the separate
motion of their particles, when the stimulus of light and warmth
occasions a renewal of the activity. It is well to collect the speci-
mens about midday, that being the time when the rotation is most
active, and the movement is usually quickened by artificial warmth,
which, indeed, is a necessary condition in some instances to its being
seen at all. The most convenient method of applying this warmth,
while the object is on the stage of the Microscope, is to blow a
stream of air upon the thin-glass cover, through a glass or metal
tube previously heated in a spirit-lamp.
325. The walls of the Cells of Plants are frequently thickened by
* The above statement is called in question by Mr. Wenham, who affirms
that " whenever he has observed such a ' nucleus,' it has either been formed by
an accidental conglomeration of some of the cell-contents, or by morbid condi-
tions." The Author is satisfied, however, from the constancy with which the
1 nucleus ' is the centre of the diverging lines of protoplasm, in those cells
which have several streams radiating from one point, that it can neither be an
accidental nor a morbid conglomeration.
424 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
Fig. 216.
internal deposits, which may present very different appearances
according to the manner in which they are arranged. In its simplest
condition, such a deposit
forms a thin uniform layer
over the whole internal sur-
face of the cellulose-wall
(probably on the outside
of the primordial utricle),
scarcely detracting at all
from its transparence, and
chiefly distinguishable by
the ' dotted ' appearance
which the membrane then
presents (Fig. 211, a). These
dots, however, are not pores,
as their aspect might natu-
rally suggest, but are merely
points at which the deposit
is wanting, so that the ori-
ginal cell-wall there remains
unthickened. When the Cel-
lular tissue is required to
possess unusual firmness, a deposit of sclerogen (a substance which,
when separated from the resinous and other matters that are
commonly associated with it, is found to be allied in chemical
composition to cellulose) is formed in successive layers, one within
Tissue of the
Star-Anise .- — A, a
seen on the surface.
Testa or Seed-coat of
s seen in section; b, as
Fig. 217.
Fig. 218.
w. ^
Section of Cherry-stone, cutting
the cells transversely.
Section of Coquilla-nut,
in the direction of the long
diameters of the cells.
THICKENING DEPOSITS :— FIBRE-CELLS.
425
another (Fig. 216, a), which present themselves as concentric rings
when the cells containing them are cut through ; and these layers
are sometimes so thick and numerous as almost to obliterate the
original cavity of the cell. By a continuance of the same arrange-
ment as that which shows itself in the single layer of the dotted
cell — each deposit being deficient at certain points, and these points
corresponding with each other in the successive layers — a series
of passages is left, by which the cavity of the cell is extended
at some points to its membranous wall ; and it commonly
happens that the points at which the deposit is wanting on the
walls of two contiguous cells, are coincident, so that the mem-
branous partition is the only obstacle to the communication between
their cavities (Figs. 216-218). It is of such tissue that the ' stones '
of stone-fruit, the gritty substance which surrounds the seeds and
forms little hard points in the fleshy substance of the Pear, the
shell of the Cocoa-nut, and the albumen of the seed of Phytel&pkas
(known as ' vegetable ivory '), are made up ; and we see the use of
this very curious arrangement, in permitting the cells, even after
they have attained a considerable degree of consolidation, still to
remain permeable to the fluid required for the nutrition of the parts
which such tissue encloses and protects.
326. The deposit sometimes assumes, however, the form of
definite fibres, which lie coiled up in the interior of cells, so as to
form a single, a double, or even a triple or quadruple spire (Fig.
Fig. 220.
Fig. 219.
Spiral cells of leaf of Ontidium.
Spiral fibres of Seed-coat of Collomia.
219). Such spvral cells are found most abundantly in the leaves of
certain Orchideous plants, immediately beneath the cuticle, where
they are brought into view by vertical sections ; and they may be
426 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
obtained in an isolated state by macerating the leaf and peeling off
the cnticle so as to expose the layer beneath, which is then easily
separated into its components. In an Orchideous plant, named
Saccolabium gutiatum, the spiral cells are unusually long, and
have spires winding in opposite directions ; so that, by their mutual
intersection, a series of diamond- shaped markings is produced.
Spiral cells are often found upon the surface of the testa or outer
coat of Seeds ; and in the Collomia grandiflora, the Salvia verbe-
naca (Wild Clary), and some other plants, the membrane of these
cells is so weak, and the elasticity of their fibres so great, that when
the membrane is softened by the action of water the fibres suddenly
uncoil and elongate themselves (Fig. 220), springing out, as it were,
from the surface of the seed, to which they give a peculiar floccu-
lent appearance. This very curious phenomenon, which is not un-
frequently spoken of by persons ignorant of its true nature as the
'germination' of the seed, may be best observed in the following man-
ner .- — A very thin transverse slice of the seed should first be cut, and
laid upon the lower glass of the Aquatic-box; the cover should then
be pressed down, and the box placed upon the Stage, so that the
body of the Microscope may be exactly focussed to the object, the
power employed being the 1-inch, 2-3rds inch, or the -|-inch Objec-
tive. The cover of the Aquatic-box being then removed, a small
drop of water should be placed on that part of its internal surface
with which the slice of the seed had been in contact ; and the cover
being replaced, the object should be immediately looked at. It is
important that the slice of the seed should be very thin, for two
reasons ; first, that the view of the spires may not be confused by
their aggregation in too great numbers ; and second, that the drop
of water should be held in its place by capillary attraction, instead
of running down and leaving the object, as it will do if the glasses
be too widely separated.
327. In some part or other of most Plants, we meet with cells
containing granules of Starch. These granules are sometimes
minute and very numerous, and are so closely packed together as
to fill the cavity (Fig. 221) ; in other instances they are of much
larger dimensions, so that only a small number of them can be in-
cluded in any one cell ; while in other cases, again, they are both
few and minute, so that they form but a small proportion of the
cell-contents. Their nature is at once detected by the addition of a
solution of Iodine, which gives them a beautiful blue colour. Each
granule exhibits a peculiar spot, termed the hilum, which marks
the point at which, in its early state, it is attached to the cell-wall ;
and it also presents, when highly magnified, a set of circular lines,
which are for the most part concentric (or nearly so) with the
hilum. When viewed by Polarized light, each grain exhibits a
dark cross, the point of intersection being at the hilum (Fig. 222) ;
and when a Selenite-plate is interposed, the cross becomes
beautifully coloured. Opinions are very much divided regarding
the internal structure of the Starch-grain ; for whilst some affirm
STARCH-GRANULES. 427
the concentric lines to indicate the existence of a number of con-
centric lamellae, one enclosing another, others consider that they
are dne to the peculiar plaiting or involution of a single vesicular
wall ;* and among those who consider it to be concentrically lamel-
lated, some hold that each lamella is formed outside or wpon that
which preceded it, while others consider that each is formed inside
Fig. 221.
Fig. 22:
Cells of P atony, filled with Starch.
Granules of Starch, as seen under
Polarized Light.
or within its predecessor. The centre of the granule i3 often
occupied by starchy matter in an unconsolidated state ; and the
appearance arising from the different refractive power of this has
caused some observers to describe the starch-grain as possessing a
nucleus. — Although the dimensions of the starch-grains produced
by any one species of Plant are by no means constant, yet there is
a certain average for each, from which none of them depart "very
widely ; and by reference to this average, the starch-grains of
different Plants that yield this product in abundance may be
microscopically distinguished from one another, a circumstance of
considerable importance in commerce. The largest starch-grains
in common use are those of the plant (a species of Canna) known
as Tons les mois ; the average diameter of those of the Potato
is about the same as the diameter of the smallest of the Tous les
mois ; and the size of the ordinary starch-grains of Wheat and
* The first of these opinions is the one which was generally received, until
Mr. G. Busk supported the latter by new observations made upon the unfolding
of the stare h-granules by dilute sulphuric acid ; since when, Prof. Allman,
after repeating Mr. Busk's observations, has been led to affirm them to be falla-
cious, and to revert to the first of the above-mentioned doctrines. — See Mr.
Busk's memoir in " Trans, of Microsc. Soc," 2nd Ser. Vol. i. (1853), p. 58, and
that of Prof. Allman in " Quart. Jo urn. of Microsc. Science," Vol. ii. (185-4),
p. 163 ; also Cruger, on the Development of Starch, in the same volume, p. 173 ;
Grundy in "• Pharmaceutical Journal," April, 1855; Henfrey in Ann. of
"Nat. Hist." Ser. 2, Vol. xv. p. 246 ; and Eainey in "Quart. Journ. of Microsc.
Science," Vol. viii. (186U), p. 1. Nageli regards the internal layers as formed
by a process of intussusception; see " Pflanzenphysiologische Untersuchungen,"
by Nageli and Cramer, 1858 ; and his Papers in " Sitzungsberichte der Kon.
Baler. Akad. der Wissenschaften," 1862 and 1863.
428 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
of Sago is about the same as that of the smallest grains of Potato-
starch; whilst the granules of .Rice-starch are so very minute as to
be at once distinguishable from any of the preceding.
328. Deposits of Mineral matter in a crystalline condition, known
as Raphides, are not -[infrequently found in Vegetable cellsr;
where they are at once brought into view by the use of Polarized
light. Their designation (derived from pacfus, a needle) is very
appropriate to one of the most common states in which these
bodies present themselves, that, namely, of bundles of needle-
like crystals, lying side-by- side in the cavity of the cells ; such
bundles are well seen in the cells lying immediately beneath the
cuticle of the bulb of the medicinal Squill. It does not apply,
however, to other forms which are scarcely less abundant ; thus,
instead of bundles of minute needles, single large crystals, octo-
hedral or prismatic, are frequently met with ; and the prismatic
crystals are often aggregated in beautiful stellate groups. One
of the most common materials of raphides is Oxalate of Lime,
which is generally found in the stellate form; and no plant
yields these stellate raphides so abundantly as the common
Rhubarb, the best specimens of the dry medicinal root containing
as much as 35 per cent, of them. In the cuticle of the bulb of
the Onion the same material occurs under the octohedral or the
prismatic form. In other instances, the Calcareous base is com-
bined with Tartaric, Citric, or Malic acid ; and the acicular
raphides are said to consist usually of Phosphate of Lime. Some
Raphides are as long as l-40th of an inch, while others measure
no more than 1 -100th. They occur in all parts of plants, — the
Wood, Pith, Bark, Root, Leaves, Stipules, Sepals, Petals, Fruit,
and even in the Pollen. They are always situated in cells, and
not, as some have stated, in intercellular passages ; the cell-
membrane, however, is often so much thinned away as to be
scarcely distinguishable. Certain plants of the Cactus tribe,
when aged, have their tissues so loaded with raphides as to
become quite brittle ; so that when some large specimens of G.
senilis, said to be a thousand years old, were sent to Kew
Gardens from South America, some years since, it was found
necessary for their preservation during transport to pack them
in cotton, like jewellery. It is not yet known what office the
raphides fulfil in the economy of the plant, or whether they are
to be considered in any other light than as non-essential results
of the Vegetative processes. For as all these processes require
the introduction of Mineral bases from the soil, and themselves
produce Organic acids in the substance of the plant, it may
be surmised that the accidental union of such components will
occasion the formation of raphides wherever such union may
occur ; and this view is supported by the fact, that the late Mr.
E. Quekett succeeded in artificially producing raphides within the
cells of Rice-paper (§ 320), by first filling these with Lime-water
by means of the air-pump, and then placing the paper in weak
WOODY FIBEE. 429
solutions of Phosphoric and Oxalic acids. The artificial raphides
of Phosphate of Lime were rhombohedral ; while those of Oxalate of
Lime were stellate, exactly resembling the natural raphides of the
Ehubarb*
329. A large proportion of the denser parts of the fabric of the
higher Plants is made-up of the substance which is known as
Ligneous Tissue or Woody Fibre. This, however, can only be
regarded as a very simple variety of Cellular tissue ; for it is com-
posed of peculiarly-elongated cells (Fig. 234), usually pointed at
their two extremities so as to become spindle-shaped, whose walls
have a special tendency to undergo consolidation by the internal
deposit of sclerogen. It is obvious that a tissue consisting of
elongated cells, adherent together by their entire length, and
strengthened by internal deposit, must possess much greater
tenacity than any tissue in which the cells depart but little from
the primitive spherical form ; and we accordingly find Woody fibre
present wherever it is requisite that the fabric should possess
not merely density, but the power of resistance to tension. In the
higher classes of the Vegetable Kingdom it constitutes the chief
part of the stem and branches, where these have a firm and durable
character ; and even in more temporary structures, such as the
herbaceous stems of annual Plants, and the leaves and flowers of
almost every tribe, this tissue forms a more or less important con-
stituent, being especially found in the neighbourhood of the Spiral
Vessels and Ducts, to which it affords protection and support.
Hence the bundles of fasciculi composed of these elements, which
form the ' veins' of leaves, and which give ' stringiness' to various
esculent vegetable substances, are commonly known under the
name of fibro-vascular tissue. In their young and unconsolidated
state, the ligneous cells seem to conduct fluids with great facility
in the direction of their length ; and in the Coniferous tribe, whose
stems and branches are destitute of ducts, they afford the sole
channel for the ascent of the sap. But after their walls have
become thickened by internal deposit, they are no longer subservient
to this function ; nor, indeed, do they then appear to fulfil any
other purpose in the Vegetable economy than that of affording
mechanical support. It is this which constitutes the difference
between the alburnum or ' sap-wood,' and the duramen or ' heart-
wood,' of Exogenous Stems (§ 339).
330. A peculiar set of markings seen on the Woody fibres of the
* The materials of the above paragraph are derived from the excellent section
on this subject in Prof. Quekett's " Lectures on Histology." — Besides the Vege-
table structures therein named as affording good illustrations of different kinds of
Eaphides, may be mentioned the parenchyma of the leaf of Agave, Aloe, Cycas,
Encephalartos, &c. ; the cuticle of the bulb of the Hyacinth, Tulip, and Garlic
(and probably of other bulbs) ; the bark of the Apple, Cascarilla, Cinchona, Lime.
Locust, and many other trees ; the pith of Eleagnus, and the testa of the seeds
oiAnagallis and the Elm. — The Eaphides characteristic of the different Natural
Orders of Plants have been carefully studied by Mr. Gulliver ; who has given
an account of them in successive Papers in "Ann. Nat. Hist.," 1861 et seq.
430 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
Fig. 223.
Coniferoz, and of some other tribes, is represented in Fig. 223 ; in
each of these spots the inner circle appears to mark a deficiency of
the lining deposit, as in the porons cells of other plants ; whilst the
outer circle indicates the boundary of a lenticular cavity which
intervenes between the adjacent cells
at this point, and which contains a
small globular body that may be some-
times detached. Of the purpose of
these minute bodies interposed between
the Wood-cells, nothing is known ; there
can be no doubt, however, from the
definiteness and constancy of their ar-
rangement, that they fulfil some im-
portant object in the economy of the
Plants in which they occur ; and there
are varieties in this arrangement so
characteristic of different tribes, that it
is sometimes possible to determine, by
the microscopic inspection of a minute
fragment, even of a Fossil wood, the
tribe to which it belonged. The Woody
fibre thus marked is often designated as
glandular.
331. All the more perfect forms of
Phanerogamia contain, in some part of
their fabric, the peculiar structures
which are known as Spiral vessels.*
These have the elongated shape of
Woody fibres ; but the internal deposit,
as in the spiral cells (§ 326), takes
the form of a spiral fibre winding from end to end, remaining
distinct from the cell-wall, and retaining its elasticity ; this fibre
may be single, double, or even quadruple — this last character pre-
senting itself in the very large elongated fibre-cells of the Nepenthes
(Chinese Pitcher-plant). Such cells are especially found in the
delicate membrane (medullary sheath) surrounding the pith of
Exogens, and in the midst of the woody bundles occurring in the
stem of Endogens ; thence they proceed in each case to the leaf-
stalks, through which they are distributed to the leaves. By
careful dissection under the Microscope, they may be separated
entire ; but their structure may be more easily displayed by cutting
round, but not through, the leaf-stalk of the Strawberry, Geranium,
&c, and then drawing the parts asunder. The membrane com-
posing the tubes of the vessels will thus be broken across ; but the
* So long, however, as they retain their original cellular character, and do
not coalesce -with each other, these fusiform spiral cells cannot be regarded as
having any more claim to the designation of vessels, than have the elongated
cells of the ligneous tissue.
Section of Coniferous Wood
in the direction of the Fi-
bres, showing their 'glan-
dular ' dots : — a a a, Medullary
Rays crossing the fibres.
SPIRAL, ANNULAR, AND DOTTED DUCTS. 431
fibres within, being elastic, will be drawn-out and unrolled. Spiral
vessels are sometimes found to convey liquid, whilst in other cases
they contain air only ; the conditions of this difference are not yet
certainly known.
332. Although fluid generally finds its way with tolerable facility
through the various forms of Cellular tissue, especially in the
direction of the greatest length of their cells, a more direct means
of connection between distant parts is required for its active trans-
mission. This is afforded by what has been termed Vasiform tissue,
which consists merely of cells laid end-to-end, the partitions between
them being more or less obliterated, so that a continuous Duct is
formed. The origin of these Ducts in cells is occasionally very-
evident, both in the contraction of their calibre at regular intervals,
and in the persistence of remains of their partitions (Plate XII.,
fig. 2, b, b) ; but in most cases it can only be ascertained by study-
ing the history of their development, neither of these indications
being traceable. The component Cells appear to have been some-
times simply membranous, but more commonly to have been of
the fibrous type (§ 326). Some of the Ducts formed from the latter
(Fig. 224, 2) are so like continuous spiral vessels as to be scarcely
distinguishable from them, save in the want of elasticity in their
spiral fibre, which causes it to break when the attempt is made to
draw it out. This rupture would seem to have taken place, in
some instances, from the natural elongation of the cells by growth ;
the fibre being broken-up into rings, which lie sometimes close
together, but more commonly at considerable intervals ; such a
duct is said to be annular (Fig. 224, 1). Intermediate forms be-
tween the Spiral and Annular ducts, which show the derivation
of the latter from the former, are very frequently to be met -with.
The spires are sometimes broken-up still more completely, and the
fragments of the fibre extend in various directions, so as to meet
and form an irregular network lining the duct, which is then said
to be reticulated. The continuance of the deposit, however, gra-
dually contracts the meshes, and leaves the walls of the duct
marked only by pores like those of porous cells (§ 325) ; and canals
upon this plan, commonly designated as dotted ducts, are among
the most common forms of vasiform tissue, especially in parts of
most solid structure and least rapid growth (Fig. 224, 3). The
scalariform ducts of Ferns (§ 314) are for the most part of the
spiral type ; but spiral ducts are frequently to be met with also in
the rapidly growing leaf-stalks of Flowering-plants, such as the
Rhubarb. Xot unfrequently, however, we find all forms of Ducts
in the same bundle, as seen in Fig. 224. The size of these ducts
is occasionally so great as to enable their openings to be distin-
guished by the unaided eye; they are usually largest in stems
whose size is small in proportion to the surface of leaves which they
support, such as the common Cane, or the Vine ; and, generally
speaking, they are larger in woods of dense texture, such as Oak
432 MICKOSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
or Mahogany, than in those of which the fibres, remaining un-
consolidated, can serve for the conveyance of fluid. They are
entirely absent in the Conifer ce.
Fig. 224.
eft
Longitudinal section of stem of Italian Feed: — a, Cells of the
Pith ; ft, Fibro-vascular bundle, containing 1, Annular duct ;
2, Spiral duct ; 3, Dotted duct, with Woody fibre ; c, Cells
of the integument.
333. The Vegetable tissues whose principal forms have been now
described, but among which an immense variety of detail is found,
may be either studied as they present themselves in thin sections
of the various parts of the plant under examination, or in the
isolated conditions in which they are obtained by dissection. — The
former process is the most easy, and yields a large amount of in-
formation ; but still it cannot be considered that the characters of
any tissue have been properly determined, until it has been dis-
sected-out. Sections of some of the hardest Vegetable substances,
such as ' vegetable ivory,' the ' stones ' of fruit, the ' shell ' of the
Cocoa-nut, &c. (§ 325), can scarcely be obtained except by slicing
and grinding (§ 154) ; and these may be mounted either in Canada
balsam or in Glycerine jelly. In cases, however, in which the
tissues are of only moderate firmness, the section may be most
readily and effectually made with the 'Section-instrument' (§ 153);
and there are few parts of the Vegetable fabric which may not be
STEUCTUEE OF STEM AXD EOOT. 433
advantageously examined by this means, any very soft or thin
portions being placed in it between two pieces of cork. In certain
cases, however, in which even this compression wonld be injurious,
the sections must be made with a sharp knife, the substance being laid
upon a slip of glass. — In dissecting the Vegetable Tissues, scarcely
any other instrument will be found really necessary than a pair of
needles (in handles), one of them ground to a cutting edge. The
adhesion between the component cells, fibres, &c, is often sufficiently
weakened by a few hours' maceration to allow of their readily
coming apart, when they are torn-asunder by the needle-points
beneath the simple lens of a Dissecting-microscope. But if this
should not prove to be the case, it is desirable to employ some
other method for the sake of facilitating their isolation. Isone is so
effectual as the boiling of a thin slice of the substance under ex-
amination, either in dilute nitric acid, or in a mixture of nitric acid
and chlorate of potass. This last method (which was devised by
Schultz) is the most rapid and effectual, requiring only a few
minutes for its performance ; but as oxygen is liberated with such
freedom as to give an almost explosive character to the mixture, it
should be put in practice with extreme caution. After being thus
treated, the tissue should be boiled in Alcohol, and then in Water ;
and it will then be found very easy to tear-apart the individual
Cells, Ducts, &c, of which it may be composed. These may be
preserved by mounting in weak Spirit.
334. Structure of the Stem and Root. — It is in the Stems and
Boots of Plants that we find the greatest variety of tissues in con-
bination, and the most regular plans of structure ; and sections of
these viewed under a low magnifying power are objects of peculiar
beauty, independently of the scientific information which they
afford. The Axis (under which term is included the Stem with its
branches, and the Boot with its ramifications) always has for the
basis of its structure a dense Cellular parenchyma ; though this,
in the advanced stage of development, may constitute but a small
proportion of it. In the midst of the parenchyma we generally
find fibro-vascular bundles ; that is, fasciculi of Woody fibre, with
Ducts of various kinds, and (very commonly) Spiral vessels. It is
in the mode of arrangement of these bundles, that the fundamental
difference exists between the stems which are commonly designated
as Endogenous (growing from within), and those which are more
correctly termed Exogenous (growing on the outside) ; for in the
former the bundles are dispersed throughout the whole diameter of
the axis without any peculiar plan, the intervals between them being
filled-up by cellular parenchyma; whilst in the latter they are
arranged side by side in such a manner as to form a hollow cylinder
ofivood, which includes within it the portion of the cellular substance
known as pith, whilst it is itself enclosed in an envelope of the same
substance that forms the bark. These two plans of Axis-formation,
respectively characteristic of those two great groups into which the
434 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
Phanerogamia are subdivided — namely, the Monocotyledons and
the Dicotyledons — will now be more particularly described.
335. When a transverse section (Fig. 225) of a monocotyledonous
Stem is examined microscopically, it is found to exhibit a number
of fibro-vascular bundles, disposed without any regularity in the
midst of the mass of cellular tissue, which forms (as it were) the
matrix or basis of the fabric. Each bundle contains two, three, or
more large Ducts, which are at once distinguished by the size of
their openings; and these are surrounded by Woody fibre and
Spiral vessels, the transverse diameter of which is so extremely
Fig. 225.
-3 "vv;
Transverse Section of Stem of young Palm.
small, that the portion of the bundles which they form is at once
distinguished in transverse section by the closeness of its texture
(Fig. 226). The bundles are least numerous in the centre of the
stem, and become gradually more approximated towards its cir-
cumference ; but it frequently happens that the portion of the area
in which they are most compactly arranged is not absolutely at its
exterior, this portion being itself surrounded by an investment
composed of Cellular tissue only; and sometimes we find the central
portion, also, completely destitute of Fibro-vascular bundles ; so
that a sort of indication of the distinction between Pith, Wood, and
Bark is here presented. This distinction, however, is very imper-
fect ; for we do not find either the central or the peripheral portions
ever separable, like Pith and Bark, from the intermediate Woody
layer. In its young state the centre of the stem is always filled-up
STRUCTURE OF ENDOGENOUS STEM.
425
Fig. 226.
with cells ; but these not infrequently disappear after a time, ex-
cept at the nodes, leaving the stem hollow, as we see in the whole
tribe of Grasses. When a vertical section is made of a woody stem
(as that of a Palm) of sufficient length to trace the whole extent of
the fibro-vascular bundles, it is found that whilst they pass at
their upper extremity into the
leaves, they pass at the lower end
towards the surface of the stem, and
assist, by their interlacement with
the outer bundles, in forming that
extremely tough investment which
the lower ends of these stems pre-
sent. The fibro-vascular bundles
once formed receive no further
additions ; and the augmentation
of the stem in diameter depends
upon the development of fresh
woody bundles, in continuity with
the leaves which are successively
evolved at its summit. It was for-
merly supposed that these succes-
sively-formedbundles descend in the
interior of the stem through its
entire length until they reach the
roots ; and as the successive develop-
ment of leaves involves a succes-
sive development of new bundles,
the stem was imagined to be con-
tinually receiving additions to its interior, whence the term
endogenous was given to this type of stem-structure. From the
fact just stated, however, regarding the course of the fibro-vascular
bundles, it is obvious that such a doctrine cannot be any longer
admitted ; for those which are most recently formed only pass into
the centre of the stem during the higher part of their course, and
usually make their way again to its exterior at no great distance
below ; and thus the lower and older portions of a Palm-stem really
do receive very little augmentation in diameter, while a rapid
elongation is taking place at its summit. In fact, the dense un-
yielding nature of the fabric which is formed by the interlacement
of the fibro-vascular bundles at or near the surface of the trunk,
would prevent any considerable augmentation by expanding pres-
sure from within.
336. In the Stems of dicotyledonous Phanerogamia, on the other
hand, we find a method of arrangement of the several parts, which
must be regarded as the highest form of the development of the
Axis, being that in which the greatest differentiation exists. A
distinct division is always seen in a transverse section (Fig. 227)
between three concentric areae, — the pith, the wood, and the
lark ; the first (a) being central, the last (6) peripheral, and
r f 2
Portion of Transverse Section of
Stem of Wanghie Cane.
436 MICKOSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
Fig. 227.
these having the wood interposed between them, its circle being
made up of wedge-shaped bundles (d, d), kept apart by the bands
(c, c) that pass between the pith and the bark. The Pith (Fig. 229,
a), is almost invariably composed of
cellular tissue only, which usually
presents (in transverse section) a
hexagonal areolation. When newly
formed it has a greenish hue, and its
cells are filled with fluid ; but it gra-
dually dries-up and loses its colour ;
and not unfrequently its component
cells are torn apart by the rapid
growth of their envelope, so that
irregular cavities are found in it ; or, if
the stem should increase with ex-
treme rapidity, it becomes hollow, the
pith being reduced to fragments, which
are found adhering to its interior wall.
The pith is immediately surrounded
Diagram of the first formation by a delicate membrane consisting
1 ' almost entirely of Spiral vessels, which
is termed the medullary sheath.
337. The woody portion of the
stem (Fig. 228, h, b), is made^ up
of Woody fibres, usually with the addition of Ducts of various
kinds ; these, however, are absent in one large group, the Coniferce
or Fir tribe with its allies (Fig. 232-235), in which the Woody
an Exogenous Stem :— a, Pith ,
b b, Bark ; c c, plates of cellular
tissue (Medullary Eays) left be-
Woody Bundles d d.
tween
Transverse Section of Stem of Clematis .--—a, pith ; 6, 6, 6, woody bundles ;
c, c, c, medullary rays.
fibres are of unusually large diameter, and have the peculiar
glandular markings already described (§ 330). In any stem or
branch of more than one year's growth, the Woody structure
presents a more or less distinct appearance of division into concen-
tric rings, the number of which varies with the age of the tree
CONCENTRIC KINGS OF EXOGENOUS WOOD.
437
(Fig. 229). The composition of the several rings, which, are the
sections of so many cylindrical layers, is nniformly the same, how-
ever different their thickness ; but the arrangement of the two
Fig. 230.
Fig. 229.
Transverse Section of Stem of
Bkamnus (Buckthorn), showing
concentric layers of Wood.
Portion of the same, more
highly magnified.
principal elements, — namely, the "Woody fibre and the Dncts, —
varies in different species : the Dncts being sometimes almost
nniformly diffused through the whole layer, but in other instances
being confined to its inner part ; while in other cases, again, they
are dispersed with a certain regular irregularity (if such an expres-
sion may be allowed), so as to give a curiously -figured appearance
to the transverse section (Figs. 229, 230). The general fact, how-
ever, is, that the Ducts predominate towards the inner side of the
ring (which is the part of it first formed), and that the outer portion
of each layer is almost exclusively composed of Woody tissue :
such an arrangement is shown in Fig. 228. This alternation of
Ducts and Woody fibre frequently serves to mark the succession
of layers, when, as it is not uncommon, there is no very distinct
line of separation between them.
338. The number of layers is usually considered to correspond
with that of the years during which the stem or branch has been
growing ; and this is, no doubt, generally true in regard to the
trees of temperate climates, which thus ordinarily increase by
annual layers. There can be no doubt, however, that such is not the
universal rule ; and that we should be more correct in stating that
each layer indicates an epoch of vegetation ; which, in temperate
climates, is usually (but not invariably) a year, but which is com-
monly much less in the case of trees flourishing in tropical regions.
Thus among the latter it is very common to find the leaves regularly
438 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
shed and replaced twice or even thrice in a year, or five times in
two years ; and for every crop of leaves there will be a correspond-
ing layer of wood. It sometimes happens, even in temperate
climates, that trees shed their leaves prematurely in consequence
of continued drought, and that, if rain then follow, a fresh crop of
leaves appears in the same season ; and it cannot be doubted that
in such a year there would be two rings of Wood produced, which
would probably not together exceed the ordinary single layer in
thickness. That such a division may even occur as a consequence
of an interruption to the processes of vegetation produced by
seasonal changes, — as by heat and drought in a tree" that flourishes
best in a cold damp atmosphere, or by a fall of temperature in a
tree that requires heat,— would appear from the frequency with
which a double or even a multiple succession of rings is found in
transverse sections of wood to occupy the place of a single one.
Thus in a section of Hazel stem (in the Author's possession), of
which a portion is represented in Fig. 231, between two layers of
Fig. 231.
-^=*v£T?:
■m
Portion of Transverse Section of Stem of Hazel, showing, in the portion
o, 6, c, six narrow layers of Wood.
the ordinary thickness there intervenes a band whose breadth is
altogether less than that of either of them, and which is yet com-
posed of no fewer than six layers, four of them (c) being very
narrow, and each of the other two (a, b) being about as wide as
these four together.
339. The inner layers of Wood are the oldest, and the most
solidified by matters deposited within their component Cells and
Vessels ; hence they are spoken of collectively under the designation
duramen or ' heart-wood,' On the other hand, it is through the
Cells and Ducts of the outer and newer layers that the sap rises
from the roots towards the leaves; and these are consequently
designated as alburnum or ' sap-wood.' The line of demarcation
between the two is sometimes very distinct, as in Lignum-vita3 and
Cocos wood ; and as a new layer is added every year to the exterior
of the alburnum, an additional layer of the innermost part of the
alburnum is every year consolidated by internal deposit, and is
thus added to the exterior of the duramen. More generally, how-
ever, this consolidation is gradually eff ected, and the alburnum and
duramen are not separated by any abrupt line of division.
EXOGENOUS STEM : — MEDULLAEY RAYS.
439
340. The Medullar]) Bays which cross the successive rings of
Wood, connecting the cellular substance of the Pith with that of
the Bark, and dividing each ring of Wood into wedge-shaped
segments, are thin plates of cellular tissue (Fig. 228, c, c), not usually
extending to any great depth in the vertical direction. It is not
Fig. 232.
JR. I
Fig. 23S
Portion of Transverse Section of the Stem of Cedar ,
6, b, b, woody layers ; c, bark.
often, however, that their character can be so clearly seen in a
transverse section, as in the diagram just referred to ; for they are
usually compressed so closely as to appear darker than the wedges
of Woody tissue between which they intervene (Figs. 230, 232);
and their real nature is
best understood by a com-
parison of longitudinal sec-
tions made in two different
directions, — namely radial
and tangential, — with the
transverse. Three such sec-
tions of a fossil Coniferous
wood in the Author's pos-
session are shown in Figs.
233-235. The Stem was of
such large size, that, in so
small a part of the area of
its transverse section as is
represented in Fig. 233, the
Medullary Bays seem to
run parallel to each other, -n ,. , m n £. ,. a,
• ± a ~£ ~a-~±- .g. rortion of Transverse Section of large Stem
instead of radiating from a of coniferous Wood (fossil), showing part of
common centre, lney are two annual layers, divided at a, a, and tra-
very narrow ; but are so versed by very thin but numerous Medullary
closely set together, that EaJs-
only two or three rows of
440 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
Woody fibres (no ducts being here present) intervene between
any pair of them. In the longitndinal section taken in a radial di-
rection (Fig. 234), and consequently passing in the same conrse with
the medullary rays, these are seen as thin plates (a, a, a) made-np
Fig. 231.
Portion of Vertical Section of the same
wood, taken in a radial direction, show-
ing the glandular Woody fibres, without
Ducts, crossed by the Medullary Rays, a, a.
Portion of Vertical Sec-
tion of the same wood,
taken in a tangential di-
rection, so as to cut across
the Medullary Rays.
of superposed cells very much elongated, and crossing in a
horizontal direction the woody fibres which lie parallel to one
another vertically. And in the tangential section (Fig. 235), which
passes a direction at right angles to that of the Medullary Rays,
and therefore cuts them across, we see that each of the plates thus
formed has a very limited depth from above downwards, and is
composed of no more than one thickness of horizontal cells. — A
section of the stem of Mahogany taken in the same direction as
the last (Fig. 236), gives a very good view of the cut ends of the
Medullary Rays, as they pass between the woody fibres ; and they
are seen to be here of somewhat greater thickness, being composed
of two or three rows of cells, arranged side by side.
341. In another Fossil Wood, whose transverse section is shown
in Plate XII., fig. 1, and its tangential section in fig. 2, the Medul-
lary Rays are seen to occupy a much larger part of the substance
of the stem ; being shown in the transverse section as broad bands
(a a, a a) intervening between the closely-set woody fibres, among
which some large ducts are scattered ; whilst in the tangential, they
are observed to be not only deeper than the preceding from above
PLATE XII.
Fig. 1.
Fig. 2.
Fig. 3.
Sections of Exogenous Stems
[To face p. 440.
MEDULLARY RAYS :— BARK.
441
Fig. 236.
downwards, but also to have a much greater thickness. This section
also gives an excellent view of the ducts b b, b b, which are here
plainly seen to be formed by the coalescence of large cylindrical
cells, lying end-to-end.— In another Fossil Wood in the Author's
possession, the Medullary Eays constitute a still larger proportion
of the stem ; for in the transverse section (Plate XII., fig. 3) they
are seen as very broad bands (b, b), al-
ternating with plates of woody structure
(a, a), whose thickness is often less than
their own; whilst in the tangential
section (fig. 4) the cut extremities of
the Medullary Kays occupy a very large
part of the area, having apparently de-
termined the sinuous course of the
woody fibres ; instead of looking (as in
Fig. 235) as if they had forced their way
between the woody fibres, which there
hold a nearly straight and parallel
course on either side of them. — The
function of the Medullary Eays appears
to be to maintain a connection between
the external and the internal parts of
the Cellular basis of the stem, which
have been separated by the interposi-
tion of the Wood.
342. The Bark may be usually found
to consist of three principal layers ; the
external, or epipliloziirn, also termed the
suberous (or corky) layer ; the middle,
or mesophlceum, also termed the cel-
lular envelope ; and the internal, or Vertical Section of Mahogany,
endojMoeum, which is more commonly
known as the liber. The two outer layers are entirely cellular ;
and are chiefly distinguished by the form, size, and direction of
their cells. The epi/phlceum is generally composed of one or more
layers of colourless or brownish cells, which usually present a
cubical or tabular form, and are arranged with their long diameters
in the horizontal direction ; it is this which, when developed to an
unusual thickness, forms Cork, sl substance which is by no means
the product of one kind of tree exclusively, but exists in greater or
less abundance in the bark of every Exogenous stem. The
viesophlmum consists of cells, usually of green colour, prismatic in
their form, and disposed with their long diameters parallel to the
axis ; it is more loosely arranged than the preceding, and contains
intercellular passages, which often form a network of canals that
have been termed Laticiferous Vessels ; and although usually less
developed than the suberous layers, it sometimes constitutes the
chief thickness of the bark. The liber or ' inner bark,' on the other
hand, usually contains woody fibre in addition to the cellular tissue
442 MICROSCOPIC STEUCTUEE OF PHANEEOGAMIC PLANTS.
and laticif erous canals of the preceding ; and thus ajDproaches more
nearly in its character to the woody layers, with which it is in
close proximity on its inner surface. The Liber may generally be
found to be made up of a succession of thin layers, equalling in
number those of the Wood, the innermost being the last formed ;
but no such succession can be distinctly traced either in the cellular
envelope or in the suberous layer, although it is certain that they
too augment in thickness by additions to their interior, whilst their
external portions are frequently thrown-off in the form of thickish
plates, or detach themselves in smaller and thinner laminae. — The
bark is always separated from the wood by the cambium-layer,
which is the part wherein all new growth takes place : this seems
to consist of mucilaginous semi-fluid matter ; but it is really made-
up of cells of a very delicate texture, which gradually undergo
transformation, whereby they are for the most part converted into
Woody fibres, Ducts, Spiral vessels, &c. These materials are so
arranged as to augment the Fibro-vascular bundles of the Wood
on their external surface, thus forming a new layer of Alburnum
which encloses all those that preceded it ; whilst they also form a
new layer of Liber, on the interior of all those which preceded it :
they also extend the Medullary Rays, which still maintain a con-
tinuous connection between the pith and the bark ; and a portion
remains unconverted, so as always to keep apart the Liber and the
Alburnum. — This type of Stem-structure is termed Exogenous ; a
designation which applies very correctly to the mode of increase of
the Woody layers, although (as just shown) the Liber is formed
upon a truly Endogenous plan.
343. Numerous departures from the normal type are found in
particular tribes of Exogens. Thus in some the Wood is not'
marked by concentric circles, their growth not being interrupted
by any seasonal change. In other cases, again, each Woody zone
is separated from the next by the interposition of a thick layer of
Cellular substance. Sometimes Wood is formed in the Bark (as in
Galycanthus), so that several woody columns are produced, which
are quite independent of the principal woody axis, but cluster
around it. Occasionally the woody Stem is divided into distinct
segments by the peculiar thickness of certain of the Medullary
Rays; and in the stem of which Fig. 237 represents a transverse
section, these cellular plates form four large segments, disposed in
the manner of a Maltese cross, and alternating with the four
woody segments, which they equal in size.
344. The Exogenous Stem, like the so-called Endogenous, con-
sists in its first-developed state of Cellular tissue only ; but after
the leaves have been actively performing their functions for a short
time, we find a circle of Fibro-vascular bundles, as represented in
Fig. 227, interposed between the central (or medullary) and the
peripheral (or cortical) portions of the cellular matrix ; these fibro-
vascular bundles being themselves separated from each other by
plates of cellular tissue, which still remain to connect the central
DEVELOPMENT OF EXOGENOUS STEM.
443
and the peripheral portions of the matrix. This first stage in the
formation of the Exogenous axis, in which its principal parts —
the Pith, Wood, Bark, and Medullary Rays — are marked out, is
seen even in the stems of herbaceous Plants, which are destined
to die down at the end of the season (Fig. 238) ; and sections
Fig. 238.
Fig.
m
■*
Transverse section of the stem of a Portion of transverse section
climbing-plant (Aristolochiaf) from New of Arctium (Burdock), sho-vring
Zealand. one of the Fibro-vascular bun-
dles that lies beneath the cellu-
lar integument.
of these, which are very easily prepared, are most interesting
Microscopic objects. In such stems, the difference between the
Endogenous and the Exogenous types is manifested in little else than
the disposition of the Fibro-vascular layers ; which are scattered
through nearly the whole of the cellular matrix (although more
abundant towards its exterior) in the former case; but are
limited to a circle within the peripheral portion of the cellular
tissue in the latter. It is in the further development which
takes place during succeeding years in the woody stems of
perennial Exogens, that those characters are displayed, which
separate them most completely from the Ferns and their allies,
whose stems contain a cylindrical layer of Fibro-vascular bundles,
as well as from (so-called) Endogens. For whilst the Fibro-
vascular layers of the latter, when once formed, undergo no
further increase, those of Exogenous stems are progressively
augmented by the metamorphosis of the cambium-layer ; so that
each of the bundles which once lay as a mere series of parallel
cords beneath the cellular investment of a first-year's stem, may
become in time the small end of a wedge-shaped mass of wood,
Ui MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
extending continuously from the centre to the exterior of a trunk
of several feet in diameter, and becoming progressively thicker
as it passes outwards. The Fibro-vascular bundles of Exogens
are therefore spoken of as 'indefinite;' whilst those of Exogens
and Acrogens (Ferns, &c.) are said to be ' definite' or ' closed/
345. The structure of the Boots of Endogens and Exogens is
essentially the same in plan with that of their respective Stems.
Generally speaking, however, the roots of Exogens have no pith,
although they have medullary rays ; and the succession of distinct
rings is less apparent in them, than it is in the stems from which
they diverge. In the delicate radical filaments which proceed
from the larger root-fibres, a central bundle of vessels will be seen,
enveloped in a sheath of cellular substance ; and this investment
also covers-in the end of the fibril, which is usually somewhat
dilated, and composed of peculiarly succulent tissue, forming what
is termed the spongioid. The structure of the radical filaments
may be well studied in the common Duckweed, every floating leaf
of which has a single fibril hanging down from its lower surface.
346. The structure of Stems and Boots cannot be thoroughly
examined in any other way, than by making sections in different
directions with the Section-instrument. The general instructions
already given (§ 153) leave little to be added respecting this
special class of objects ; the chief points to be attended to being
the preparation of the Stems, &c, for slicing, the sharpness of the
knife and the dexterity with which it is handled, and the method of
mounting the sections when made. The Wood, if green, should
first be soaked in strong alcohol for a few days, to get rid of
the resinous matter ; and it should then be macerated in water for
some days longer, for the removal of its gum, before being sub-
mitted to the cutting-process. If the wood be dry, it should first
be softened by soaking for a sufficient length of time in water, and
then treated with spirit, and afterwards with water, like green
wood. Some Woods are so little affected even by prolonged mace-
ration, that boiling in water is necessary to bring them to the
degree of softness requisite for making sections. ISTo Wood that
has once been dry, however, yields such good sections as that
which is cut fresh. When a piece, of the appropriate length, has
been placed in the grasp of the Section-instrument (wedges of deal
or other soft wood being forced-in with it, if necessary for its firm
2 fixation), a few thick slices should first be taken, to reduce its sur-
face to an exact level; the surface should then be wetted with
spirit, the Micrometer- screw moved through a small part of a
revolution, and the slice taken off with the razor, the motion given
to which should partake both of drawing and pushing. A little
practice will soon enable the operator to discover, in each case,
how thin he may venture to cut his sections without a breach of
continuity ; and the Micrometer-screw should be turned so as to
give the required elevation. If the surface of the wood has been
sufficiently wetted, the section will not curl-up in cutting, but will
MOUNTING SECTIONS OF WOOD.— CUTICLE. 445
adhere to the surface of the razor, from which it is best detached
by dipping the razor in water so as to float away the slice of wood,
a camel-hair jDencil being used to push it off, if necessary. All the
sections that may be found sufficiently thin and perfect, should be
put aside in a bottle of weak spirit until they be mounted. For
the minute examination of their structure, they may be either
mounted in fluid, none being preferable to weak spirit, or in
glycerine jelly. Where a mere general view only is needed, dry-
mounting answers the purpose sufficiently well ; and there are
many stems, such as the Clematis, of which transverse sections
rather thicker than ordinary make very beautiful opaque objects,
when mounted dry on a black ground. Canada Balsam should
not be had recourse to, except in the case of very opaque sections,
as it usually makes the structure too transparent. Transverse
sections, however, when slightly charred by heating between two
plates of glass until they turn brown, may be mounted with
advantage in Canada balsam, and are then very showy specimens
for the Gas-Microscope. The number of beautiful and interesting
objects which may be thus obtained, at the cost of a very
small amount of trouble, can scarcely be conceived save by those
who have made a special study of these wonderful structures.
Even the commonest Trees, Shrubs, and herbaceous Plants, yield
specimens that exhibit a varied elaboration of arrangement, which
cannot but strike with astonishment even the most cursory ob-
server ; and there is none in which a careful study of sections
made in different parts of the stem, and especially in the neigh-
bourhood of the ' growing point,' will not reveal to the eye of the
scientific Physiologist some of the most important phenomena
of Yegetation. — Fossil Woods, when well preserved, are almost
invariably siUcified, and require, therefore, to be cut and polished by
a Lapidary. Should the Microscopist be fortunate enough to
meet with a portion of a calcified stem in which the organic
structure is preserved, he should proceed with it after the manner
of other hard substances which need to be reduced by grinding
(§§ 155-157).
347. Structure of the Cuticle and Leaves. — On all the softer
parts of the higher Plants, save such as grow under water, we find
a surface-layer, differing in its texture from the parenchyma
beneath, and constituting a distinct membrane, known as the
Cuticle. This membrane is composed of cells, the walls of which
are flattened above and below, whilst they adhere closely to each
other laterally, so as to form a continuous stratum (Figs. 243, 245,
a, a). Their shape is different in almost every tribe of plants ;
thus in the cuticle of the Yucca (Fig. 239), Indian Com (Fig. 240),
Iris (Fig. 244), and most other Monocotyledons, the cells are elon-
gated, and present an approach to a rectangular contour ; their
margins being straight in the Yucca and Iris, but minutely
sinuous or crenated in the Indian Corn. In most Dicotyledons, on
the other hand, the cells of the cuticle depart less from the form of
446 MICEOSCOPIC STEUCTUKE OF PHANEEOGAMIC PLANTS.
circular disks ; but their margins usually exhibit large irregular
sinuosities, so that they seem to fit together like the pieces of a
dissected map, as is seen in the cuticle of the Apple (Fig. 241, b, b).
Even here, however, the cells of that portion of the cuticle (a, a)
Fig. 239.
Fig. 240.
Cuticle of Leaf of Yucca.
Cuticle of Leaf of Indian
Corn (Zea Mais).
Fig. 241.
Portion of the Cuticle of the inferior surface of the Leaf of
the Apple, with the layer of parenchyma in immediate con-
tact with it : — a, a, elongated cells of the cuticle overlying
the veins of the leaf; b, 6, ordinary cuticle-cells, overlying
the parenchyma ; c, c, stomata ; d, d, green^cells of the paren-
chyma, forming a very open network near the lower surface
of the leaf.
STRUCTURE OF CUTICLE. 447
which, overlies the ' veins ' of the leaf, have an elongated form, ap-
proaching that of the wood-cells of which these veins are chiefly
composed; and it seems likely, therefore, that the elongation
of the ordinary cuticle-cells of Monocotyledons has reference to
that parallel arrangement of the veins which their leaves almost
constantly exhibit.
348. The cells of the Cuticle are colourless, or nearly so, no
chlorophyll being formed in their interior; and their walls are
generally thickened by secondary deposit, especially on the side
nearest the atmosphere. This deposit is of a waxy nature, and
consequently renders the membrane very impermeable to fluids, the
retention of which within the soft tissue of the leaf is obviously
the purpose to be answered by the peculiar organization of the
cuticle. In most European plants the cuticle contains but a single
Fig. 242.
c
Portion of the Cuticle of the upper surface of the Leaf of
JRochea falcata, as seen at A from its inner side, and at B from
its outer side : — a, a, small cells forming the inner layer of
the cuticle ; 6, 6, large prominent cells of the outer layer ;
c, c, stomata disposed between the latter.
row of cells, which, moreover, are usually thin-sided ; whilst in the
generality of tropical species, there exists two, three, or even four
layers of thick-sided cells ; this last number being seen in the
Oleander, the cuticle of which, when separated, has an almost
leathery firmness. This difference in conformation is obviously
adapted to the conditions of growth under which these plants re-
spectively exist; since the cuticle of a plant indigenous to tempe-
rate climates, would not afford a sufficient protection to the
interior structure against the rays of a tropical sun ; whilst the
less powerful heat of this country would scarcely overcome the
resistance presented by the dense and non-conducting tegument of
a species formed to exist in tropical climates.
349. A very curious modification of the Cuticle is presented by
the Bochea falcata, which has the surface of its ordinary cuticle
(Figs. 242, 243, a, a) nearly covered with a layer of large pro-
minent isolated cells, h, 6. A somewhat similar structure is found
448 MICROSCOPIC STEUCTUEE OF PHANEEOGAMIC PLANTS.
in the Mesenibryanthemum crystallinum, commonly known as the
Ice-plant ; a designation it owes to the pecnliar appearance of its
surface, which looks as if it were covered with frozen dewdrops.
In other instances, the cuticle is partially invested by a layer of
scales, which are nothing else than flattened cells, often having a
very peculiar form ; whilst
Fig. 243. in numerous cases, again,
we find the surface "beset
with hairs, which occasion-
ally consist of single elon-
gated cells, but are more
commonly made up of a
linear series, attached end
to end, as in Fig. 215.
Sometimes these hairs bear
little glandular bodies at
their extremities, by the
secretion of which a peculiar
Portion of vertical section of Leaf oi lHochea, ^acidity is given to the
showing the small cells, a. a, of the inner £ p ±i i £
layer of cuticle ; the large cells, 6, 6, of the surface ot the leaf, as in
outer layer; c, one of the stomata ; d, d, the Sundew (Drosera) ; m
cells of the parenchyma ; L, cavity between other instances, the hair has
the parenchymatous cells, into which the a glandular body at its base,
stoma opens. ^^ whoge secretion ft js
moistened, so that when
this secretion is of an irritating quality, as in the Nettle, it consti-
tutes a ' sting.' A great variety of such organs may be found, by
a microscopic examination of the surface of the leaves of plants
having any kind of superncial'mvestnient to the cuticle. Many
connecting links present themselves between Hairs and Scales,
such as the stellate hairs of the Deutzia scabra, which a good deal
resemble those within the air-chambers of the Yellow Waterlily
(Fig. 213).
350. The Cuticle in many plants, especially those belonging
to the Gh'ass tribe, has its cell-walls impregnated with silex, like
that of the Equisetum (§ 317) ; so that when the organic matter
seems to have been got rid-of by heat or by acids, the forms of the
Cuticle-cells, Hairs, Stomata, &c, are still marked out in silex, and
(unless the dissipation of the organic matter has been most
perfectly accomplished) are most beautifully displayed by Polarized
light. Such silicified cuticles are found in the husks of the grains
yielded by these plants : and there is none in which a larger pro-
portion of mineral matter exists, than that of Bice, which contains
some curious elongated cells with toothed margins. The hairs
with which the palece (chaff-scales) of most Grasses are furnished,
are strengthened by the like siliceous deposit ; and in the Festuca
pratensis, one of the common meadow-grasses, the paleas are also
beset with longitudinal rows of little cup-like bodies formed of
silica. The cuticle and scaly hairs of Deutzia scahra also contain
STRUCTURE OF CUTICLE: — STOMATA,
449
Fig. 244.
a large quantity of silex; and are remarkably beautiful objects
for the Polariscope.
351. Externally to the Cuticle there usually exists a very
delicate transparent pellicle, without any decided traces of organi-
zation, though occasionally somewhat granular in appearance, and
marked by lines that seem to be impressions of the junctions of
the cells with which it was in contact. When detached by macera-
tion, it not only comes off from the surface of the cuticle, but also
from that of the hairs, &c, which this may bear. This membrane
is obviously formed by the agency of the cells of the cuticle ; and
it seems to consist of the external layers of their thickened cellu-
lose walls, which have coalesced with each other, and have sepa-
rated themselves from the subjacent layers, by a change somewhat
analogous to that which occurs in the Palmelleae (§ 263), the outer
walls of whose original cells seem to melt away into the gelatinous
investment that surrounds the ' broods ' which have originated in
their subdivision.
352. In nearly all plants which possess a distinct Cuticle, this
is perforated by the minute openings termed Stomata (Figs. 241,
242, c, c) ; which are bor-
dered by cells of a peculiar
form, distinct from those of
the cuticle, and more re-
sembling in character those
of the tissue beneath. These
boundary-cells are usually
somewhat kidney - shaped,
and lie in pairs (Fig. 244,
b, b), with an oval opening
between them ; but by an
alteration in their form, the
opening may be contracted
or nearly closed. In the
Cuticle of Yticca, however,
the opening is bounded by
two pairs of cells, and is
somewhat quadrangular
(Fig. 239) ; and a like dou- _
bling of the boundary -cells, away with it a portion of the parenchymatous
with a narrower slit between layer in immediate contaet with it : — a, a,
them, is seen in the cuticle elongated cells of the cuticle ; 6, 6, cells of
of the Indian Com (Fig. ^stomata; crc, cells of the parenchyma ;
QAtw t xt_ 4. j. £ <*i a, impressions on the epidermic cells
240). In the stomata of no formed by their contact; e, e, cavities in the
Phanerogamic plant, how- parenchyma, corresponding to the stomata.
ever, do we meet with any
conformation at all to be compared in complexity with that which
has been described in the humble Marchantia (§ 306). — Stomata
are usually found most abundantly (and sometimes exclusively)
in the cuticle of the lower surfaces of leaves, where they open into
G &
Portion of the Cuticle of the Leaf of the Iris
germanica, torn from its surface, and carrying
450 MICEOSCOPIC STEUCTUEE OF PHANEEOGAMIC PLANTS.
the air-chambers that are left in the parenchyma which lies next
the inferior cnticle ; in leaves which float on the surface of water,
however, they are found in the cnticle of the upper surface only ;
whilst in leaves that habitually live entirely submerged, as there is
no distinct cuticle, so there are no stomata. In the erect leaves of
Grasses, the Iris tribe, &c, they are found equally (or nearly so) on
both surfaces. As a general fact, they are least numerous in suc-
culent plants, whose moisture, obtained in a scanty supply, is des-
tined to be retained in the system ; whilst they abound most in
those which exhale fluid most readily, and therefore absorb it most
quickly. It has been estimated that no fewer than 160,000 are
contained in every square inch of the under surface of the leaves
of Hydrangea and of several other plants ; the greatest number
seeming always to present itself in species, the upper surface of
whose leaves is entirely destitute of these organs. In Iris ger-
manica, each surface has nearly 12,000 stomata in every square
inch ; and in Yucca, each surface has 40,000. In Oleander,
Banksia, and some other plants, the Stomata do not open directly
upon the lower surface of the cuticle, but lie in the deepest part of
little pits or depressions, which are excavated in it and lined with
hairs ; the mouths of these pits, with the hairs that line them, are
well brought into view by taking a thin slice from the surface of
the cuticle with a sharp knife ; but the form of the cavities and
the position of the stomata can only be well made out in vertical
sections of the leaves.
353. The internal structure of Leaves is best brought into view
by making vertical sections, that shall traverse the two layers of
cuticle and the interme-
diate cellular parenchy-
ma ; portions of such sec-
tions are shown in Figs.
243, 245, and 246. In
close apposition with the
cells of the upper cuticle
(Fig. 245, a, a), which
may or may not be per-
forated with stomata (c,
c, d, d), we find a layer
of soft thin-walled cells,
containing a large quan-
Vertical section of the Cuticle, and of a por- ^ ,-, i i
tionof the subjacent parenchyma, of a leaf of generally press so closely
Iris germanica, taken in a transverse direction : — one against another, that
o, a, cells of the cuticle ; 6, 6, cells at the sides their sides become mutu-
of the Stomata ; c, c, small green cells placed ally flattened, and no
within these ; d d, openings of the stomata; spacesare left, save where
e, e, cavities in the parenchyma into which the A • j„q, ;+„ •„
stomata open ; /,/, cells of the parenchyma. there is a definite air-
r chamber into which the
Stoma opens (Fig. 245, e) ;
Fig. 245.
INTERNAL STRUCTURE OF LEAVES. 451
and the compactness of this superficial layer is well seen, when,
as often happens, it adheres so closely to the cuticle, as to be
carried away with this when it is torn off (Fig. 244, c, c). Beneath
this first layer of leaf-cells, there are usually several others rather
less compactly arranged ; and the tissue gradually becomes more
and more lax, its cells not being in close apposition, and large
intercellular passages being left amongst them, until we reach the
lower cuticle, which the parenchyma only touches at certain points,
its lowest layer forming a set of network (Fig. 241, d, d) with large
interspaces, into which the stomata open. It is to this arrange-
ment that the darker shade of green almost invariably presented
by the superior surfaces of leaves is principally due ; the colour of
the component cells of the parenchyma not being deeper in one
part of the leaf than in another. — In those plants, however, whose
Fig. 246.
I Kf $
Portion of a vertical longitudinal section of the Leaf of Iris,
extending from one of its flattened sides to the other: — a, a,
elongated cells of the cuticle; b, 6, Stomata cut through
longitudinally; c, c, green cells of the parenchyma; d, d,
colourless tissue, occupying the interior of the leaf.
leaves are erect instead of being horizontal, so that their two sur-
faces are equally exposed to light, the parenchyma is arranged on
both sides in the same manner, and their cuticles are furnished
with an equal number of stomata. This is the case, for example,
with the leaves of the common garden Iris (Fig. 246) ; in which,
moreover, we find a central portion (d, d) formed by thick-walled
colourless tissue, very different either from ordinary leaf-cells
or from woody fibre. The explanation of its presence is to be
found in the peculiar conformation of the leaves ; for if we pull
one of them from its origin, we shall find that what appears to be
the flat expanded blade really exposes but half its surface ; the
blade being doubled together longitudinally, so that what may be
considered its under surface is entirely concealed. The two halves
are adherent together at their upper part, but at their lower they
are commonly separated by a new leaf which comes-up between
them ; and it is from this arrangement, which resembles the posi-
g g 2
452 MICROSCOPIC STEUCTUEE OF PHANEROGAMIC PLANTS.
tion of the legs of a man on horseback, that the leaves of the Iris
tribe are said to be equitant. ISTow b y tracing the middle layer of
colourless cells, d, d, down to that lo wer portion of the leaf where
its two halves diverge from one another, we find that it there
becomes continuous with the cuticle, to the cells of which
(Fig. 244, a) these bear a strong resemblance in every respect save
the greater proportion of their breadth to their length. — Another
interesting variety in leaf- structure is presented by the Water-Lily
and other Plants whose leaves float on the surface ; for here
the usual arrangement is entirely reversed, the closely-set layers of
green leaf-cells being found in contact with the lower surface,
whilst all the upper part of the leaf is occupied by a loose spongy
parenchyma, containing a very large number of air-spaces that
give buoyancy to the leaf ; and these spaces communicate with the
external air through the numerous stomata, which, contrary to the
general rule (§ 352), are here found in the upper cuticle alone.
354. The examination of the foregoing structures is attended
with very little difficulty. Many Cuticles may be torn off, by the
exercise of a little dexterity, from the surfaces of the leaves they
invest, without any preparation : this is especially the case with
Monocotyledonous plants, the veins of whose leaves run parallel,
and with such Dicotyledons as have very little woody structure
in their leaves ; in those, on the other hand, whose leaves are
furnished with reticulated veins to which the cuticle adheres (as
is the case in by far the larger proportion), this can only be
detached by first macerating the leaf for a few days in water;
and if their texture should be particularly firm, the addition of
a few drops of nitric acid to the water will render their cuti-
cles more easily separable. Cuticles may be advantageously
mounted in weak spirit, or in Glycerine-jelly, if it be desired to
preserve them. — Very good sections of most Leaves may be made
by a sharp knife, handled by a careful manipulator ; but it is
generally preferable to use Valentin's knife (§ 152) or the Section-
instrument (§ 153) ; taking care in the former case to cut-down
upon a piece of fine cork ; and in the latter not to crush the
leaf between the two pieces of cork that hold it, very soft cork
being used whenever the delicacy of the leaf renders this
desirable. In order to study the structure of leaves with the
fulness that is needed for scientific research, numerous sections
should be made in different directions ; and slices taken parallel
to the surfaces, at different distances from them, should also be
examined. There is no known medium in which such sections
can be preserved altogether without change ; but some one of
the methods formerly described ('§ 181) will generally be found
to answer sufficiently well.
355. Structure of Flowers. — Many small Flowers are, when
looked-at entire with a low magnifying power, very striking
Microscopic objects ; and the interest of the young in such obser-
vations can scarcely be better excited, than by directing their
STRUCTURE OF FLOWERS.
453
attention to the new view thej thus acquire of the ' composite'
nature of the humble down-trodden Daisy, or to the beauty of the
minute blossoms of many of those Umbelliferous Plants which
are commonly regarded only as rank weeds. The scientific
Microscopist, however, loots more to the organization of the
separate parts of the Flower ; and among these he finds abundant
sources of gratification, not merely to his love of knowledge, but
also to his taste for the beautiful. The general structure of the
sepals and petals, which constitute the ' perianth' or floral envelope,
closely corresponds with that of leaves ; the chief difference lying
in the peculiar change of hue which the chlorophyll almost in-
variably undergoes in the latter class of organs, and very frequently
in the former also. There are some petals, however, whose cells
exhibit very interesting pe-
culiarities, either of form or Fig. 247-
marking, in addition to their
distinctive coloration ;* such
are those of the Geranium
(Pelargonium), of which a
small portion is represented
in Pig. 247. _ The different
portions of this petal, — when
it has been dried after strip-
ping it of its cuticle, im-
mersed for an hour or two
in oil of turpentine, and
then mounted in Canada
balsam, — exhibit a most
beautiful variety of vivid
coloration, which is seen to
exist chiefly in the thickened partitions of the cells ; whilst the
surface of each cell presents a very curious opaque spot with
numerous diverging prolongations. This method of preparation,
however, does not give a true idea of the structure of the cells ;
for each of them has a peculiar mammillary protuberance, the base
of which is surrounded by hairs ; and this it is which gives the
velvety appearance to the surface of the petal, and which, when
altered by drying and compression, occasions the peculiar spots
represented in Fig. 247. Their real character may be brought into
view by Dr. Inman's method ; which consists in drying the petal
(when stripped of its cuticle) on a slip of glass, to which it adheres,
and then placing on it a little Canada balsam diluted with Tur-
pentine, which is to be boiled for an instant over the spirit-lamp,
after which it is to be covered with a thin glass. The boiling
' blisters' it, but does not remove the colour ; and on examination
Cells from tlie Petal of the Geranium.
(Pelargonium).
* See especially Mr. Tuffen West ' On some Conditions of the Cell- Wall in
the Petals of Flowers,' in " Quart. Journ. of Microsc. Science," Vol. vii. (1859),
p. 22.
454 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
many of the cells will be found showing the mammilla very distinctly,
with a score of hairs surrounding its base, each of these slightly
curved, and pointing towards the apex of the mammilla. — The petal
of the common Scarlet Pimpernel (Anagallis arvensis), that of the
common Chickweed (Stellaria media), together with many others
of a small and delicate character, are also very beautiful microscopic
objects ; and the two just named are peculiarly favourable
subjects for the examination of the Spiral vessels in their natural
position. For the ' veins' which traverse these petals are entirely
made-up of spiral vessels, none of which individually attain any
great length ; but one follows or takes the place of another, the
conical commencement of each somewhat overlapping the like ter-
mination of its predecessor ; and where the ' veins' seem to branch,
this does not happen by the bifurcation of a spiral vessel, but by
the ' splicing-on' (so to speak) of one to the side of another, or by
the ' splicing-on' of two new vessels diverging from one another, to
the end of that which formed the principal vein.*
356. The Anthers and Pollen-grains, also, present numerous
objects of great interest, both to the scientific Botanist and to the
amateur Microscopist. In the first place, they afford a good oppor-
tunity of studying that form of ' free' Cell-development which
seems peculiar to the parts concerned in the Eeproductive process,
and which consists in the development of a new cell- wall round an
isolated mass of protoplasm forming part of the contents of a
' parent-cell ;' so that the new cell lies free within its cavity, instead
of being developed in continuity with it, as in the ordinary
methods of multiplication (§ 273). If the Anther be examined by
thin sections at an early stage of its development within the young
flower -bud, it will be found to be made-up of ordinary cellular
parenchyma in which no peculiarity anywhere shows itself : but
a gradual ' differentiation' speedily takes-place, consisting in the
development of a set of very large cells in two vertical rows, which
occupy the place of the loculi or ' pollen chambers' that afterwards
present themselves ; and these cells give origin to the pollen-grains,
whilst the ordinary parenchyma remains to form the walls of the
pollen-chambers. The first change consists in the multiplication
of the cells of the primary row by cell-division, in correspondence
with the general increase in the size of the anther ; until at length
they form masses of considerable size, composed of large squarish
cells, filled with granular contents, well-defined as constituting a
distinct tissue from the walls of the pollen-chambers. The history
of the development of the pollen-grains in their interior is thus
described by Mr. Henfrey, who made a special study of it. " The
contents of each of these cells secrete a layer of cellulose, which
does not adhere to the wall of the parent-cell to form a layer of
secondary deposit, but lies free against it, so that a new free cell is
formed within each old one nearly filling it. The walls of the old
* See Mr. K. H. Solly's description and figure of the petal of the Anagallis,
in " Trans, of Soc. of Arts," Vol. xlviii.
DEVELOPMENT OF POLLEN-GEAINS. 455
cells then dissolve, so that the free cells become free, no longer in
their parent-cells, but in a cavity which is to constitute the pollen-
chamber or loculus of the anther. These free cells are the ' parent-
cells of the pollen' of authors. A new phenomenon soon occurs
in these. These parent-cells divide into four by ordinary cell-
division ; either by one or two successive partings, by septa at right
angles to each other, but both perpendicular to an imaginary axis
(as when an orange is quartered) ; or by simultaneously formed
septa, which cut-off portions in such a manner, that the new cells
stand in the position of cannon balls piled into a pyramid
(tetrahedrally). These new cells are the ' special parent-cells of
the pollen ;' and in each of these the entire protoplasmic contents
secrete a series of layers, which in the ordinary course, by the
solution of the primary walls of the special parent- cells upon which
they were applied, become the walls of free-cells, which constitute
the simple ordinary pollen-cells. These subsequently increase in
size, and their outer coat assumes its characteristic form and ap-
pearance, while free in the chamber of the anther."* This history
bears a very close parallel with that of the development of the
spores within the thee a of Mosses (§ 311); and it is not a little
curious that the layer of cells which lines the pollen chambers should
exhibit, in a considerable proportion of plants, a strong resem-
blance in structure, though not in form, to the elaters of Marchantia
(Fig. 193). For they have in their interior a fibrous deposit; which
sometimes forms a continuous spiral (like that in Fig. 219), as in
ISTarcissus and Hyoscyamus ; but is often broken-up, as it were, into
rings, as in the Iris and Hyacinth ; in many instances forms an
irregular net-work, as in the Yiolet and Saxifrage ; in other cases,
again, forms a set of interrupted arches, the fibres being deficient
on one side, as in the Yellow Water-lily, Bryony, Primrose, &c. ;
whilst a very peculiar stellate aspect is often given to these cells,
by the convergence of the interrupted fibres towards one point of
the cell-wall, as in the Cactus, Geranium, Madder, and many other
well-known plants. Various intermediate modifications exist ; and
the particular form presented often varies in different parts of the
wall of one and the same anther. It seems probable that, as in
Hepaticae, the elasticity of these spiral cells may have some share
in the opening of the pollen-chambers and in the dispersion of the
pollen-grains.
357. The form of the Pollen-grains seems to depend in part upon
the mode of division of the cavity of the parent-cell into quarters ;
generally speaking it approaches the spheroidal, but it is sometimes
elliptical, and sometimes tetrahedral. It varies more, however,
when the pollen is dry, than when it is moist ; for the effect of the
imbibition of fluid, which usually takes-place when the pollen is
placed in contact with it, is to soften-down angularities, and to
bring the cell nearer to the typical sphere. The pollen-cell (save
in a few submerged plants) has a thick outer coat surrounding a
* "Hi orographic Dictionary," 2nd Edition, p. 558.
456 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
thin interior wall ; and this often exhibits very curious markings,
which seem due to an increased thickening at some points and a
thinning-away at others. Sometimes these markings give to the
surface-layer so close a resemblance to a stratum of cells (Fig. 248,
b, c, d), that only a very
Fig. 248. careful examination can
detect the difference.
The roughening of the
surface by spines or
knobby protuberances, as
shown at a, is a very
common feature ; and
this seems to answer the
purpose of enabling the
pollen-grains more rea-
dily to hold to the surface
whereon they may be
cast. Besides these and
other inequalities of the
surface, most pollen
grains have what appear
to be pores or slits in
their outer coat (varying
in number in different
species), through which
the inner coat protrudes
itself as a tube, when
the bulk of its contents
has been increased by
imbibition ; it seems probable, however, that the outer coat is
not absolutely deficient at these points, but is only thinned-away.
Sometimes the pores are covered by little disk-like pieces or lids,
which fall-off when the pollen- tube is protruded. This action takes
place naturally when the pollen-grains fall upon the surface of the
stigma, which is moistened with a viscid secretion ; and the pollen
tubes, at first mere protrusions of the inner coat of their cell,
insinuating themselves between the loosely-packed cells of the
stigma, grow downwards through the style, sometimes even to the
length of several inches, until they reach the ovarium. The first
change, — namely, the protrusion of the inner membrane through
the pores of the exterior, — may be made to take-place artificially, by
moistening the pollen with water, thin syrup, or dilute acids
(different kinds of pollen-grains requiring different modes of treat-
ment) ; but the subsequent extension by growth will only take
place under the natural conditions.
358. The darker kinds of Pollen may be generally rendered
transparent by mounting in Canada balsam ; or, if it be desired to
avoid the use of heat, in the Benzine solution of Canada balsam
(§ 174), setting aside the slide for a time in a warm place. For the
Pollen-grains of, — A, Althcea rosea ; B, Cobcea
scandens ; c, Passiflora ccerulea ; D, Ipomcea pur-
purea.
POLLEN-GEAINS :— OVULES. 457
less opaque pollens, the Damar solution (§ 179) is preferable. The
more delicate pollens, however, become too transparent in either
of these media ; and it is consequently preferable to mount them
either dry or (if they will bear it without rupturing) in fluid. The
most interesting forms are found, for the most part, in plants of
the orders Amarantacem, Giclioraceoe, Cucurbitacece, Malvaceae, and
Passiflorece; others are furnished also by Convolvulus, Campanula^
(Enothera, Pelargonium (Geranium), Polygonum, Sedum, and many
other plants. It is frequently preferable to lay-down the entire
anther, with its adherent pollen-grains (where these are of a kind
that hold to it), as an opaque object ; this may be done with great
advantage in the case of the common Mallow (Malva sylvestris) or
of the Hollyhock (Althcea rosea) ; the anthers being picked soon
after they have opened, whilst a large proportion of their pollen is
yet undischarged ; and being laid down as flat as possible, before
they have begun to wither, between two pieces of smooth blotting-
paper, then subjected to moderate pressure, and finally mounted
upon a black surface. They are then, when properly illuminated,
most beautiful objects for Objectives of 2-3rds, 1, If, or 2 in. focus,
especially with the Binocular Microscope.
359. The structure and development of the Ovules that are pro-
duced within the ovarium at the base of the pistil, and the operation
in which their fertilization essentially consists, are subjects of in-
vestigation which have a peculiar interest for scientific Botanists,
but which, in consequence of the special difficulties that attend the
inquiry, are not commonly regarded as within the province of ordi-
nary Microscopists. — The Ovule, in its earliest condition, is, like the
anther, a mass of cells in which no part is differentiated from the
rest ; gradually this body, which is termed the nucleus, is found to
be enveloped in one, two, or three coats, which are formed by the
multiplication of cells that at first constitute merely an annular
enlargement at its base ; these coats, however, do not entirely close
in around the nucleus, at the point of which there always remains
a small aperture called the micropyle. In the interior of the
nucleus a large cavity is formed, apparently by the enlargement of
one of its cells at the expense of those which surround it ; and this
cavity, which is called the embryo-sac, is at first filled only with a
liquid protoplasm. Some little time before fecundation, however,
a small number of peculiar corpuscles, which seem to be unwalled
masses of viscid protoplasm, are seen lying freely in this liquid,
near the apex of the embryo-sac ; these are incipient germ-cells, of
which one only, the embryonal corpuscle, is ordinarily destined to
be fertilized. This act is accomplished by the penetration of the
pollen-tube, which, when it has made its way down to the ovarium,
enters the micropyle of the ovule, and impinges upon the apex of
the embryo-sac, which it sometimes pushes before it in such a
manner as to have given origin to the idea that the tube enters its
cavity ; no such penetration, however, really takes place ; and it is
only by transudation through the membrane of the embryo-sac, as
458 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.
well as that of the pollen-tube, that the contents of the latter
can reach the interior of the former. As a consequence of this
transudation (the influence of which seems to be the same as that
of the contact of the antherozoids in the Cryptogamia) the ' em-
bryonal corpuscle ' is completed into a cell by the development of a
cellulose-wall around it ; and the production of this ' primordial cell '
lays the foundation of the fabric of the embryo, which is developed
from it like the brood that springs from the ' oo-spore ' of the Pro-
tophytes (§ 218).
360. The early processes of Embryonic Development correspond
closely with those which have been described as taking place
through the whole of the inferior tribes ; for the ' primordial cell '
gives origin by binary subdivision to a pair, this again to four, and
so on; it being usually in the terminal cell of the filament so
generated, that the process of mutiplication chiefly takes place, as in
the Confervae (§ 273). The filament then begins to enlarge at its
lower extremity, where its cells are often multiplied into a some-
what globular mass ; of this mass, by far the larger proportion is
destined to be evolved into the cotyledons, or ' seed-leaves,' whose
function is limited to the earliest part of the life of the young plant ;
the small remainder is the rudiment of the plumula, which is to be
developed into the stem and leaves ; while the prolonged extremity
of the embryonic filament which is directed towards the micropyle,
is the original of the radicle or embryonal root. The mucilaginous
protoplasm filling the embryo- sac, in which the embryonal corpuscle
was imbedded, becomes converted by the formation of free cells,
soon after fecundation, into a loose cellular tissue, which constitutes
what is known as the endosperm ; this, however, usually deli-
quesces again, as the embryonal mass increases in bulk and presses
upon it.
361. In tracing the origin and early history of the Ovule, very
thin sections should be made through the flower-bud, both vertically
and transversely ; but when the ovule is large and distinct enough
to be separately examined, it should be placed on the thumb-nail
of the left hand, and very thin sections made with a sharp razor ;
the ovule should not be allowed to dry -up, and the section should
be removed from the blade of the razor by a wetted camel-hair
pencil. The tracing-downwards the pollen-tubes through the tissue
of the style, may be accomplished by sections (which, however, will
seldom follow one tube continuously for any great part of its
length), or, in some instances, by careful dissection with needles.
Plants of the Orchis tribe are the most favourable subjects for this
kind of investigation, which is best carried-on by artificially apply-
ing the pollen to the stigma of several flowers, and then examining
one or more of the styles daily. " If the style of flower of an
Epipactis (says Schacht), to which the pollen has been applied
about eight days previously, be examined in the manner above
mentioned, the observer will be surprised at the extraordinary
number of pollen-tubes, and he will easily be able to trace them
FERTILIZATION OF OVULE: —SEEDS.
459
in large strings, even as far as the ovnles. Viola tricolor (Hearts-
ease) and Bibes nigrum and rubruru (Black and Eed Currant) are
also good plants for the purpose ; in the case of the former plant,
withered flowers may be taken, and branched pollen-tubes will not
unfrequently be met with." The entrance of the pollen-tube into
the micropyle may be most easily observed in Orchideous plants
and in Euphrasia ; it being only necessary to tear-open with a
needle the ovary of a flower which is just withering, and to detach
from the placenta the ovules, almost every one of which will be
found to have a pollen-tube sticking in its micropyle. These ovules,
however, are too small to allow of sections being made, whereby
the origin of the embryo may be discerned ; and for this purpose,
(Enothera (Evening Primrose) has been had recourse to by
HofTmeister, whilst Schacht recommends Lathrcea squamaria,
Pedicularis palustris, and particularly Pedicularis sylvatica.
362. We have now, in the last place, to notice the chief points
of interest to the Microscopist which are furnished by mature
Seeds. Many of the smaller kinds of these bodies are very curious,
and some are very beautiful objects, when looked-at in their
natural state under a low magnifying power. Thus the seed of
the Poppy (Fig. 249, a) presents a regular reticulation upon its
Fig. 249.
Seeds, as seen under a low magnifying power : — A, Poppy ,■
B, Amaranthus (Prince's feather) ; C, Antirrhinum majus (Snap-
dragon) ; D, Caryophyllum (Clove-pink) ; E, Bignonia.
surface, pits for the most part hexagonal being left between pro-
jecting walls ; that of Caryophyllum (d) is regularly covered with
curiously -jagged divisions, every one of which has a small bright
460 MICROSCOPIC STEUCTUEE OF PHANEROGAMIC PLANTS.
black hemispherical knob in its middle; that of AmarantJius
hypochondriacus has its surface traced with extremely delicate
markings (b) ; that of Antirrhinum is strangely irregular in
shape (c), and looks almost like a piece of furnace-slag ; and that
of many Bignoniacece is remarkable for the beautiful radiated
structure of the translucent membrane which surrounds it (e).
This structure is extremely well seen in the seed of the Eccre-
mocarpus scaler, a half-hardy climbing plant now common in our
gardens; and when its membranous 'wing' is examined under a
sufficient magnifying power, it is found to be formed by an extra-
ordinary elongation of the cells of the seed-coat at the margin of
the seed, the side-walls of which cells (those, namely, which lie in
contact with one another) are thickened so as to form radiating
ribs for the support of the wing, whilst the front and back walls
(which constitute its membranous surface) retain their original
transparence, being marked only with an indication of spiral
deposit in their interior. In the seed of Dictyoloma Peruviana,
besides the principal 'wing' prolonged from the edge of the seed-
coat, there is a series of successively smaller wings, whose margins
form concentric rings over either surface of the seed ; and all
these wings are formed of radiating fibres only, composed, as in
the preceding case, of the thickened walls of adjacent cells ; the
intervening membrane, originally formed by the front and back
walls of these cells having disappeared, apparently in consequence
of being unsupported by any secondary deposit.* Several other
seeds, as those of Sphenogyne speciosa and Lophospermum eru-
lescens, possess wing-like appendages ; but the most remarkable
development of these organs is said by Mr. Quekett to exist in
a seed of Calosanthes Indica, an East Indian plant, in which the
wing extends more than an inch on either side of the seed. — Some
seeds are distinguished by a peculiarity of form, which, although
readily discernible by the naked eye, becomes much more striking
when they are viewed under a very low magnifying power ; this is
the case, for example, with the seeds of the Carrot, whose
long radiating processes make it bear, under the Microscope, no
trifling resemblance to some kinds of Star-fish ; and with those of
Cyanthus minor, which bear about the same degree of resemblance
to shaving-brushes. In addition to the preceding, the following
may be mentioned as seeds easily to be obtained, and as worth
mounting for opaque objects : — Anagallis, Anethum graveolens,
Begonia, Carum carui, Goriopsis tinctoria, Datura, Delphinium,
Digitalis, Elatine, Erica, Gentiana, Gesnera, Hyoscyamus, Hype-
ricum, Lepidium, Limnocharis, Linaria, Lychnis, Mesemlryan-
themum, Nicotiana, Origamme onites, Orolanche, Petunia, Peseda,
Saxifraga, Scrophularia, Sedum, Sempervivum, Silene, Stellaria,
Symphytum asperrimum, and Verbena. The following may be
* See Brady in " Transactions of Microsc. Society," N.S., VoL ix. (1861),
p. 65.
STRUCTURE OF SEED-COATS. 461
mounted as transparent objects in Canada balsam : — Drosera,
Hydrangea, Monotrojia, Orchis, Parnassia, Pyrola, Saxifraga*
The seeds of Umbelliferous plants generally are remarkable for the
peculiar vittce, or receptacles for essential oil, which are found in
their coats. "Various points of interest respecting the structure of
the testce or envelopes of seeds, — such as the Fibre-cells of Cobcea
and Collomia, the Stellate cells of the Star-Anise, and the densely-
consolidated tissue of the ' shells ' of the Coquilla-nut, Cocoa-nut,
&c, — having been already noticed, we cannot here stop to do more
than advert to the peculiarity of the constitution of the husk of
the Corn-grains, In these, as in other Grasses, the ovary itself
continues to envelope the seed, giving a covering to it that sur-
rounds its own testa : this covering (which forms the ' bran ' that
is detached in grinding) is oomposed of hexagonal cells of remark-
able regularity and density ; and these are so little altered by
a high temperature, as still to be readily distinguishable when the
grain has been ground after roasting, — thus enabling the Micro-
scopist to detect even a small admixture of roasted Corn with
Coffee or Chicory, without the least difficulty .f
* These lists have been chiefly derived from the " Micrographic Dictionary."
f In a case in which the Author was called-upon to make such an investi-
gation, he found as many as thirty distinctly-recognizable fragments of this
cellular envelope, in a single grain of a mixture consisting of Chicory with
only 5 per cent, of roasted Corn.
CHAPTER IX.
MICROSCOPIC FORMS OF ANIMAL LIFE: — PROTOZOA; ANIMALCULES.
363. Passing-on, now, to the Animal Kingdom, we begin by-
directing onr attention to those minnte and simple forms, which
correspond in the Animal series with the Protophyta in the
Vegetable (Chap. VI.) ; and this is the more desirable, since the
formation of a distinct gronp to which the name of Protozoa (first
proposed by Siebold) may be appropriately given, is not merely
one of the most interesting results of recent Microscopic inquiry,
but is a subject on which it is particularly important that the
Microscopic observer should know what the Physiologist believes
himself to have ascertained. This group, which must be placed at
the very base of the Animal scale, beneath the great Sub-King-
doms marked-out by Cuvier, is characterized by the extreme sim-
plicity that prevails in the structure of the beings composing it ;
for in the lowest of them there is absolutely nothing that can be
properly called ' organization,' while even in the highest there is
no such differentiation of parts as constitutes the ' organs ' of the
very simplest Zoophyte or Worm. — As we have seen (§ 202) that
among the lowest Protophytes all the essential processes of Vege-
tative life may be carried on by a minute mass of ' protoplasm '
which is not even bounded by a distinct limitary membrane, so as
to constitute a cell, — the differentiation between cell-wall and cell-
contents not having yet manifested itself, — so amongst the lowest
Protozoa, we find the power of maintaining an independent exis-
tence of a kind essentially similar to that of the higher Animals, to
be possessed by similar particles of that peculiar blastema or for-
mative substance, to which the name sarcode (expressive of its
rudimentary relation to the flesh of higher animals) was given by
Dujardin, who first drew attention to its extraordinary endow-
ments. This Animal ' sarcode ' very closely resembles the ' proto-
plasm ' of Vegetables in chemical composition and behaviour with
re-agents, and in many of its vital manifestations ; *but without
affirming that there is a strict and absolute boundary between
Animals and Vegetables, we may generally recognise a distinction
between a simple Protophyte and a simple Protozoon, in regard alike
to the nature of the aliment on which each respectively is sup-
ported, and to the means by which that aliment is introduced
§ 198).
DISTINCTION BETWEEN PLANTS AND ANIMALS. 463
364. Hence these simplest members of the two Kingdoms, which
can scarcely be distinguished from each other by any structural
characters, seem (as a general rule) to be physiologically separable,
by the mode in which they perform those actions wherein their
life most essentially consists : for the Protophyte decomposes
Carbonic acid under the influence of Light, and generates Chloro-
phyll and Albuminous compounds, in a manner in all respects
comparable to that in which the same operations are performed by
the leaf-cells of the most perfect Plant ; whilst the Protozoon
ingests and digests both Vegetable and Animal food, and applies
it to the nutrition of its body, no less effectively than an Aiimal
possessing the most complex digestive and circulating apparatus.
And in the present state of our knowledge, we seem justified
in laying it down as the most ready and certain differential
character we are acquainted with, between those Protophytes and
Protozoa which are apparently most closely related to each other
in the simplicity of their structure, that the former (with the
exception of the Fungi) decompose carbonic acid under the in-
fluence of light, and acquire a red or green colour from the new
compounds which they form, in their interior ;* whilst the latter,
having no such power, receive animal and vegetable organisms, or
particles of such, into the interior of their bodies, where they
extract from them the ready -prepared nutriment they are fitted to
yield. The most marked exception to this general principle seems
to be presented by the Amoeba-like zoospores of the Myxogastric
Fungi (§ 300), which, during their active state, seem to take in
and to appropriate solid organic particles. And according to the
observations of Cienkowski,f the same is true of the Amoeba-like
bodies which constitute one stage in the life of Monads. For they
are observed to lose their long cihum, by the lashing action of
which they were rapidly propelled (like the motile forms of Proto-
coccus, § 208), and to become amcebiform ; and in this state they
are seen to feed like true Amoeba (§ 376). After a time, however,
they cease to move, become enclosed in a cellulose envelope, and
become coloured with chlorophyll ; their life thus becoming truly
vegetal. The endochrome-mass contained within the cyst breaks
up into four or more segments, each of which on its escape from
the envelope becomes a new Monad. — These observations render it
* Many instances have been cited of Animalcules acquiring a green colour
by the decomposition of Carbonic acid under the influence of light ; but there
can be no doubt in the mind of any one who is familiar with the results of re-
cent Microscopic research, that in most of these cases, if not in all, the supposed
Animalcules were really Protojrfiytes. There is, however, more difficulty in
regard to the Spongllla, or fresh-water Sponge (§ 465), which, while unquestion-
ably allied in its general structure and development to marine Sponges (whose
animality cannot be doubted), seems to have the vegetable attribute of decom-
posing Carbonic acid, and of generating Chlorophyll, under the influence of
light. (See Hogg, in "Linnasan Transactions," vol. xviii.)
f "Beitrage zur kentniss der Monaden," in Schultze's "Archiv fur Mi-
kroskop. Anai," Bd. I. (1865), p. 203.
464 MICROSCOPIC FORMS OF ANIMAL LIFE.
probable that the production of amoebiform bodies observed by
Dr. Hicks to take place within the Volvox- sphere, constitutes one
mode of the reproduction of that type (§ 217).
365. It has been proposed by Prof. Hasckel to revive the old idea of
a Kingdom of Nature intermediate between Animals and Plants,
for which he proposes the name Protista. But nothing seems to
be really gained by such an arrangement ; and of the groups in-
cluded in it by Prof. Hseckel, some, as Diatomacem and Volvocince,
are unquestionably Plants ; whilst others, as Bhizopods, Sponges,
and Noctilucce are as certainly Animals. When we know the whole
life-history of each type, we shall be able pretty certainly to rank
it on the one side or the other of the boundary-line between the two
Kingdoms ; notwithstanding that in some phase of its existence it
may cross that line, and take upon itself a mode of life different
from that by which it is usually characterized. There seems good
reason, however, for adopting Prof. Hseckel's proposal to institute a
group even lower than the Rhizopods, which have been usually
regarded as the simplest types of Protozoa ; these Moners, as he
designates them, being simply particles of living jelly, having
neither ' nucleus' nor ' contractile vesicle,' and showing no differen-
tiation into ' ectosarc' and ' endosarc' (§ 369), and yet possessing
the power not only of changing their forms by contraction and ex-
tension, but also of putting forth ' pseudopodia,' like those of Rhizo-
pods, and of thereby drawing minute particles from without into
their own substance, so as (presumably) to be nourished by them.*
It is impossible to conceive anything simpler ; and the existence of
such Monerozoa clearly indicate that Life is a property of the
molecules of the matter which exhibits it, and does not depend upon
that arrangement which we call Organization, — this being simply
the result of a differentiation of parts, whereby the attributes that
here belong to the generalized sarcode, are specialized in particular
structures.
366. To this group it would seem that we are to refer these in-
definite expansions of Protoplasmic substance, which there is much
reason to regard as generally spread over the Deep- Sea-bed. When
examining, in 1868, the ' globigerina-mud' brought up by the
Cyclops soundings in 1857, Prof. Huxley was struck with its
peculiar viscidity ; and found this to be due to the presence of " in-
numerable lumps of a transparent gelatinous substance, which are
of all sizes, from patches visible with the naked' eye to excessively
minute particles :"f diffused through this substance, he found heaps
of very minute granules from 1 -40,000th to l-8000th of an inch in
diameter ; and also the larger particles of more definite form which
he had first noticed in 1847, and had designated as coccoliths, as
well as the larger spherical aggregations first observed by Dr.
* See his " Mono grap hie der Moneren," in " Jenaische Zeitschrif t," Bd. iv.
Heft 1 ; translated in " Quart. Journ. of Microsc. Sci.," N. S., vol. ix. (1869).
t "Qnart. Journ. of Microsc. Sci.," N. S., vol. viii. p. 205, et seq.
PLATE XIII.
■**
CoSOUTOMSCOTj PoiUEl-SCiLEi BilHYBinS AND COCCOLIIHS.
[To face p. 4li5.
COCCOLITHS AND COCCOSPHERES. 465
Wallich (1860), and designated by him as coccosplieres. Eegardmg
the gelatinous matrix as a new form of those simple animated
beings which have been so well described by Haeckel, he proposed
to confer on it the generic name Bathybius, indicative of its habitat
in the depths of the sea. His idea of its characters has been fully
accepted and confirmed by Haeckel ;* whose representation of a
living specimen of Bathybius is given in Plate XIII., fig. 4.
367. Two distinct types are recognizable among the Coccoliths,
which Prof. Huxley has designated respectively discoliths and
cyatholiths. The ' discoliths' are round or oval disks, having a thick
strongly -refracting rim, and a thinner internal portion, the greater
part of which is occupied by a slightly-opaque, cloud-like patch lying
round a central corpuscle (Plate XIII., fig. 5). In general, the disco-
liths are slightly convex on one side, slightly concave on the other,
and the rim is raised into a prominent ridge on the more convex
side ; so that when viewed edgewise, they present the appearances
shown in figs. 8, 9. The ordinary length of the discoliths is between
l-400Uth and l-5000th of an inch ; but they range between l-2700th
and l-ll,000th. The largest are commonly free ; but the smallest
are generally found imbedded among heaps of granular particles
of which some are probably discoliths in an early stage of develop-
ment.— The ' cyatholiths,' also, when full grown, have an oval
c tntour ; though they are often circular when immature. They
are convex on one face, and fiat or concave on the other ; and when
left to themselves, they lie on one or other of these two faces. In
either of these aspects, they seem to be composed of two concentric
zones (Plate XIII., fig. 6, 2, 3) surrounding an oval thick-walled
central corpuscle (1), in the centre of which is a clear space
sometimes divided into two. The zone (2) immediately surround-
ing the central corpuscle is usually more or less distinctly granular,
and sometimes has an almost bead-like margin. The narrower
outer zone (3) is generally clear, transparent, and structureless ;
but sometimes shows radiating striae. When viewed sideways or
obliquely, however, the ' cyatholiths' are found to have a form
somewhat resembling that of a shirt-stud (figs. 7, 10, 11). Each
consists of a lower plate, shaped like a deep saucer or watch-
glass ; of a smaller upper plate, which is sometimes flat, some-
times more or less concavo-convex ; of the oval, thick- walled,
flattened corpuscle, which connects these two plates together at
their centres ; and of an intermediate granular substance, which
more or less completely fills up the interval between the two plates.
The length of these cyatholiths ranges from about l-160oth to
l-8000th of an inch, those of 1 -3000th of an inch and under being
always circular. It appears from the action of dilute acids upon
the coccoliths, that they must mainly consist of calcareous matter,
as they readily dissolve, leaving scarcely a trace behind. When
the cyatholiths are treated with very weak acetic acid, the central
* " Jenaische Zeitsclirift," Bd. v. p. 499 et seq.
H K'
466 MICROSCOPIC FOKMS OF ANIMAL LIFE.
corpuscle rapidly loses its strongly refracting character ; and
there remains an extremely delicate, finely-granulated membra-
nous framework. When treated with iodine, they are stained,
but not very strongly ; the intermediate substance being the
most affected. Both discoliths and cyatholiths are completely
destroyed by strong hot solutions of caustic potass or soda.
— The Coccospheres (fig. 5) are made up by the aggregation of
bodies resembling ' cyatholiths' of the largest size in all but the
absence of the granular zone ; they sometimes attain a diameter
of 1 -760th of an inch.
368. What is the relation of the Coccospheres to the Coccoliths,
and that of both to the Bathybius in which they are found im-
bedded, are questions whereon no positive judgment can be at
present given. By Prof, Ltuxley (loc. cit.) it was surmised " that
they are not independent organisms, but that they stand in the
same relation to the protoplasm of Bathybius, as the spicula
of Srjonges or Radiolaria do to the soft part of those animals."
But Prof. Haeekel has since described a very curious Radiolarian
organism, Myxobrachia rhopalum* furnished with diverging ap-
pendages, at the ends of which he has detected accumulations
of bodies closely resembling, if not identical with ordinary
' coccoliths ' and ' coccospheres ; ' and he suggests it as a possible
explanation of their presence, that they may be accumulations of
an indigestible residue of the organism (whatever may be its
nature) to which these particles really belong, after the absorp-
tion of all its available nutriment. It seems difficult to believe,
however, that such accumulations should be disposed with the
remarkable regularity which we find them to present in Myxo-
brachia;' and the question must be left open for further inquiry.
It is one fraught with interest, not merely on account of the
enormous extent of this Monerozoic type, and the probability that
it is at the present time serving as the basis of all Marine
Life ; but also from the fact that ' coccoliths' and ' coccospheres,'
differing in no essential particular from those now existing, are
found in great abundance in Chalk, of which the ' globigerina mud'
of the ISTorth Atlantic may be regarded as a continuation, and that
they can also be recognised even in very early Limestones ; showing
that, whatever may be the form of life in which they originate,
that form has probably been continuously persistent in the
Deep Sea from the remotest periods of Geological history. (See
Chap. XIX.).
369. Rhizopoda. — This designation (which means 'root-footed')
was given by Dujardin to a group of minute animals which were
formerly ranked among Infusoria, as an appropriate expression of
the leading feature in their organization,— namely the extension of
their sarcode-body into long processes, termed jpseudopodia (false
* "Jenaisehe Zeitschrift," Bd. v. p. 519 ; and "Quart. Journ. of Microsc.
Sci./' N.B., Vol. xi. (1871), p. 63.
GENERAL CHARACTERS OF RHIZOPODS. 467
feet) which, serve at the same time as instruments of locomotion,
and as prehensile organs for obtaining food. The other characters
by which this group is distinguished from ordinary Animalcules are
for the most part negative ; consisting in the absence of any definite
mouth or digestive cavity, and in the want of an enveloping mem-
brane sufficiently firm to resist the introduction of particles from
without into the substance of the body at any point. That body
may be almost entirely enclosed within a shelly or horny casing ;
but one or more apertures always exist in that casing, through
which the prolongations of the sarcode-body are put forth ; and the
particles of food introduced by their instrumentality no more enter
into the interior of that body by any definite mouth, than they do
in the naked or shell -less forms. In the lowest Ehizopods, indeed,
there seems no distinction whatever between the containing and the
contained portion of the sarcode-body, the whole being apparently
composed of a viscid homogeneous protoplasm. In the highest,
which most nearly approach those more elevated Protozoa that
exhibit a more or less definite organization, there is a decided
differentiation between the external or containing and the internal
or contained portion of the sarcode-body ; to the former, which
sometimes has an almost membranous firmness, the name ecto-
sarc has been given ; whilst the latter, which is a liquid of almost
watery thinness, has received the name of endosarc. Now upon
the degree of this differentiation between the ' ectosarc' and the
' endosarc' depends the character of the pseudopodial prolongations ;
and these may present themselves under three distinct conditions ;
namely (1), as indefinite extensions of the viscid homogeneous pro-
toplasm, freely branching and subdividing into threads of extreme
tenuity, and undergoing complete mutual coalescence wherever
they come into contact (Fig. 250), so as to form an irregular net-
work that may be likened to an animated spider's-web ; (2) as more
definite rod-like extensions of the ectosarc, having a more or less
regular radiating arrangement (Fig. 251), and exhibiting little
disposition either to ramify or to coalesce, so as almost constantly
to maintain their distinctness ; (3) as lobose extensions of the body
itself, having like it an almost membranous ectosarc with a ver}^
liquid endosarc, and exhibiting an entire absence of any tendency
either to ramify or to coalesce when they come into mutual contact
(Figs. 252, 253). > To the first of the Orders thus marked-out, the
name lleticularia seems appropriate ; the second have been dis-
tinguished as Radiolaria ; and the third may be designated Lobosa.
It must be freely admitted, however, that these groups cannot
be distinctly marked out ; the typical examples which will now
be described being connected by many intermediate forms. This
is not to be wondered at, when the extreme indefiniteness which
characterizes this lowest type of Animal existence is duly borne
in mind.*
* For a more detailed exposition of his " Systematic Arrangement of the
H H 2
468 MICEOSCOPIC FOEMS OF ANIMAL LIFE.
370. Beticularia. — The peculiarities of this type have been
most fully studied in a remarkable naked form, which has been
described by MM. Claparede and Lachmann* under the name
of Lieberhilhnia. The whole substance of the body of this
animal and its pseudopodial extensions is composed of a homo-
geneous, semifluid, granular protoplasm ; the particles of which,
when the animal is in a state of activity, are continually per-
forming a circulatory movement, which may be likened to the
rotation of the particles in the protoplasmic network within the
cell of a Tradescantia (§ 324). The entire absence of anything
like a membranous envelope is evinced by the readiness with
which the pseudopodian extensions coalesce whenever they come
into contact, and with which the principal branches subdivide into
finer and yet finer threads, by whose continual inosculations a
complicated network is produced. Any small alimentary particles
that may come into contact with the glutinous surface of the
pseudoj^odia, are retained in adhesion by it, and speedily partake
of the general movement going on in their substance. This move-
ment takes place in two principal directions ; from the body
towards the extremities of the pseudopodia, and from these
extremities back to the body again. In the larger branches a
double current may be seen, two streams passing at the same
time in opposite directions ; but in the finest filaments the
current is single, and a granule may be seen to move in one of
them to its very extremity, and then to return, perhaps meeting
and carrying back with it a granule that was seen advancing in
the opposite direction. Even in the broader processes, granules
are sometimes observed to come to a stand, to oscillate for a
time, and then to take a retrograde course, as if they had been en-
tangled in the opposing current, — just as is often to be seen
in Ghara. "When a granule arrives at a point where a filament
bifurcates, it is often arrested for a time until drawn into one or
Rhizopoda," see the Author's Memoir on that subject in the " Natural History
Review," October, 1861 ; and his "Introduction to the Study of the Forami-
nifera," published by the Ray Society, 1862. — Another Classification has been
more recently proposed by Dr. Wallich, Avhose Memoir on the Structure and
Affinities of the Polycysiina ("Transact, of Microsc. Society," N.S., Vol. xiii.,
1865, p. 57) contains much important information derived from personal obser-
vation. An important Memoir on the Rhizopods has been recently published
by Dr. Hartwig and E. Lesser, in Schultze's " Archiv fur Mikroskop. Anat.,"
Bd. x. (1874) Supplement-heft ; in which several new and interesting forms are
described, and much is added to our knowledge of the group. So much yet
remains to be learned, however, in regard to the life-history of the Rhizopods,
and especially as to their sexual Generation, that the Author does not think it
worth while yet to abandon his own classification, which he looks upon as
purely provisional, for another system which may prove to be equally destitute
of the characters of permanence.
* "Etudes stir les Infusoires et les Rhizopodes ;" Geneva, 1850-1861. The
beautiful figure of Lieberkiihnia, given by M. Claparede, has been reproduced
by the Author in Plate 1 of his " Introduction to the Study of the Foramini-
fera."
RETICULAEIAN EHIZOPODS :— GBOMIA.
469
tlie otter current ; and when carried across one of the bridge-
like connections into a different band, it not unfrequently meets a
current proceeding in the opjoosite direction, and is thus carried
back to the body without having proceeded very far from it.
Fig. 250.
Gromia oviformis, with its pseudopodia extended.
The p seudopodial network along which this ' cyclosis ' takes place,
is con tinually undergoing changes in its own arrangement ; new
filaments being put forth in different directions, sometimes from
its margin, sometimes from the midst of its ramifications, whilst
470 MICROSCOPIC FOEMS OF ANIMAL LIFE.
others are retracted. ISTot unfrequently it happens that to a spot
where two or more filaments have met, there is an afflux of the
protoplasmic substance that causes it to accumulate there as a
sort of secondary centre, from which a new radiation of filamen-
tous processes takes place, just as in Fig. 250. The entire absence
of differentiation in the protoplasmic substance, the freedom of
the mutual inosculation of its pseudopodial extensions, and the
active cyclosis incessantly going- on between these and the body,
are three mutually -related conditions, which not only serve to
characterize the group of Animals that exhibits them, but to
differentiate that group from others. There is, moreover, a nega-
tive character of much importance, which is naturally associated
with the absence of differentiation, — namely, the deficiency of the
s nucleus ' and of the ' contractile vesicle ' that present themselves
alike in the Radiolaria and in the Lobosa.
371. It is by Animals belonging to this Order, that those very
remarkable minute Shells are formed, which are known under the
designation Fobaminifera. These constitute a group of organisms
altogether so peculiar, and presenting so many features of interest,
as to call for a somewhat detailed, account of them, which will be
most conveniently given in a separate Chapter (Chap. X.).
372. In G-romia, however, we have an example of a Ehizopod
which very characteristically exhibits the Eeticularian type in the
disposition of its pseudopodia (Fig. 250), but which, as Dr. Wal-
lich first pointed out (op. cit. p. 60), possesses both nucleus and
contractile vesicle, and thus shows a transition to the higher
orders. The sarcode-body of this animal is enclosed in an egg-
shaped brownish-yellow membranous ' test,' which seems to be
composed of the horny substance termed chitine ; and this has a
single round orifice, whence issue very long pseudopodia that
spread at their base over the external surface of the ' test ' so as to
form a continuous layer, from any portion of which fresh pseudo-
podia may extend themselves. The smooth coloured ' test ' of Gromia,
which commonly attains a diameter of from 1-1 0th to 1-1 2th of an
inch, looks to the naked eye very much like the egg of a Zoophyte
or the seed of an aquatic Plant ; and its real nature would not be
suspected until, after an interval of rest, the animal begins to
creep about by means of its pseudopodia, and to mount along the
sides of the glass vessel that contains it. Some Gromice are
marine, and are found among tufts of Corallines and Algse ;
whilst others inhabit fresh water, adhering to Confervas and other
plants of running streams.
373. Radiolaria. — A characteristic example of this Order is
presented by the Actinophrys sol (Fig. 251), a minute creature
which is not uncommon in ponds and lakes, occurring for the
most part amongst Confervas and other aquatic plants, and
distinguishable with the naked eye as a whitish-grey motionless
spherical particle. The sarcode of which the body and pseudo-
podia of Actinophrys are composed, is less homogeneous than that
EADIOLAEIAN EHIZOPODS :— ACTIXOPKRYS.
471
of Gromia and its allies ; its external layer or ' ectosarc ' "being
more condensed, while its contained substance or ' endosarc ' is
more liquid. Although the existence of a ' nucleus ' in Actinophiy s
has been denied, yet its presence (in certain species at least) must
be regarded as a well-established fact. It presents itself as a flat-
tened vesicular body with a well-defined margin, usually of circular
outline, and very pellucid ; and its central portion is occupied by
an aggregation of granular particles, less defined at its margin
and less regular in shape. It may be brought into view either by
crushing the body of the animalcule, or by treating it with dilute
Fig. 251.
Actinophrys sol, in different states : — A, in its ordinary svm-
like form, with a prominent contractile vesicle o ,• B, in the
act of division or of conjugation, with two contractile vesicles
o, o ; C, in the act of feeding ; D, in the act of discharging
faecal (?) matters, a and 6.
acetic acid. Throughout the body, but more particularly near its
surface, there are to be observed ' vacuoles ' occupied by a watery
fluid ; these have no definite boundary, and may easily be arti-
ficially made either to coalesce into larger ones, or to subdivide
into smaller ; sometimes they have such a regularity of arrange-
ment as to give to the intervening sarcocle-substance the appear-
ance of a cellular structure. A 'contractile vesicle,' pulsating
rhythmically with considerable regularity, is always to be dis-
tinguished either in the midst of the sarcode-body or (more com-
monly) at or near its surface ; and it sometimes projects con-
siderably from this in the form of a flattened sacculus with a deli-
cate membranous wall, as shown at o. It has been stated by
various observers that the cavity of this sacculus is not closed
472 MICROSCOPIC FORMS OF ANIMAL LIFE.
externally, but communicates with the surrounding medium ;
and this appears to have been fully established by the careful
observations of Dr. Zenker .* There does not seem to be any
distinct and permanent orifice ; but the membraniform wall
gives way when the vesicle contracts, and then closes-over again.
This alternating action seems to serve a respiratory purpose, the
water thus taken in and expelled being distributed through a
system of channels and vacuoles excavated in the substance of the
body ; some of the vacuoles which are nearest the surface being
observed to undergo distension when the vesicle contracts, and to
empty themselves gradually as it re-fills.
374. The body of this animal is nearly motionless, but it is sup-
plied with nourishment by the instrumentality of its pseudopodia ;
its food being derived not merely from Vegetable particles, but
from various small Animals, some of them (as the young of Ento-
mostraca) possessing great activity as well as a comparatively high
organization. When any of these happen to come into contact
with one of the pseudopodia, this usually retains it by adhesion;
but the mode in which the particle thus taken captive is intro-
duced into the body, differs according to circumstances. When the
prey is large and vigorous enough to struggle to escape from its
entanglement, it may usually be observed that the neighbouring
pseudopodia bend over and apply themselves to it, so as to assist
in holding it captive, and that it is slowly drawn by their joint
retraction towards the body of its captor. Any small particle not
capable of offering active resistance, on the other hand, may be
seen after a little time to glide towards the central body along the
edge of the pseudopodium, without any visible movement of the
latter, much in the same manner as in Gromia. When in either of
these modes the food has been brought to the surface of the body,
this extends over it on either side a prolongation of its own
sarcode- substance ; and thus a marked prominence is formed
(Fig. 251, c) which gradually subsides as the food is drawn more
completely into the interior. The struggles of the larger Animals,
and the ciliary action of Infusoria and Botifera, may sometimes
be observed to continue even after they have been thus received
into the body ; but these movements at last cease, and the process
of digestion begins. The alimentary substance is received into one
of the vacuoles of the ' endosarc,' where it lies in the first instance
surrounded by liquid ; and its nutritive portion is gradually con-
verted into an undistinguishable gelatinous mass, which becomes
incorporated with the material of the sarcode-body, as may be
seen by the general diffusion of any colouring particles it may con-
tain. Several vacuoles may be thus occupied at one time by
alimentary particles ; frequently four to eight are thus dis-
tinguishable, and occasionally ten or twelve ; Ehrenberg, in one
* See Schultze's "Arch. f. Mikrosk. Anatomie," Bd. ii. p. 232; and " Quart.
Journ. of Microsc. Science," Vol. vii., N.S. (1867), p. 263.
EADIOLAEIAN EHIZOPODS : — POLYCYSTINA.. 473
instance, connted as many as sixteen, which he described as mul-
tiple stomachs. Whilst the digestive process, which usually
occupies some hours, is going on, a kind of slow circulation takes
place in the entire mass of the endosarc with its included vacuoles.
If, as often happens, the body taken in as food possesses some
hard indigestible portion (as the shell of an Entomostracan or
Eotifer), this, after the digestion of the soft parts, is gradually
pushed towards the surface, and is thence extruded by a process
exactly the converse of that by which it was drawn in : if the par-
ticle be large, it usually escapes at once by an opening which (like
the mouth) extemporizes itself for the occasion (Fig. 251, d) ; but
if small, it sometimes glides along a pseudopodium from its base
to its point, and escapes from its extremity. What is known
regarding the reproduction of Adinoplinjs will be presently stated
(§§ 381, 382) *
375. The Order Badiolaria includes various forms of Rhizopods
which agree with Actinophrys in the leading peculiarities of its
structure, but which differ in having the body included in an
envelope of more or less firm consistence. This may be formed
simply of a membranous or a chitinous exudation, as in certain
genera which represent in this order the Groin la among the
Reticularis., and the Arcella and BifHugia among the Lobosa. But
the types in this group -that are of most general interest to the
Microscopist are the Polycystina and Marine Ra.diolae.ia, whose
bodies are furnished with Siliceous skeletons of most wonderful
beauty and variety of form and structure ; these will be de-
scribed, with the Foraminifera, in a separate chapter (Chap. X.).
Some beautiful fresh-water forms, bearing a strong resemblance
to the marine Radiolaria, have been described by Mr. Archer.f
376. Lobosa. — No example of the Rhizopod type is more common
in streams and ponds, vegetable infusions, &c, than the Amoeba
(Fig. 252) ; a creature which cannot be described by its form, for
this is as changeable as that of the fabled Proteus, but which may
yet be definitely characterized by peculiarities that separate it from
the two groups already described. The distinction between
' ectosarc ' and ' endosarc ' is here clearly marked, so that the body
approaches much more closely in its characters to an ordinary cell
composed of cell-wall and cell-contents. It is through the ' endo-
sarc ' alone that those coloured and granular particles are diffused,
on which the hue and opacity of the body depend ; its central por-
tion seems to have an almost watery consistence, the granular par-
ticles being seen to move quite freely upon one another with every
* The following Memoirs should be consulted by such as vrish to apply
themselves to the study of this interesting organism": — Kolliker and Cohn, in
uSiebold and Kolliker's Zeitschrift," 1849 and 1851 ; Claparede, in "Ann. of
Nat. Hist.," 2nd Ser., Vol xv. pp. 211, 285, and in his "Etudes sur les
Infusoires " (1S65), 2ieme Partie; Weston, in "Quart. Journ. of Microsc.
Science," Vol. iv. p. 116.
t "Quart. Journ. of Microsc. Sci.," Vol. ix., N.S. (1869), p. 250.
474
MICKOSCOPIC FOEMS OF ANIMAL LIFE.
change in the shape of the body; hat its superficial portion is
more viscid, and graduates insensibly into the firmer substance of
the ' ectosarc' The ectosarc, which is perfectly pellucid, forms an
almost membranous investment to the endosarc ; still it is not pos-
sessed of such tenacity as to oppose a solution of its continuity at
any point, for the introduction of alimentary particles, or for the
«
Fig. 252.
Amoeba princeps, in different forms, A, B, c.
extrusion of effete matter; and thus there is no evidence, in
Amoeba and its immediate allies, of the existence of any more
definite orifice, either oral or anal, than exists in other Bhizopods.
The more advanced differentiation of the ectosarc and the endo-
sarc of Amoeba is made evident by the effects of re-agents. If, as
Auerbach has shown, an Amoeba radiosa be treated with a dilute
alkaline solution, the granular and molecular endosarc shrinks
together and retreats towards the centre, leaving the radiating
extensions of the ectosarc in the condition of cascal tubes, of
which the walls are not soluble, at the ordinary temperature,
either in acetic or mineral acids or in dilute alkaline solutions ;
thus agreeing with the envelope noticed by Cohn as possessed by
Paramecium and other ciliated Infusoria, and with the containing
membrane of ordinary animal cells. A 'nucleus' is always
distinctly visible in Amoeba, adherent to the inner portion of the
ectosarc, and projecting from this into the cavity occupied by the
endosarc ; when most perfectly seen, it presents the aspect of a
clear flattened vesicle surrounding a solid and usually spherical
nucleolus ; it is readily soluble in alkalies, and first expands and
then dissolves when treated with acetic or sulphuric acid of
moderate strength ; but when treated with diluted acids it is
LOBOSE EHIZOPODS :— AMCEBA. 475
rendered darker and more distinct, in conseqnence of the precipita-
tion of a finely granular substance in the clear vesicular space
that surrounds the nucleolus. A 'contractile vesicle' seems also
to be uniformly present ; though it does not usually make itself so
conspicuous by its external prominence as it does in Actinoplirys.
377. In all these particulars, therefore, the Amcebina, present a
nearer approach to Infusoria than is discernible among other
Ehizopods ; and they tend towards Infusoria, also, in their higher
locomotive powers, obtaining their food by actively going in search
for it, instead of entrapping it and drawing it into the substance of
their bodies by the agency of their extended pseudopodia. The
pseudopodia, which are not so much appendages, as lobate exten-
sions of the body itself, are few in number, short, broad, and
rounded ; and their outlines present a sharpness which indicates
that the substance of which their exterior is composed possesses
considerable tenacity. No movement of granules can be seen to
take place along the surface of the pseudopodia ; and when two of
these organs come into contact, they scarcely show any disposition
even to mutual cohesion, still less to a fusion of their substance.
Sometimes the protrusion seems to be formed by the ectosarc alone,
but more commonly the endosarc also extends into it, and an active
current of granules may be seen to pass from what was previously
the centre of the body into the protruded portion, when the latter is
undergoing rapid elongation ; whilst a like current may set towards
the centre of the body from some other protrusion which is being
withdrawn into it. It is in this manner that an Amoeba moves
from place to place ; a protrusion like the finger of a glove being
first formed, into which the substance of the body itself is gradually
transferred ; and another protrusion being put forth, either in the
same or in some different direction, so soon as this transference has
been accomplished, or even before it is complete. The kind of pro-
gression thus executed by an Amoeba is described by most observers
as a ' rolling' movement, this being certainly the aspect which it
commonly seems to present ; but it is maintained by MM. Claparede
and Lachmann that the appearance of rolling is an optical illusion,
for that the nucleus and contractile vesicle always maintain the
same position relatively to the rest of the body, and that ' creeping'
would be a truer description of their mode of progression. It is in
the course of this movement from place to place, that the Amoeba
encounters particles which are fitted to afford it nourishment ; and
it appears to receive such particles into its interior through any
part of the ectosarc, whether of the body itself or of any of its
lobose expansions, insoluble particles which resist the digestive
process being got rid of in the like primitive fashion.
378. Although several different forms of Amceboe have been
specified by authors as distinguished by what seemed well marked
peculiarities, yet the longer the study of them has been continuously
carried on, the more obvious has it become that these peculiarities are
transitory, so that the reputed species may be merely phases in the
476 MICROSCOPIC FORMS OF ANIMAL LIFE.
life of one and the same organism. Thus Dr. Wallich, having met
in the course of his very careful study of this type, with a form
that seemed uniformly distinguished by the presence of a set of
villous processes at one end, which it sometimes used as instru-
ments of prehension, at first assigned to it a distinct specific rank
under the name of A. villosa ; but he subsequently came to regard
this as only a peculiar development of the ordinary type, perhaps
depending on some special condition of the water it inhabits.
379. Bearing in mind what has been already stated (§§ 217, 300,
364) as to the amoebiform condition of the germinal granules of
many very dissimilar organisms, it can scarcely be thought im-
probable that what we are accustomed to regard as true Amoabce
should pass at some period of their existence into a phase altogether
different. The following statement, recently put on record by an
observer whose statements in regard to another type (§ 273) bear
the marks of care and intelligence, deserves attention, and may
stimulate further inquiry : — " On one of the bright days during
last spring, I collected in one of the pieces of fresh water in the
Central Park of New York, a mass of matter containing numerous
individuals belonging plainly to the group of organisms ranked as
Amoebce. To ascertain the origin of these wandering masses of
protoplasm, I watched them at intervals for the better part of
two days, and saw the following changes take place. From an
almost hyaline condition the Amoebce became more and more
granular ; the granules increasing in dimensions until the indi-
viduals appeared to be packed almost full of dense oil-globules.
Then they came to a rest, or at least their hitherto lively move-
ments were arrested ; and presently, near one end, cilia appeared
to be evolved (so to speak) from the mass, one after the other,
until a crown of them was seen surrounding what was plainly now
a defined locality. At the same time a change was going on all over
the Amoeba, by reason of which at last from this simple mass of
albuminoid material a true ciliated Animalcule, belonging, I believe,
to either the genus Kolpoda or Paramecium (which resemble each
other very much), was evolved."*
380. The Amoeban like the Actinophryan type shows itself in the
testaceous as well as in the naked form; the commonest examples
of this being known under the names Arcella and Difflugia. The
body of the former is enclosed in a ' test' composed of a horny
membrane, apparently resembling in constitution the chitine which
gives solidity to the integuments of Insects ; it is usually discoidal
(Fig. 253, c, d) with one face flat and the other arched, the aper-
ture being in the centre of the flat side; and its surface is often
marked with a minute aud regular pattern. The test of Difflugia,
on the other hand, is more or less pitcher- shaped (Fig. 253, a, b), and
is chiefly made up of minute particles of gravel, shell, &c, cemented
* Prof. A. M. Edwards (U. S. A.), in "Monthly Microsc. Journ.," Vol. viii.
(1872), p. 29.
LOBOSE EHIZOPODS; — AMCEBINA.
477
together. In each of these genera, the sarcode-body resembles that
of Amoeba in every essential particular ; the contrast between its
large, distinct, lobose extensions, and the ramifying and inosculating
pseudopoclia of Gromia (Fig. 256), being as obvious as the difference
between an Amoeba and a Lieberhilhnia. A marine example of
Fig. 253.
Testaceous forms of Amctban Bhizopods: — A, Difflugia • pro-
tefformis ; B, Dijflttgia oblcnga; C, Arcella acuminata; D,Arctlla
dentata.
this type, remarkable alike for its extraordinary size and for the
nature of its 'test,' has been described by Dr. 0. Sandahl under
the name Astrorltiza Umicola.* Its form is lenticular, with ir-
regular radiating extensions which occasionally branch ; the
diameter of its central disk sometimes attains 1 -5th of an inch ;
and its ' test' is composed of a spongy substance intermingled with
more solid particles.f This order, however, is not represented by
any group of Calcareous-shelled organisms like the Foramiuiftra,
or by any Siliceous- shelled organisms like the Polycystina.+
381. Reproduction of Rhizopoda. — Yery little is certainly known
respecting the processes by which the multiplication of Ehizopods
is effected. It may often be seen that portions of the sarcode-body
detached from the rest can maintain an independent existence ;
and it is probable that such separation of fragments is the ordinary
mode of increase in this group. Thus when the pseudopoclian lobe
* " Ofversight af Vet. Akad. Forhandl.," 1857, p. 299.
f Prof. Loven, of Stockholm, to wliom the Author was indebted for his first
specimens of this remarkable organism, assured him that it is not uncommon ;
so that it might piobably be found on our own coasts, if carefully looked for.
The Author has since met with it in his deep-sea dredgings, in association
with what seems an allied form having a ' test ' made up by the loose aggrega-
tion of sand- grains, which apparently leads towards the ArenaceotisFor&wmiiem
(§ 432>
£ For more detailed information respecting Amceba and its allies, the reader
may be specially referred to the Memoir of Dr. Auerbach in " Siebold und
Koiliker's Zeitschrift," Band vii., 1856 ; to the "Etudes sur les Infusoires " of
MM. Claparede and Lachmann ; and to the elaborate series of Papers by Dr.
Wallich in the " Annals of Natural History," 3rd Ser., Vols, xi., xii., xiii., 1863
and 1864.
478 MICEOSCOPIC FOEMS OF ANIMAL LIFE.
of an Amoeba has been pnt-forth to a considerable length, and has
become enlarged and fixed at its extremity, the subsequent con-
traction of the connecting portion, instead of either drawing the
body towards the fixed point, or retracting the pseudopodian lobe
into the body, causes the connecting band to thin-away until it
separates ; and the detached portion speedily shoots out pseudo-
podian processes of its own, and comports itself in all respects as
an independent Amoeba. It is an interesting exemplification of the
intimacy of the relation between the form of the pseudopodia and
the properties of the sarcode-body of the Ehizopoda, that any small
separated portion of that body will behave itself after the characte-
ristic fashion of its type ; thus, if the shell of an Arcella be crushed,
so as to force out a portion of its sarcode, and this be detached from
the rest, it will soon begin to put forth lobose extensions like those
of an Amoeba; whilst if the like operation be performed upon a
Polystomella, or any other of the Foraminifera, the detached frag-
ments of the protoplasm will extend itself into delicate ramifying
and inosculating pseudopodia resembling those of Ghromia. We
shall find that the production of the ' polythalamous' (many-
chambered) shells of Foraminifera is due to a repeated gemmation
or budding of the sarcode-body ; and there can be no reasonable
doubt that in such ' monothalamous' (single-chambered) forms as
Gromia, Arcella, and Difflugia, similar buds are put forth, but
become detached before they develope their testaceous envelopes.
Dr. Hartwig has described, under the name of Cyromia socialis, a
type in which ' colonies' are formed by the separation of portions
of the sarcode extruded from the mouth, each of them becoming an
independent organism.* There is evidence, again, that in such
naked forms as Actinophrys and Amoeba, multiplication takes place
by a binary subdivision resembling that of Protophytes. Thus it
may often be observed that the spherical body of Actinophrys is
marked by an annular constriction, which gradually deepens so as
to separate its two halves by a sort of hour-glass contraction ; and
the connecting band becomes more and more slender, until the two
halves are completely separated. This process of fission, which
may be completed within half an hour from its commencement,
seems to take place first in the contractile vesicle ; for each segment
very early shows itself to be provided with its own (Fig. 233, b, o, o),
and the two vesicles are commonly removed to a considerable
distance from one another. The segments thus divided are not
always equal, and sometimes their difference in size is very con-
siderable.
382. The junction of two individuals, which has been seen to take
place in Actinophrys, has been supposed to correspond to the
' conjugation' of Protophytes ; it is very doubtful, however, whether
this junction really involves a complete fusion of the substance of
the bodies which take part in it, and there is not sufficient evidence
* Scliultze's " Archiv fur Mikrosk. Anat.," Bd. x. (1874), Supple meiitheft.
EEPEODUCTION OF EHIZOPOBS ; — GEEGAEIXIDA. 479
tliat it lias any relation to the act of Reproduction. Certain it is
that such a junction or ' zygosis' may occur, not between two only,
but between several individuals at once, their number being
recognised by that of their contractile vesicles ; and that, after
remaining thus coherent for several hours, they may separate again
without having undergone any discoverable change. — It appears,
however, from the observations of Mr. H. J. Carter,* that a dis-
tinction of sexes exists among Amcebina and Adinophryna ; bodies
resembling spermatozoa being developed from the nucleus in certain
individuals, whilst in others ova seems to be dispersed through the
general substance of the body. And these observations derive an
increased significance from the discoveries which have been lately
made by M. Balbiani respecting the sexual propagation of Infusoria
(§ 398). But Mr. Carter has not yet succeeded either in tracing
any relation between the ' zygosis' just mentioned as occurring
between two or more individuals, and the fertilization of the ova
by the spermatozoids ; or in ascertaining with certainty whether
the product of each ovum is a single Bhizopod, or an aggregation
of independent Bhizopods ; and these problems have still to be
worked out.
383. Geegarinida. — A very curious animal parasite is often to be
met with in the intestinal canal of Insects, Centipedes, &c, and
sometimes in that of higher animals, the simplicity of whose struc-
ture requires that it should be ranked among the Protozoa. It is
not yet certain, however, that we know the entire life-history of
this parasite, the Gregarina ; and it may possibly be only a phase
in the existence of some higher kind of Entozoon. Each indi-
vidual (Fig. 254, a) essentially consists of a single cell, usually
more or less ovate in form, and sometimes considerably elongated ;
a sort of beak or proboscis frequently projects from one extremity ;
and in some instances this is furnished with a circular row of
hooklets, closely resembling that which is seen on the head of
Taenia. There is here a much more complete differentiation
between the cell-rnembrane and its contents, than exists either in
Actinoplirijs or in Amoeba ; and in this respect we must look upon
Gregarina as representing a decided advance in organization. Being
nourished upon the juices already prepared for it by the digestive
operations of the animal which it infests, it has no need of any
such apparatus for the introduction of solid particles into the
interior of its body, as is provided in the ' pseudopodia ' of the
Bhizopods and in the oral cilia of the Infusoria. Within the
cavity of the cell, whose contents are usually milk-white and
minutely granular, there is generally seen a pellucid nucleus ; and
this becomes first constricted and then cleft, when, as often
happens, the cell subdivides into two, by a process exactly analo-
gous to that which takes-place in the simplest Protophytes (§ 204).
* 'Notes on the Freshwater Infusoria in the Island of Bombay,' in "Annals
of Nat. Hist.," 2nd Ser., Vol. xviii. (1856), pp. 223-233.
480
MICROSCOPIC FORMS OF ANIMAL LIFE.
The membrane and its contents, except the nucleus, are soluble in
acetic acid. Cilia have been detected both upon the outer and the
inner surface ; but these would seem destined, not so much to give
motion to the body, as to renew the stratum of fluid in contact
with it ; for such change of place as the animal does exhibit, is
effected by the contractions and extensions of the body generally,
as in the Amoeba (§ 377). An ' encysting process,' very much re-
sembling that of the lower Protophytes, is occasionally observed to
Fig. 254.
Gregarina of the Earthworm : — A, in its ordinary aspect ; B,
in its encysted condition ; c, D, showing uivision of its con-
tents into pseudo-navicelhe ; E, F, free pseudo-navicellse ; H,
free anioeboids produced from them.
take place in Gregdrince, and seems to be preparatory to their mul-
tiplication. Whatever the original form of the bodj may be,
it becomes globular, ceases to move, and becomes invested by a
structureless ' cyst ' within which the substance of the body under-
goes a singular change. The nucleus disappears ; and the sarcodic
mass breaks up into a series of globular particles, which gradually
resolve themselves (as shown at b, c, Fig. 254) into forms so like
those of Navicular (§ 256) as to have been mistaken for them ;
though their walls are destitute of silex, and there is no further
resemblance between the two kinds of bodies than that of figure.
These ' pseudo-navicellse ' are set-free, in time, by the bursting of
GEEGAEIXIDA. — THALASSICOLLIDA.
481
the capsule that encloses them ; and they develope themselves into
a new generation of Gregarinae, first passing through an Aruoeba-
like form. A sort of 'conjugation' has been seen to take place
between two individuals, whose bodies, coming into contact with
each other by corresponding points, first become more globular in
shape, and are then encysted by the formation of a capsule around
them both ; the partition-walls between their cavities disappear ;
and the substance of the two bodies becomes completely fused
together. As the product of this ' zygosis ' is the same as that of
the ordinary encysting process, there seems no reason for regarding
it, like the ' conjugation ' of Protophytes, as a true Generative
act ; and the resolution of the sarcodic body into ' pseudo-navi-
cellee ' must thus be regarded as analogous to the resolution of the
endochrome-mass of an Ulva or Aclilya into zoospores (§§ 265, 271).*
384. Thalassicollida. — A very curious type of composite
Rhizopods, discovered by Prof. Huxley, seems to connect the
preceding forms with Sponges and Polycystina. The Thalas-
sicollce, or Sea-jellies, are gelatinous rounded bodies, of very
variable size and shape, but usually either globular or discoidal.
€J
Sphcerozoum ouodimare.
Externally they are invested by a layer of condensed sarcode,
which sends forth pseudopodial extensions that commonly stand
out like rays, but sometimes inosculate with each other so as
* See the Memoir by M. Nat. Lieberkiitm in " Mem. de l'Acad. Boy. de
Belgique," torn. xvi.
I I
482 MICKOSCOPIC FOKMS OF ANIMAL LIFE.
to form networks. Towards the inner surface of this coat are
scattered a great number of oval bodies resembling cells, having a
tolerably distinct rnembraniforrn wall and a conspicnons ronnd
central nnclens, thus corresponding closely with the Gregarina
type. Each of these bodies appears to be withont any direct con-
nexion with the rest ; but it serves as a centre aronnd which
a number of minnte yellowish-green vesicles are disposed. Each
of these groups is protected by a siliceous skeleton, which some-
times consists of separate spicules (as in Fig. 255), bnt which
may be a thin perforated sphere like that of certain Polycystina
(§ 462), sometimes extending itself into radiating prolongations.
The internal portion of each mass is composed of an aggregation of
large vesicle-like bodies, imbedded in a softer sarcodic substance.
Notwithstanding the subsequent observations of Miiller and
Haeckel,* much obscurity still hangs over the real nature of these
bodies ; and as they so abound in the seas of warm latitudes as to
be among the commonest products of the tow-net, the Micro -
scopist who has the requisite opportunity should not neglect the
careful search-for and observation of them.
ANIMALCULES.
385. We have now to apply ourselves to the special subject of
this Chapter, namely, the assemblage of those minute forms of
Animal life which are commonly known under the designation of
Animalcules. Nothing can be more vague or inappropriate than
this title, since it only expresses the small dimensions of the beings
to which it is applied, and does not indicate any of their charac-
teristic peculiarities. In the infancy of Microscopic knowledge, it
was natural to associate together all those creatures which could
only be discerned at all under a high magnifying power, and
whose internal structure could not be clearly made out with the
instruments then in use ; and thus the most heterogeneous
assemblage of Plants, Zoophytes, minute Crustaceans (water-
fleas, &c.), larvae of "Worms and Mollusks, &c, came to be ag-
gregated with the true Animalcules under this head. The Class
was being gradually limited by the removal of all such forms
as could be referred to others ; but still very little was known
of the real nature of those that remained in it, until the study
was taken up by Prof. Ehrenberg, with the advantage of instru-
ments which had derived new and vastly improved capabilities
from the application of the principle of Achromatism. One of
the first and most important results of his study, and that
which has most firmly maintained its ground, notwithstanding
* See Huxley in "Annals of Natural History," 2nd Ser., Vol. viii. (1851),
p. 433 ; and " Quart. Joum. of Microsc. Science," Vol. iv. (1856), p. 72 j also
Miiller in his Treatise "Ueber die Thalassicollen, Polycystinen, und Acantho-
metren des Mittelnieeres," originally published in the Transactions of the Berlin
Academy for 1858 ; and the magnificent work of Haeckel, " Die Badiolarien,"
Berlin, 1862.
ANIMALCULES :— LNFUSOEIA AND EOTIFERA. 483
the overthrow of Prof. Ehrehberg's doctrines on other points,
was the separation of the entire assemblage into two distinct
groups, having scarcely any feature in common excepting their
minute size ; one being of very low, and the other of com-
paratively high organization. On the lower group he conferred
the designation of Polygastrica (many-stomached), in consequence
of having been led to form an idea of their organization which
the united voice of the most trustworthy observers now pro-
nounces to be erroneous ; and as the retention of this term must
tend to perpetuate this error, it is well to fall back on the name
Infusoria, or Infusory Animalcules, which simply expresses their
almost universal prevalence in infusions of organic matter. For
although this was applied by the older writers to the higher
group as well as to the lower, yet as the former are now
distinguished by an appropriate apioellation of their own, and are,
moreover, not found in infusions while in that state of rapid
decomposition which is most favourable to the presence of the
inferior kind of Animalcules, the designation may very well be
restricted to the forms essentially constituting the Polygastrica of
Ehrenberg, which is the sense wherein it has been used by many
recent writers.— To the higher group, Prof. Ehrenberg's name
Rot if era or Rotatoria is on the whole very appropriate, as signi-
ficant of that peculiar -arrangement of their cilia upon the
anterior parts of their bodies, which, in some of their most
common forms, gives the appearance (when the cilia are in action)
of wheels in revolution ; the group, however includes many
members in which the ciliated lobes are so formed as not to
bear the least resemblance to wheels. In their general organiza-
tion, these ' Wheel-animalcules' must certainly be considered as
members of the Articulated division of the Animal Kingdom ;
and they seem to constitute a class in that lower portion of it,
to which the designation Worms is now commonly given. — Not-
withstanding this wide zoological separation between these two
kinds of Animalcules, it seems most suitable to the plan of the
present work to treat of them in connexion with one another ;
since the Microscopist continually finds them associated together,
and almost necessarily ranges them in his own mind under one
and the same category.
386. Infusoria. — This term, as now limited by the separation
of the Rotifera, is applied to a far smaller range of forms than that
which was included by Prof. Ehrenberg under the name of ' poly-
gastric' animalcules. For a large section of these, including the
JJesmidiacece, Diatomacece, Volvocinece, and many other Proto-
phytes, have been transferred by the almost concurrent voice of
those Naturalists whose judgment is most to be relied-on, to the
Vegetable Kingdom. The Rhizopod group, again, must be excluded,
as being very distinct in its plan of organization from the true
Infusoria. And, lastly, it is not impossible that many of the
reputed Infusoria may be but larval forms of higher organisms,
n2
434
MICKOSCOPIC FOEMS OF ANIMAL LIFE.
instead of being themselves complete animals. Still an extensive
group remains, of which no other acconnt can at present be given,
than that the beings of which it is composed go through the whole
of their lives, so far as we are acquainted with them, in a grade of
existence which is essentially 'protozoic;' their lowest forms ap-
proximating closely to the highest Ehizopods, whilst even in their
most elevated types we find no such differentiation of parts as
would justify our associating them with any other class. — The
following general account of the organization of Infusoria is given
in accordance with the concurrent representations of the best
observers of the present time.
387. The bodies of Infusoria consist of ' sarcode,' of which the
outer layer possesses considerably more consistence than the
internal portion : the process of differentiation having here
advanced sufficiently far to establish a clear distinction between
the ' ectosarc' and the ' endosarc' Sometimes, as in Paramecium,
Fig. 256.
V
./////.I
A, Kerona silurus: — a, contractile vesicle; 5, mouth; c, c,
Animalcules swallowed by the Kerona, after having them-
selves ingested particles of indigo. B, Paramecium caudatum .-
— a, a, contractile vesicles ; 6, mouth.
a distinct pellicle may be recognised on the surface of the ectosarc
or ' cortical layer' of the body ; and this pellicle, which is studded
with regularly-arranged markings like those of Diatomacea?,
seems to be the representative of the carapace of Arcella, &c.
(§ 380), as of the cellulose coat of Protophytes. In certain
Infusoria, as Paramecium (Loxodes) hursaria, the surface of the
body is beset with ' trichocysts' resembling those of Zoophytes
STRUCTURE AND ACTIONS OF INFUSORIA.
485
Fig. 257.
in miniature (§ 486) ; bnt it is remarkable that these are not
present in all the individuals of the species in which they occur.
Sometimes, again, the tegumentary membrane is hardened, so as
to form a shield that protects the body on one side only, or a
' lorica' that completely invests it ; and there are other cases in
which it is so prolonged and doubled upon itself as to form a sheath
resembling the ' cell' of a Zoophyte, within which the body of the
Animalcule lies loosely, being attached only by a stalk at the bottom
of the case, and being able either to project itself from the outlet
or to retract itself into the interior. The form of the body is usually
much more definite than
that of Amoeba or Acti-
nophrys, each species hav-
ing its characteristic shape,
which is only departed from,
for the most part, when the
Animalcule is subjected to
pressure from without, or
when its cavity has been
distended by the ingestion
of any substance above the
ordinary size. The body
does not seem to possess
much contractile power in
its own substance, its move-
ments being principally exe-
cuted by the instrumen-
tality of locomotive appen-
dages ; one remarkable in-
stance of contractility, how-
ever, is presented by the
stalk of Vorticella (Fig. 257).
The locomotive appendages,
which may all be considered
as prolongations of the te-
gumentary layer, are desti-
tute of any more minute
organization ; being, in fact,
of the nature of cilia, though
sometimes of much larger
dimensions, and employed
in a different manner. The
vibration of ciliary filaments, which are either disposed along
the entire margin of the body, as well as around the oral aperture
(Fig. 256 a, b), or are limited to some one part of it, which is always
in the immediate vicinity of the mouth (Fig. 257), supi3lies the
means by far the most frequently employed by the beings of this
class, both for progression through the water and for drawing
alimentary particles into the interior of their bodies. In some
Group of Vorticella nebulifera, showing
A, the ordinary form ; B, the same with the
stalk contracted ; c, the same with the bell
closed ; D, e, p, successive stages of fissi-
parous multiplication.
486 MICEOSCOPIC FORMS OF ANIMAL LIFE.
their vibration is constant, whilst in others it is only occasional,
thns conveying the impression that the Animalcnle has a voluntary
control over them ; bnt there is strong reason for questioning
the existence of any such self-directing power. These cilia, like
those of the zoospores of Protophytes, can usually be distinctly
seen only when their movement is very much slackened in its
rate, or when it has entirely ceased. Sometimes, however, instead
of a multitude of short cilia, we find a small number of long
slender filaments usually proceeding from the anterior part of
the body (that nearest the mouth), and strongly resembling the
elongated cilia of Protococcus (Plate VIII., fig. 2, h) or of Volvox
(Plate IX., figs. 9, 10, 11). But in other cases, the filaments are
comparatively short and have a bristle-like firmness ; and instead
of being kept in vibration, they are moved (like the spines of
Echini) by the contraction of the substance to which their bases
are attached, in such a manner that the Animalcule crawls by
their means over a solid surface, as we see especially in Trichoda
lynceus (Pig. 260, p, q). In Chilodon and Nassula, the mouth is
provided with a circlet of plications or folds looking like bristles,
which, when imperfectly seen, received the designation of ' teeth;'
their function, however, is rather that of laying hold of alimentary
particles by their expansion and subsequent drawing-together
(somewhat after the fashion of the tentacula of Zoophytes),
than of reducing them by any kind of masticatory process.
388. The modes of movement which Infusory Animalcules
execute by means of these instruments, are extremely varied and
remarkable. Some propel themselves directly forwards, with a
velocity which appears, when thus highly magnified, like that of
an arrow, so that the eye can scarcely follow them ; whilst others
drag their bodies slowly along like a leech. Some attach them-
selves by one of their long filaments to a fixed point, and revolve
around it with great rapidity ; whilst others move by undulations,
leaps, or successive gyrations : in short, there is scarcely any kind
of animal movement which they do not exhibit. There is no suffi.-
cient reason, however, to regard such actions as indicative of con-
sciousness ; indeed, the very fact that they are performed by the
instrumentality of Cilia seems to imply the contrary, since we know
that ciliary action takes-place to a large extent in our own bodies
without the least dependence upon our consciousness, and that it
is also used as a means of dispersion among the zoospores of the
lowest Plants, which cannot for a moment be supposed to be en-
dowed with this attribute. We can only regard it, therefore, as
indicative of a wonderful adaptation, on the part of these simple
organisms, to a kind of life which enables them to go in quest of
their own nutriment, and to introduce it when obtained into the
interior of their bodies. — The curious contraction of the foot- stalk
of the Vorticella (Pig. 257), however, is a movement of a different
nature, and is due to the contractility of the tissue that occupies
the interior of the tubular pedicle. This stalk serves to attach the
STRUCTURE AND ACTIONS OF INFUSORIA. 487
bell-shaped body of the Animalcule to some fixed object, such as
the leaf or stem of duck-weed ; and when the animal is in search of
food, with its cilia in active vibration, the stalk is fully extended.
If, however, the Animalcule should have drawn to its mouth any
particles too large to be received within it, or should be touched
by any other that happens to be swimming near it, or should be
' jarred' by a smart tap on the stage of the Microscope, the stalk
suddenly contracts into a spiral, from which it shortly afterwards
extends itself again into its previous condition. The central cord,
to whose contractility this action is due, has been described as
muscular, though not possessing the characteristic structure of
either kind of muscular fibre ; it possesses, however, the special irri-
tability of muscle, being instantly called into contraction (accord-
ing to the observations of Kuhne) by electrical excitation. The
same character is assigned by Stein to the longitudinal bands or
stripes seen in Stentor and some other large Infusoria ; which may
be considered as modifications of ordinary sarcode specially endowed
with contractility. — The only special organs of sense with the
possession of which Infusoria can be credited, are the delicate bristle-
like bodies which project in some of them from the neighbourhood of
the mouth, and in Stentor from various parts of the surface ; these
may be conceived to be organs of touch. The red spots seen in many
Infusoria, which have been designated as eyes by Prof. Ehrenberg
from their supposed correspondence with the eye-spots of Botifera
(§ 410), really bear a much greater resemblance to the red spots
which are so frequently seen among Protophytes (§ 207). If these
creatures are really endowed with consciousness, as their movements
seem to indicate, though other considerations render it very doubtful,
they must derive their perceptions of external things from the im-
pressions made upon their general surface, but more particularly
upon their filamentous appendages.
389. The interior of the body does not always seem to consist
of a simple undivided cavity occupied by soft sarcode ; for the
tegumentary layer appears in many instances to send prolonga-
tions across it in different directions, so as to divide it into chambers
of irregular shape, freely communicating with each other, which
may be occupied either by sarcode, or by particles introduced from
without. The alimentary particles which can be distinguished in
the interior of the transparent bodies of Infusoria, are usually Pro-
tophytes of various kinds, either entire or in a fragmentary state.
The Diatomacese seem to be the ordinary food of many ; and the
insolubility of their loricce enables the observer to recognise them
unmistakably. Sometimes entire Infusoria are observed within
the bodies of others not much exceeding them in size (Fig. 260, b) ;
but this is only when they have been recently swallowed, since
the prey speedily undergoes digestion. It would seem as if these
creatures do not feed by any means indiscriminately, since
particular kinds of them are attracted by particular kinds of
aliment ; the crushed bodies and eggs of Entomostraca, for
488 MICEOSCOPIC FOEMS OF ANIMAL LIFE.
example, are so voraciously consumed by the Coleps, that its body
is sometimes quite altered in shape by the distension. This cir-
cumstance, however, by no means proves, as some have considered
it to do, that such creatures possess a sense of taste and a
power of determinate selection ; for many instances might be cited,
in which actions of the like apparently-conscious nature are per-
formed without any such guidance.
390. The ordinary process of feeding, as well as the nature and
direction of the ciliary currents, may be best studied by
diffusing through the water containing the Animalcules a few
particles of indigo or carmine. These may be seen to be carried
by the ciliary vortex into the mouth, and their passage may be
traced for a little distance down a short (usually ciliated)
oesophagus. There they commonly become aggregated together,
so as to form a little pellet of nearly globular form ; and this,
when it has attained the size of the hollow within which it is
moulded, is projected into the ' general cavity of the body,' where
it lies in a vacuole of the sarcode, its place in the oesophagus being
occupied by other particles subsequently ingested. This ' mould-
ing,' however, is by no means universal ; the aggregations of
coloured particles in the bodies of these animals being often desti-
tute of any Regularity of form. A succession of such particles
being thus introduced into the interior of the body, each aggrega-
tion seems to push-on its predecessors ; and a kind of circulation
is thus occasioned in the contents of the cavity. The pellets that
first entered make their way out after a time (after yielding up
their nutritive materials), generally by a distinct anal orifice, some-
times, however, by any part of the surface indifferently, and some-
times by the mouth. A circumstance which seems clearly to
indicate that they cannot be enclosed (as maintained by Prof.
Ehrenberg) in distinct stomachal cavities, is that, when the pellets
are thus moving round the body of the Animalcule, two of them
sometimes appear to become fused together, so that they obviously
cannot have been separated by any membranous investment.
When the Animalcule has not taken food for some time, ' vacuoles,'
or clear spaces, extremely variable both in size and number, filled
only with a very transparent fluid, are often seen in its sarcode ;
their fluid sometimes shows a tinge of colour, and this seems to be
due to the solution of some of the vegetable chlorophyll upon which
the Animalcule may have fed last.
391. Contractile Vesicles (Fig. 256, a, a), usually about the size
of the ' vacuoles,' are found, either singly or to the number of from
two to sixteen, in the bodies of most Animalcules ; and may be
seen to execute rhythmical movements of contraction and dilata-
tion at tolerably regular intervals, being so completely obliterated
when emptied of their contents as to be quite indistinguishable,
and coming into view again as they are refilled. These vesicles
do not change their position in the individual, and they are pretty
constant, both as to size and place, in different individuals of the
STEUCTUEE AND MULTIPLICATION OF INFUSOEIA. 489
same species ; hence they are obviously quite different in character
from the ' vacuoles.' In Paramecium there are always to be
observed two globular vesicles (Fig. 256, b, a, a), each of them sur-
rounded by several elongated cavities, arranged in a radiating
manner, so as to give to the whole somewhat of a star-like aspect
(Plate XIV., fig. 1, v, v) ; and the liquid contents are seen to be pro-
pelled from the former into the latter, and vice versa. Further, in
Stentor, a complicated network of canals, apparently in connexion
with the contractile vesicles, has been detected in the substance of
the 'cortical layer;' and traces of this maybe observed in other
Infusoria. In some of the larger Animalcules it may be distinctly
seen that the contractile vesicles have permanent valvular orifices
opening outwards, and that an expulsion of fluid from the body
into the water around is effected by their contraction. Hence
it appears likely that their function is of a respiratory nature,
and that they serve, like the gill-openings of Fishes, for the expul-
sion of water which has been taken-in by the mouth, and which
has traversed the interior of the body. (See § 373.)
392. Of the Reproduction of the Infusoria our knowledge has
lately received a great accession in the discovery of their true
sexual Generation (§ 398) ; the attention of observers having,
until a comparatively recent period, been fixed almost exclusively
upon the act of binary subdivision, which, though by far the
most frequent method of propagation, is not a true generative
operation. This act seems to be effected in the same general mode
as the subdivision of Protophyta : and has been observed in many
instances to commence in the ' nucleus,' which may usually be dis-
Fig. 258.
Fissiparous multiplication of Chilodon cucullulus : — A, B, C,
successive stages of longitudinal fission (?) ; D, E, F, succes-
sive stages of transverse fission.
tinguished in the bodies of the Infusoria. The division
place in some species longitudinally, that is, in the direction of the
greatest length of the body (Fig. 257, d, e, f), in other species
transversely (Fig. 260, a, d), whilst in some, as in Chilodon
cucullulus (Fig. 258), it has been supposed to occur in either direc-
tion indifferently ; but it seems most probable from recent disco-
veries, that what has been here supposed to be longitudinal fission
490
MICEOSCOPIC FOEMS OF ANIMAL LIFE.
(a, b, c) is really an act of 6 conjugation' (§ 398), and that the real
fission is transverse only (d, e, f). This operation is performed
with such rapidity, under favourable circumstances, that, according
to the calculation of Prof. Ehrenberg, no fewer than 268 millions
might be produced in a month by the repeated subdivisions of a
single Paramecium. When this fission occurs in Vorticetta (Fig.
257), one of the divisions is usually smaller than the other, some-
times so much so as to look like a bud ; and this generally detaches
itself when mature from the main body, and swims freely about
until it developes a new footstalk for itself. But sometimes the
two parts are equal in size, and the fission extends down the stalk,
which thus becomes double for a greater or less part of its length ;
and thus a whole bunch of Yorticellse may spring (by a repetition
of the same process) from one base. In some members of the
same family, indeed, an arborescent structure is produced, just as
in certain Diatoms (Fig. 152), by the like processes of division and
gemmation.
393. Many Infusoria at certain times undergo an encysting pro-
cess, resembling the passage of Protophytes into the ' still ' condi-
tio. 259.
Encysting process in Vorticetta microstoma .- — A, full-grown
individual in its encysted state ; a, retracted oval circlet of
cilia ; &, nucleus ; c, contractile vesicle ; — B, a cyst separated
from its stalk ; — c, the same more advanced, the nucleus
broken-up into spore-like globules ; — D, the same more deve-
loped, the original body of the Vorticella, d, having become
sacculated, and containing many clear spaces ; — E, one of the
sacculations having burst through the enveloping cyst, a
gelatinous mass, e, containing the gemmules, is discharged.
tion (§ 209), and apparently serving, like it, as a provision for
their preservation under circumstances which do not permit the
continuance of their ordinary vital activity. Previously to the
IXFUSOEIA: — ENCYSTING PEOCESS. 491
formation of the cyst, the movements of the animalcule diminish
in vigour, and gradually cease altogether ; its form becomes more
rounded ; its oral aperture closes ; and its cilia or other filamentous
prolongations are either lost or retracted, as is well seen in
Vorticella (Fig. 259, a). The surface of the body then exudes a
gelatinous excretion that hardens around it so as to form a
complete coffin-like case, within which little of the original struc-
ture of the animal can "be distinguished. Even after the comple-
tion of the cyst, however, the contained animalcule may often be
observed to move freely within it, and may sometimes be caused to
come forth from its prison by the mere application of warmth and
moisture. In the simplest form of the ' encysting process,' indeed,
the animalcule seems to remain altogether quiescent through the
whole period of its torpidity ; so that, however long may be the
duration of its imprisonment, it emerges without any essential
change in its form or condition. But in other cases, this process
seems to be subservient either to multiplication or to metamor-
phosis. For in Vorticella the substance of the encysted body ("b)
appears to break up (c, d) into numerous gemmules, which are
analogous to the ' zoospores ' of Protophytes, and which, like them,
are set free by the bursting of the parent-cyst (e), swimming forth
to develop themselves into new individuals of the same kind,
though at first, perhaps, bearing little or no resemblance to the
type from which they sprang.
394. In Triclwda lynceus, on the other hand, the ' encysting
process ' appears subservient to a kind of metamorphosis of the
individual (like the somewhat parallel passage of Insects through
the pupa-stage) ; the Animalcule which emerges from the cyst
having characters in many respects different from those of the
animalcule which became encysted, but no multiplication being
effected either by subdivision or gemmatiou . According to M. Jules
Haime, by whom this history was very carefully studied,* the form
to be considered as the larval one is that shown in Fig. 260, a — e,
which has been described by Prof. Ehrenberg under the name of
Oxytricha. This possesses a long, narrow, flattened body, fur-
nished with cilia along the greater part of both margins, and
having also at its two extremities a set of larger and stronger
hair-like filaments ; and its mouth, which is an oblique slit on the
right-hand side of its fore-part, has a fringe of minute cilia on each
lip. Through this mouth, large particles are not unfrequently
swallowed, which are seen lying in the midst of the gelatinous con-
tents of the general cavity of the body, without any surrounding
' vacuole;' and sometimes even an Animalcule of the same species,
but in a different stage of its life, is seen in the interior of one of
these voracious little devourers (b). In this phase of its existence,
the Triclwda undergoes multiplication by transverse fission, after
the ordinary mode (c, d) ; and it is usually one of the short-bodied
* " Annales des Sci. Nat.," S6\ 3, Tom. xix. p. 109.
492
MICROSCOPIC FORMS OF ANIMAL LIFE.
' doubles' (e), thus produced that passes into the next phase. This
phase consists in the assumption of the globular form, and the
almost entire loss of the locomotive appendages (f) ; in the escape
of successive portions of the granular sarcode, so that ' vacuoles '
make their appearance (g) ; and in the formation of a gelatinous
envelope or cyst, which, at first soft, afterwards acquires increased
Metamorphoses of Trichoda lynceus : — A, larva (Oxytricha) ;
B, a similar larva, after swallowing the animalcule repre-
sented at M ; c, a veiy large individual on the point of under-
going fission ; D, another in which the process has advanced
further ; E, one of the products of such fission ; F, the same
body become spherical and motionless ; G, aspect of this
sphere fifteen days afterwards ; H, later condition of the
same, showing the formation of the cyst ; I, incipient separa-
tion between living substance and exuvial matter ; K, partial
discharge of the latter, with flattening of the sphere ; L, more
distinct formation of the confined animal ; M, its escape from
the cyst ; N, its appearance some days afterwards ; o, more
advanced stage of the same ; P, Q, perfect individuals, one as
seen sideways, moving on its bristles, the other as seen from
below (these are magnified twice as much as the preceding
figures).
firmness (h). After remaining for some time in this condition, the
contents of the cyst become clearly separated from their envelope ;
and a space appears on one side, in which ciliary movement can be
distinguished (i). This space gradually extends all round, and a fur-
ther discharge of granular matter takes -place from the cyst, by
which its form becomes altered (k) ; and the distinction between the
newly-formed body to which the cilia belong, and the effete residue
of the old, becomes more and more apparent (l). The former
INFUSORIA :— ENCYSTING PROCESS. 493
increases in size, whilst the latter diminishes ; and at last the
former makes its escape through an aperture in the wall of the
cyst, a part of the latter still remaining within its cavity (ii). The
body thus discharged (n) does not differ much in appearance from
that of the Oxytricha before its encystment (f), though only of
about two-thirds its diameter ; but it soon developes itself (o, p, q)
into an animalcule very different from that in which it originated.
First it becomes still smaller, by the discharge of a portion of its
substance ; numerous very stiff bristle-like organs are developed,
on which the animalcule creeps, as by legs, over solid surfaces ;
the external integument becomes more consolidated on its upper
surface, so as to become a kind of carapace ; and a mouth is
formed by the opening of a slit on one side, in front of which is a
single hair-like filament, which is made to turn round and round
with great rapidity, so as to describe a sort of inverted cone,
whereby a current is brought towards the mouth. This latter form
had been described by Prof. Ehrenberg under the name of Aspiclisca.
It is very much smaller than the larva ; the difference being, in
fact, twice as great as that which exists between a and p, q (Fig.
260), since the last two figures are drawn under a magnifying
power twice as great as that employed for the preceding. How the
Aspiclisca- form in its turn gives origin to the Oxijtricha-form, has
not yet been made-out. . A Sexual process, it may be almost cer-
tainly concluded, intervenes somewhere ; but other transformations
may not improbably take place, before the latter of these types is
reproduced.
395. The ' encysting process ' has been observed to take place
among several other forms of Infusoria ; so that, considering the
strong general resemblance in kind and degree of organization
which prevails throughout the group, it does not seem unlikely
that it may occur at some stage of the life of nearly all these
Animalcules, just as the ' still ' condition alternates with the
'motile' in the most active Protophytes (§§ 207-211). And it is
not improbably in the ' encysted ' condition that their dispersion
takes place ; since they have been found to endure desiccation in
this state, although in their ordinary condition of activity they
cannot be dried-up without loss of lif e. "When this circumstance is
taken into account, in conjunction with the extraordinary rapidity
of multiplication of these Animalcules, and with the fact that a
succession of different forms may be presented by one and the
same being, the difficulty of accounting for the universality of
their diffusion, which has led some Naturalists to believe in their
' spontaneous generation,' and others to regard them as isolated
particles of higher organisms set-free in their decomposition so as
to constitute an ' equivocal generation,' is as readily got-over as we
have seen it to be in the case of the Fungi (§ 289). Although it
may be stated as a general fact, that wherever decaying Organic
matter exists in a liquid state and is exposed to air and warmth, it
speedily becomes peopled with these minute inhabitants, yet it
494 MICROSCOPIC FORMS OF ANIMAL LIFE.
may be fairly presumed that, as in the case of the Fungi, the dried
cysts or germs of -Infusoria are everywhere floating about in the
air, ready to develop themselves wherever the appropriate condi-
tions are presented ; and all our knowledge of their history, as well
as the strong analogy of the Fungi, seems further to justify the
belief, that the same germs may develop themselves into several
different forms, according to the nature of the liquid in which they
chance to be deposited. — This is a subject peculiarly worthy of the
attention of Microscopic observers ; who can scarcely be better
employed than in tracing-out the succession of phases which any
particular type may present, and in thus making a most important
extension of our knowledge of its life-history, whilst at the same
time effecting a most desirable reduction in the number of reputed
species.
396. Such a study has recently been very carefully prosecuted
with really important results, by Messrs. Dallinger and Drysdale,
who have worked not only with the highest powers, but with ap-
pliances specially devised to keep the same drop of water under
continuous view.* Their first set of observations was made upon a
Gercomonacl having a long whip-like flagellum at each end, that
abounded in water in which a cod's head had been macerated. The
multiplication of this form by transverse fission went on con-
tinuously for at least eight days, the whole process being usually
completed in less than five minutes. The cercomonads then passed
into the amoeboid condition, each giving forth a sarcodic expan-
sion round its body, and moving by the pseudopodial extensions
put forth from this, its flagella disappearing. Two of these
amceboids coming together, their bodies coalesced, and round
the united mass a cyst was developed, the contents of which were
slightly yellow in hue. After a short time the membrane of the
cyst ruptured, and gave exit to a multitude of granules of such
extreme minuteness, that even under a magnifying power of 2500
diameters they had not any appreciable dimension. A continuous
watching of the same drop enabled a progressive increase in the
size of these granules to be traced ; until, at the expiration of nine
hours, they presented the characteristic aspect, movements, and
flagella of their parent form, although still very minute in com-
parison. In a few hours more the full size was attained, and
multiplication by fission speedily commenced, thus completing the
cycle. — In another form, having two flagella at the same end,
something more like a distinction of sexes presented itself. Certain
of the individuals produced by fission become still, then amoeboid,
then round, and a small cone of sarcode shoots out, dividing and
increasing into another pair of flagella. The disk then splits ; each
half becomes possessed of a nuclear body ; and two well-formed
monads are set free. These swim freely until they meet with an
* See their succession of Papers in " Monthly Microsc. Journ.," Vols. x.
and xi;.
REPRODUCTION OF INFUSORIA. 495
ordinary form that has just completed fission ; the nuclear ends of
the two come into approximation ; their sarcode rapidly blends, so
that the nucleus -like bodies meet ; and when they come into con-
tact, the two bodies pass into one. The combined body, which is
triangular in shape, at first continues to move by the action of its
flagella, then becomes encysted and motionless, and after some
little time bursts at its angles, and emits a mass of immeasurably
minute granules, which progressively develope themselves into the
parental form. — In another case, the immediate product of the
' encysting process' was not a mass of granules, but an aggregate
of germinal particles of appreciable size ; and in another type, the
rupture of the cyst gave exit to minute bodies, which already pre-
sented the monadif orm aspect. — " In pursuing our researches," say
these excellent observers, " we have become practically convinced
of what we have theoretically assumed, — the absolute necessity for
prolonged and patient observation of the same forms. Two ob-
servers, independently of each other, examining the same Monad, if
their inquiries were not sufficiently prolonged, might, with the
utmost truthfulness of interpretation, assert opposite modes of
development. Competent optical means, careful interpretation,
close observation, and time are alone capable of solving the
problem."
397. It is a very important result of the observations of Messrs.
Dallinger and Drysdale, that the minute granular germs are able to
sustain, not merely desiccation, but exposure to a temperature
much higher than that which is fatal to the organisms that give
birth to them. In the case of the Cercomonads first described, a
temperature of 150° Fahr. sufficed to destroy all the adult forms ;
but the granular ' sporules' were not affected by it. An ordinary
slide containing adult forms and sporules, having been allowed to
evaporate slowly, was placed in a dry heat, which was raised to
250° Fahr.; it was then slowly cooled, and distilled water was
allowed to insert itself by capillary attraction. On a first exami-
nation, all the adult forms were found to be absolutely destroyed,
and no spore could be definitely identified ; but after it had been
kept moist for some hours, and watched with the l-50th inch ob-
jective, gelatinous points were seen, which were recognised as exactly
like an early stage of the developing sporule ; and the evolution of
these was traced until they reached the small flagellate stage. In
another case, the temperature of the slide was raised to 30CP Fahr.
without the destruction of the vitality of the sporules, some of
which, on being moistened, revived and developed themselves into
their adult forms. It is obvious that these facts are of fundamental
importance in the discussion of the question of ' spontaneous
generation' or Abiogenesis ; since they show (1) that germs ca-
pable of surviving desiccation, may be everywhere diffused through
the air, and may, on account of their extreme minuteness (as they
certainly do not exceed 1 -200,000th of an inch in diameter), alto-
gether escape both the most careful scrutiny and the most thorough
496 MICROSCOPIC FORMS OF ANIMAL LIFE.
cleansing-processes ; while (2) their extraordinary power of resist-
ing heat will jDrevent them from being killed, either by boiling, or
by dry-heating np to even 300° Fahr.*
398. A very important advance has recently been made in this
direction, by the discovery that a true process of sexual generation
occurs among Infusoria ; — a discovery which had been more or less
nearly approached by various observers, but of which the satisfac-
tory completion was first attained by the researches of M. Bal-
biani.f It appears from his observations, that male and female
organs are combined in each individual of the numerous genera he
has examined, but that the congress of two individuals is necessary
for the impregnation of the ova, those of each being fertilized by
the spermatozoa of the other. The ovarium (or aggregation of
germ-cells) is that organ which has been described by many obser-
vers as the 'nucleus;' whilst the testis (or aggregation of sperm-
cells) is that which has been described as the ' nucleolus.' The de-
velopment of each of these organs commences as a single minute
cell, which usually multiplies itself in the usual way by sub-
division ; and when this multiplication has proceeded to a certain
point, the cells of the ovary become converted into ova, whilst
those of the testis develope spermatozoa in their interior. The
particular form and position which these organs present, and the
nature of the changes which they undergo, vary in the several
types of Infusoria \% but as we have in the common Paramecium
aurelia an example, which, although exceptional in some par-
ticulars, affords peculiar facilities for the observation of the pro-
* The effective method devised by Messrs. Dallinger and Drysdale for "pre-
venting the evaporation of the drop of fluid under examination, so as to admit
of continuous examination of the same forms with the highest powers," and
the apparatus they used for heating their slides to any required point, are
described in the " Monthly Microsc. Journ.," Vol. xi. pp. 97—99.
f See his " Recherches sur les Phenonienes Sexuels des Infusoires," in Dr.
Brown-Sequard's " Journal de la Physiologie," for 1861. An abstract of these
researches is contained in the " Quart. Journ. of Microsc. Science," for July
and October, 1862.
X Thus, according to M. Balbiani, the ovary of CMlodon cucullulus never ad-
vances beyond the condition of a single ' primitive ovum,' formed by the diffe-
rentiation of the contents of the original ' germ-cell ' into the granular yolk-
substance and the pellucid ' germinal vesicle ' imbedded in it. But in other
Infusoria the ' germ-cell ' undergoes repeated subdivisions ; so that from 2 to 4
ova (as in Paramecium), from 8 to 15 (as in Stentor), from 20 to 25 (as in Am-
phikptus gigas), from 20 to 50 (as in Spirostomum ambiguum), and even 100 or
more (as in a species of Urostyla), may be developed in a single individual. In
some cases, again, the subdivision does not involve the entire ' germ-cells ' in
the first instance, but affects only their ' germinal vesicles ;' these being multi-
plied in the midst of the undivided granular yolk-mass, but drawing round
themselves, near the time of conjugation, their several shares of this substance,
and becoming completed into ova by the formation of an investment round their
respective yolk-segments : this is the mode in which ova are produced in the
Vorticellina. In Parameciumit seems as if the whole of the granular yolk-mass
were not thus appropriated; a number of sterile yolk-segments (a, a, Plate
XIV., fig. 5), being left after the maturation of the ova.
PLATE XIV.
iSlHiyi
10 11 12
llillllfii
13 14 15 16 17
21
<'X\,"
Sexual Reproduction of Infusoria.
[To face p.41d7.
SEXUAL GENERATION OF INFUSORIA. 497
cess, and has been most completely studied by M. Balbiani, it
is here selected for illustration. This Animalcule, as is well known,
multiplies itself with great rapidity (under favourable circum-
stances) by duplicative subdivision, which always takes place in
the transverse direction ; and the condition represented in Plate
XIV., figs. 1, 2, is not, as has been usually supposed, another form
of the same process, but is really the sexual congress of two indi-
viduals previously distinct. When the period arrives at which the
Paramecia are to propagate in this manner, they are seen assem-
bling upon certain parts of the vessel, either towards the bottom
or on the walls ; and they are soon found coupled in pairs, closely
adherent to each other, with their similar extremities turned in the
same direction, and their two mouths closely applied to one
another. The Paramecia and other free-swimming Infusoria,
while conjugated, continue moving with agility in the liquid,
turning constantly round upon their axes ; but those which, like
Stentor, are attached by a foot-stalk, remain almost motionless
(Fig. 21). This conjugation lasts for five or six days, during which
period very important changes take place in the condition of the
reproductive organs. In order to distinguish these, the Animal-
cules should be slightly flattened by compression, and treated with
acetic acid, which brings the reproductive apparatus into more dis-
tinct view, as shown in Plate XIV., figs. 1-5. In fig. 1 each indi-
vidual contains an Ovarium, a, which is shown to present in the
first instance a smooth surface ; and from this there proceeds an
excretory canal or oviduct, c, that opens externally at about the
middle of the length of the body into the buccal fissure, e. Each
individual also contains a Seminal capsule, b, in which is seen
lying a bundle of spermatozoids curved upon itself, and which com-
municates by an elongated neck with the orifice of the excretory
canal. The successive stages by which the seminal capsule ariives
at this condition from that of a simple cell, whose granular con-
tents resolve themselves (as it were) into a bundle of filaments, are
shown in figs. 6-10. In fig. 2 the surface of the ovary, a, is seen to
present a lobulated appearance, which is occasioned by the com-
mencement of its resolution into separate ova ; while the seminal
capsule is found to have undergone division into two or four
secondary capsules, b, b, each of which contains a bundle of sper-
matozoa now straightened out. This division takes place by the
elongation of the capsule into the form represented in fig. 11. and
by the narrowing of the central portion whilst the extremities
enlarge ; the further multiplication being effected by the repetition
of the same process of elongation and fission. In fig. 3, which re-
presents one of the individuals still in conjugation, the four
Seminal capsules, b, b, are represented as thus elongated in prepa-
ration for another subdivision ; whilst the Ovary, a, a, has begun
as it were to unroll itself, and to break-up into fragments which
are connected by the tube m. In this condition it is that the
object of the conjugation appears to be effected by the passage of
K K
498 MICKOSCOPIC FORMS OF ANIMAL LIFE.
the seminal capsules of each individual, previously to their com-
plete maturation, into the body of the other. In fig. 4 is shown
the condition of a Paramecium ten hours after the conclusion of
the conjugation ; the ovary has here completely broken up into
separate granular masses, of which some, a, a, remain unchanged,
whilst others, o, o, o, o, either two, four, or eight in number, are con-
verted into ovules that appear to be fertilized by the escape of the
spermatozoa from the seminal capsules, these being now seen in
process of withering. Finally, in fig. 5, which represents a Para-
mecium three days after the completion of the conjugation, are
seen four complete ova, o, o, o, o, within the connecting tube, m,m ;
whilst the seminal capsules have now altogether disappeared. In
figs. 13-18 are seen the successive stages of the development of the
ovule, which seems at first (fig. 13) to consist of a germ-cell having
within it a secondary cell containing minute granules, which is to
become the 'vitelline vesicle.' This secondary cell augments in
size, and becomes more and more opaque from the increase of its
granular contents (figs. 14, 15, 16), forming the ' vitellus' or yolk ;
in the midst of which is seen the clear ' germinal vesicle,' which
shows on its wall, as the ovule approaches maturity, the ' germinal
spot' (fig. 17). The germinal vesicle is subsequently concealed
(fig. 18) by the increase in the quantity and opacity of the vitelline
granules. The fertilized ova seem to be expelled by the gradual
shortening of the tube that contains them ; and this shortening
also brings together the scattered fragments of the granular sub-
stance of the original ovarium, so as to form a mass resembling
that shown in fig. 1, a, by the evolution of which after the same
fashion another brood of ova may be produced. The development
of the ova after their extrusion from the body has not yet been fol-
lowed out ; and its history constitutes a most important object of
inquiry.
399. A very curious case of parasitism occurs among Infusoria,
which gave rise to a grave error that gained general acceptance
for a time, through the high authority of its promulgator, Prof.
Stein. There is a curious tribe of suctorial Animalcules termed
Acinetce, which have no mouths, but put forth tubular prolongations
which act as suckers. These penetrate the bodies of other animal-
cules, either in their inactive or in their encysted condition, and
develope and multiply themselves in their interior. In fig. 20 is
seen a Paramecium containing three of these parasites, q, q, q',
which work their way into the body without rupturing its integu-
ment, pushing this before them so as to form a sort of pouch,
wherein they lie, that opens externally in a canal of which the
mouth is seen at x, x. The sexual organs of this individual,
displaced by the parasites, are shown at a, b. In fig. 19 are seen
three Acinetce in different stages of their free state ; one of them,
a, being in repose, but putting forth its suctorial appendages ;
another, B, undergoing self-division, and having cilia as well as
suckers on one-half; and a third, c, swimming actively in the
INFUSOEIA : — VORTICELLIN,£ ; OPHEYDIN^E. 499
liquid by means of its cilia* — Another parasitic growth, consisting
of a large vesicle crowded with Vibrios, has been mistaken by
some excellent observers for a spermatic cyst filled with sperma-
tozoa.
400, It is obvious that no Classification of Infusoria can be
of any permanent value, until it shall have been ascertained by
the study of their entire life-history, what are to be accounted
really distinct forms ; and the differences between them, consisting
chiefly in the shape of their bodies, the disposition of their cilia,
the possession of other locomotive appendages, the position of the
mouth, the presence of a distinct anal orifice, and the like, are
matters of such trivial importance as compared with those leading
features of their structure and physiology on which we have been
dwelling, that it does not seem desirable to attempt in this place
to give any account of them. The most remarkable departure from
the ordinary type is presented by the Vorticellince, the habit of
which is to attach themselves to the stems of aquatic plants or
some other supports : — either by the apex of their own conical body,
as is the case with Stentor (Plate XIY., fig. 21), one of the largest
of all Infusoria (being visible to the naked eye), which is very
common in ponds and ditches, attaching itself to duck- weed, de-
caying reeds, or other floating bodies, round which it forms a sort
of slimy fringe, but which is often found swimming freely, its
tnimpet-shaped body drawn together into the form of an egg ; —
or by a footstalk several times its own length, as is the case with
Vorticella (Fig. 257), which also occasionally quits its attachment
(the stalk apparently dying and being thrown-off), and swims
rapidly through the water, being propelled by the fringe of cilia,
which, when the body was fixed by its stalk, served to produce
a vortex in the surrounding fluid, that brought it both food and
air.
401. Another curious departure from the ordinary type is pre-
sented by the Family Ophrydidce ; the Animalcules of which,
closely resembling some Vorticellinas in their individual structure,
are usually found imbedded in a gelatinous mass of a greenish
colour, which is sometimes adherent, sometimes free, and may
attain the diameter of four or five inches, presenting such a
strong general resemblance to a mass of Nostoc (§ 268) or even of
Frogs' spawn, as to have been mistaken for such. The mode in
which these masses are produced closely resembles that in which
the masses of Mastogloia (§ 258) or of Palmella (§ 263) are formed ;
since they simply result from the fact that the multitude of
individuals produced by a repetition of the process of self-division,
remain connected with each other for a time by a gelatinous
exudation from the surface of their bodies, instead of at once be-
* It was supposed by Prof. Stein that the Acineta-form is a stage in the
development of the young of the Paramecza, Vorticellce, &c, in whose bodies
they are found. But this doctrine, contested from the first by many able
observers, has now been abandoned bv himself.
kk2
500 MICKOSCOPIC FOEMS OF ANIMAL LIFE.
coming completely isolated. From a comparison of the dimensions
of the individual Ophrydia, each of which is about 1 -120th of an
inch in length, with those of the composite masses, some estimate
may be formed of the number included in the latter ; for a cubic
inch would contain nearly eight millions of them, if they were
closely packed ; and many times that number must exist in the
larger masses, even making allowance for the fact that the bodies
of the Animalcules are separated from each other by their gela-
tinous cushion, and that the masses have their central portions
occupied only by water. Hence we have, in such clusters, a
distinct proof of the extraordinary extent to which multiplication
by duplicative subdivision may proceed, without the interposition
of any other operation. These Animalcules, however, free them-
selves at times from their gelatinous bed, and have been observed
to undergo an ' encysting process ' corresponding with that of the
Vorticellince (§ 393).
402. As it is among Animalcules that the action of the organs
termed Cilia has the most important connection with the vital
functions, it seems desirable to introduce here a more particular
notice of them. They are always found in connection with cells, of
whose substance, as we have seen among Protophytes (§§ 189, 194),
they may be considered as extensions. The form of the filaments
is usually a little flattened, and tapering gradually from the base
to the point. Their size is extremely variable ; the largest that
have been observed being about 1- 500th of an inch in length, and
the smallest about l-13,000th. When in motion, each filament
appears to bend from its root to its point, returning again to its
original state, like the stalks of corn when depressed by the wind ;
and when a number are affected in succession with this motion, the
appearance of progressive waves following one another is produced,
as when a corn-field is agitated by successive gusts. When the
ciliary action is in full activity, however, little can be distinguished
save the whirl of particles in the surrounding fluid ; but the
hack-stroke may often be perceived, when the forward-stroke is
made too quickly to be seen ; and the real direction of the move-
ment is then opposite to the apparent. In this back-stroke, when
made slowly enough, a. sort of ' feathering ' action may be ob-
served ; the thin edge being made to cleave the liquid, which has
been struck by the broad surface in the opposite direction. It is
only when the rate of movement has considerably slackened, that
the shape and size of the cilia, and the manner in which their
stroke is made, can be clearly seen. — It has been maintained by
some that the action of the Cilia is muscular ; but they are
generally too small to contain even the minutest fibrillge of true
muscular tissue, and no such elements can be discerned around
their base ; their presence in Plants, moreover, seems distinctly
to negative such an idea. Hence we must consider them as organs
sui generis, wherein the contractility of the cell to which they be-
long is (as it were) concentrated. We have seen that in the
CILIARY MOVEMENT.— ROTIFEKA. 501
RMzojJods, the entire rnass of whose sarcode is highly contractile, no
cilia are present ; whilst in the Infusoria, whose bodies have compara-
tively little contractility, the movements are delegated to the cilia.
403. Cilia are not confined, however, to Animalcules and Zoo-
phytes, but exist on some of the free internal surfaces, especially the
walls of the Respiratory passages, of all the higher Animals, not
excepting Man himself. Our own experience assures us that their
action takes place, not only without any exercise of will on our own
parts, but even without affecting our consciousness ; and it has been
found to continue for many hours, or even days, after the death
of the body at large. How far it is subject to any conscious con-
trol on the part of these Animalcules, in which the cilia serve as
instruments for locomotion, as well as for bringing to them food or
oxygen, it is impossible for any one to say with confidence. In
this important respect, however, the ciliary movement of Animal-
cules differs from that which is observable in the higher animals, —
that whilst in the latter it is co nst a nt, giving the idea of purely
automatic agency, in the former it is so interrupted and renewed
as almost necessarily to suggest to the observer the notion of choice
and direction.
404. Eotifera, on Wheel-Animalcules. — "We now come to that
higher group of Animalcules, which, in point of complexity of
organization, is as far removed from the preceding, as Mosses are
from the simplest Protophytes ; the only point of real resemblance
between the two groups, in fact, being the minuteness of size
which is common to both, and which was long the obstacle to the
recognition of the comparatively elevated character of the Eotifera,
as it still is to the precise determination of certain points of their
structure. Some of the Wneel- Animalcules are inhabitants of salt
water only ; but by far the larger proportion are found in collections
of fresh water, and rather in such as are free from actively decom-
posing matter, than in those which contain organic substance in a
putrescent state. Hence when they present themselves in Vegetable
infusions, it is usually after that offensive condition which is
favourable to the development of many of the Infusoria has passed-
away ; and they are consequently to be looked-for after the disap-
pearance of many successions (it may be) of Animalcules of inferior
organization. Eotifera are more abundantly developed in liquids
which have been long and freely exposed to the open air, than in such
as have been kept under shelter ; certain kinds, for example, are
to be met with in the little pools left after rain in the hollows of the
lead with which the tops of houses are partly covered ; and they
are occasionally found in enormous numbers in cisterns which are
not beneath roofs or otherwise covered over.* They are not, how-
ever, absolutely confined to collections of liquid ; for there are a few
species which can maintain their existence in damp earth ; and the
common Rotifer is occasionally found in the interior of the leaf-cells
of Sphagnum (§ 313).
* See a remarkable instance of this in p. 277 note.
502
MICEOSCOPIC FORMS OF ANIMAL LIFE.
405. The Wheel-like organs from which the class derives its
designation, are most characteristically seen in the common form
just mentioned (Fig. 262), where they consist of two disk-like lobes
or projections of the body, whose margins are fringed with long
cilia ; and it is the uninterrupted
Fig. 261. succession of strokes given by these
cilia, each row of which nearly re-
turns (as it were) into itself, that
gives rise by an optical illusion to
the notion of ' wheels.' This ar-
rangement, however, is by no means
universal ; in fact, it obtains in only
a small proportion of the group ;
and by far the more general plan is
that seen in Fig. 261, in which the
cilia form one continuous line across
/f'Wy P^^fTlSM ^e ^ody, being disposed upon the
II f^/fj'<\: ':■;.,'..' ^\||| sinuous edges of certain lobes or
projections which are borne upon its
anterior portion. Some of the chief
departures from this plan will be
noticed hereafter (§ 414).
406. The great transparence of the
Eotifera permits their general struc-
ture to be easily recognised. They
have usually an elongated form,
similar on the two sides ; but this
rarely exhibits any traces of seg-
mental division. The body is covered
with a double envelope, both layers
of which are extremely thin and
flexible in some species, whilst in
others the outer one seems to possess
a horny consistence. In the former
case the whole integument is drawn together in a wrinkled
manner when the body is shortened ; in some of the latter the
sheath has the form of a polype-cell, and the body lies loosely
in it, the inner layer of the integument being separated from the
outer by a considerable space (Fig. 264) ; whilst in others the
envelope or lorica is tightly fitted to the body, and strongly resembles
the horny casing of an Insect or the shell of a Crab, except that
it is not jointed, and does not extend over the head and tail, which
can be projected from the openings at its extremities, or completely
drawn within it for protection (Fig. 265). In those Eotifera in
which the flexibility of the body is not interfered with by the con-
solidation of the external integument, we usually find it_ capable of
great variation in shape, the elongated form being occasionally ex-
changed for an almost globular one, as is seen especially when the
animals are suffering from deficiency of water ; whilst by alternating
Brachiomis pala.
the whole integument
EOTIFERA, OR WHEEL-ANIMALCULES.
503
Fig. 262.
*cJi
movements of contraction and extension, they can mate their way
over solid surfaces, after the manner of a Worm or a Leech, with
considerable activity, — some even of the loricated species being
rendered capable of this kind of progression by the contractility of
the head and tail. All these, too, can swim readily through the
water by the action of their cilia ;
and there are some species which
are limited to the latter mode of
progression. The greater number
have an organ of attachment at
the posterior extremity of the
body, which is usually prolonged
into a tail, by which they can affix
themselves to any solid object ; and
this is their ordinary position,
when keeping their ' wheels ' in
action for a supply of food or of
water ; they have no difficulty,
however, in letting-go their hold
and moving through the water in
search of a new attachment, and
may therefore be considered as
perfectly free. The sessile species,
in their adult stage, on the other
hand, remain attached by the pos-
terior extremity to the spot on
which they have at first fixed
themselves ; and their cilia are
consequently employedfor no other
purpose than that of creating cur-
rents in the surrounding water.
407. In considering the internal
structure of Eotifera, we shall
take as its type the arrangement
which it presents in the Rotifer
vulgaris (Fig. 262) ; and specify
the principal variations exhibited
elsewhere. The body of this ani- the wheels Irawn -Id, and" al
mal, when fully extended, possesses tne wheels expanded :—a, mouth ; &,
"
Rotifer vulgaris, as seen at A with
eye-spots ; c, wheels ; d, calcar (an-
tenna ?) ; e, jaw and teeth ; /, alimen-
tary canal ; g, glandular (?) 'mass en-
closing it; h, longitudinal muscles;
i, i, tubes of water- vascular system ;
fc, young animal ; I, cloaca.
greater length in proportion to its
diameter than that of most others
of its class ; and the tail is com-
posed of three joints or segments,
which are capable of being drawn-
up, one within another, like the
sliding tubes of a telescope, each having a pair of Drones or points
at its extremity. Within the external integument of the body are
seen a set of longitudinal muscular bands (k), which serve to draw
the two extremities towards each other ; and these are crossed by a
504 MICKOSCOP1C FOKMS OF ANIMAL LIFE.
set of transverse annular bands, which also are probably muscular,
and serve to diminish the diameter of the body, and thus to increase
its length. Between the wheels is a prominence bearing towards two
red spots (&), supposed to be rudimentary eyes, and having the
mouth (a) at its extremity ; this prominence may be considered,
therefore, as a true head, notwithstanding that it is not clearly
distinguishable from the body. This head also bears upon its
under surface a projecting tubular organ (d) which was thought by
Prof. Ehrenberg to be a siphon for the admission of water to
the cavity of the body for the purpose of respiration ; this, however,
is certainly not the case, the tube being imperforate at its extremity ;
and there seems much more probability in the idea of Dujardin,
that it represents the antennae or palpi of higher Articulata, the
single organ being replaced in many Rotifera by a pair, of which
each is furnished at its extremity with a brush-like tuft of hairs
that can be retracted into the tube. The oesophagus, which is
narrow in the Rotifer, but is dilated into a crop in Steplianoceros
(Fig. 264) and in some other genera, leads to the masticating
apparatus (Fig. 262, e), which in these animals is placed far behind
the mouth, and in close proximity to the stomach.
408. The Masticating apparatus has been made the subject of
attentive study by Mr. P. H. Gosse ; who has given an elaborate
account of the various types of form which it presents in the several
subdivisions of the group.* The following description of one of
the more complicated will serve our present purpose. The various
moveable parts are included in a muscular bulb, termed the mastax
(Fig. 263, a), which intervenes between the buccal funnel (m) and the
oesophagus (p). The mastax includes a pair of organs, which, from
the resemblance of their action to that of hammers working on an
anvil, may be called mallei, and a third, still more complex, termed
the incus. Each malleus consists of two principal parts placed
nearly at right angles to each other, the manubrium (c), and the
uncus (e) ; these are articulated to one another by a sort of hinge-
joint. The former, as its name imports, serves the purpose in
some degree of a handle ; and it is the latter which is the instrument
for crushing and dividing the food. This is done by means of the
finger like processes with which it is furnished at the edge where it
meets its fellow ; these being five or six in number, set parallel to
each other like the teeth of a comb. The incus also consists of
distinct articulated portions, namely, two stout rami (a) resting on
what seems a slender footstalk (h) termed the fulcrum; when
viewed laterally, however, the fulcrum is seen to be a thin plate,
having the rami so jointed to one edge of it that they can oj^en and
close like a pair of shears. The uncus of each malleus falls into
the concavity of its respective ramus, and is connected with it by
a stout triangular muscle (i) which is seen passing from the hollow
of the ramus to the under surface of the uncus. It is difficult to
* " Philosophical Transactions," 1856, p. 419.
MASTICATING APPARATUS OF EOTIFEEA.
505
say with, certainty what is the substance of which, these firm
structures are composed ; it is not affected by solution of potass,
but is instantly dissolved without effervescence by the mineral acids
and by acetic acid. Besides the muscles already described, a thick
Fig. 263.
Masticating Apparatus of Euchlanis deflexa : — a, Mastax; c,
manubrium, and e, uncus, of Malleus ; </, rami, and h, fulcrum,
of Incus ; i, muscle connecting ramus and uncus ; j, muscle
passing from malleus to mastax ; k, muscle connecting uncus
and manubrium ; m, buccal funnel ; n, salivary glands ; p,
oesophagus.
band (j) embraces the upper and outer angle of the articulation
of the malleus ; and is inserted in the adjacent wall of the mastax ;
and a semi-crescentic band (h) is inserted by its broad end into the
inferior and basal part of the uncus, and by its slender end into the
middle of the inner side of the manubrium ; the former of these may
be considered as an extensor, and the latter as a flexor, of the
malleus. By these and other muscles which, cannot be so clearly
distinguished, the unci are made to approach and recede by a per-
pendicular motion on the hinge-joint, so that their opposing faces
come into contact, and their teeth bruise-down the particles of food;
but at the same time they are carried apart and approximated
laterally by the movement of the free extremities of the manubria.
The rami of the incus also open and shut with the working of the
mallei : and by the conjoint action of the whole, the food is effectually
comminuted in its passage downwards.
409. The form of the Alimentary Canal varies ; this being some-
times a simple tube, passing without enlargement or constriction
from the masticating apparatus to the anal orifice at the posterior
part of the body ; whilst in other instances there is a marked dis-
tinction between the stomach and intestinal tube, the former being
506 MICEOSCOPIC FOKMS OF ANIMAL LIFE.
a large globular dilatation immediately "below the jaws, whilst the
latter is cylindrical and comparatively small. The alimentary
of Rotifer (Fig. 262) most resembles the first of these types, but
presents a dilatation (I) close to the anal orifice, which may be con-
sidered as a cloaca : that of Brachionus (Fig. 261) is rather formed
upon the second. Connected with the alimentary canal are various
Glandular appendages, more or less developed ; sometimes cluster-
ing round its walls as a mass of separate follicles, which seems
to be the condition of the glandular investment (g) of the
alimentary canal in Rotifer ; in other cases having the form of
ceecal tubuli. Some of these open into the stomach close to the
termination of the oesophagus, and have been supposed to be
Salivary or Pancreatic in their character, whilst others, which
discharge their secretion into the intestinal tube, have been
regarded, and probably with correctness, as the rudiment of a
Liver. — In the genus Asplanclma (Gosse), there is a wide departure
from the ordinary Rotifer type ; as the species belonging to it
have neither intestine nor anus. The stomach consists of a large
bag at the end of the gullet, about which, when the animals
are quiet, the ovary is bent in a horseshoe form. The indiges-
tible matters are ejected through the mouth. The curious
absence of any digestive apparatus in the males of this group,
will be presently noticed (§ 411).*
410. There does not appear to be any special Circulating ap-
paratus in these animals ; but the fluid which is contained in
the ' general cavity of the body,' between the exterior of the
alimentary canal and the inner tegumentary membrane, is pro-
bably to be regarded as nutritive in its character ; and its
aeration is provided-for by a peculiar apparatus, which seems
to be a rudimentary form of the ' water- vascular system,' that
attains a high development in the class of "Worms. On either
side of the body there is usually to be observed a long flexuous
tube (Fig. 261), which extends from a contractile vesicle common
to both and opening into the cloaca (Fig. 262, i, i) towards the
anterior region of the body, where it frequently subdivides into
branches, one of which may arch-over towards its opposite side,
and inosculate with a corresponding branch from its tube.
Attached to each of these tubes are a number of peculiar organs
(usually from two to eight on each side), in which a trembling
movement is seen, very like that of a nickering flame ; these appear
to be pear-shaped sacs, attached by hollow stalks to the main tube,
and each having a long cilium in its interior, that is attached by
one extremity to the interior of the sac, and vibrates with a quick
undulatory motion in its cavity ; and there can be little doubt that
their purpose is to keep-up a constant movement in the contents of
* See Brightwell in "Ann. Nat. Hist.," Ser. 2, Vol. ii. (1848), p. 153 ; Dal-
rymple in " Philos. Transact.," 1849, p. 339; and Gosse in';i\nn. Nat. Hist.,"
Ser. 2, Vols. iii. (1848), p. 518 ; vi. (1850), p. 18 ; and viii. (1851), p. 198.
EEPEODTTCTION OF EOTIFEEA. 507
the aquiferous tubes, whereby fresh water may be continually intro-
ducedfrom without for the aeration of the fluids of the body.* — There
is much uncertainty with regard to the structures which Prof.
Ehrenberg has described as Ganglia and JSTerves ; and it seems
doubtful if there is more than a single nervous centre in the
neighbourhood of the single, double, or multiple red spots, which
are seen upon the head of the Kotifera, and which, corresponding
precisely in situation with those that in the higher Articulata are
unquestionably eyes, are probably to be regarded as rudiments of
Visual organs.
411. The Eeproduction of the Eotifera has not yet been com-
pletely elucidated. There is no instance, in this group, in which
multiplication by external gemmation or spontaneous fission is
certainly known to take place ; but the occurrence of clusters
formed by the aggregation of a number of individuals of Cono-
chilus, adherent by their tails, and enclosed within a common lorica,
would seem to indicate that these clusters, like the aggregations of
Polygastrica, Polyzoa, and Tunicata, must have been formed by
continuous growth from a single individual. It will be presently
shown, moreover, that there is strong reason for the belief that
what are commonly termed ' eggs ' are really internal gemmae.
Although the Eotifera were affirmed by Prof. Ehrenberg to be her-
maphrodite, yet the existence of distinct sexes has been detected
in so many genera (for the most part by Mr. Gossef), that it may
fairly be presumed to be the general fact. The male is inferior in
size to the female, and sometimes differs so much in organization
that it would not be recognised as belonging to the same species, if
the copulative act had not been witnessed. In all the cases yet
known, as in the Asplanclina, whose separate male was first dis-
covered by Mr. Brightwell in 1848, there is an absolute and
universal atrophy of the digestive system ; neither mastax, jaws,
oesophagus, stomach, nor intestines, being discoverable in any
male ; in fact, no other organs being fully developed than those of
generation. It would appear, therefore, quite unfit to obtain
aliment for itself ; and its existence is probably a very brief one,
being continued only so long as the store of nutriment supplied by
the egg remains unexhausted. In a remarkable six-limbed Eotifer
discovered by Dr. Hudson,* and named by him Pedalion mira,
on account of its having a large swimming limb, resembling in
appearance one belonging to a water-flea, the virgin female was
found to lay female eggs during the greater part of the year, while
male eggs, which are not found in the same individuals, " are half
* See Mr. Huxley's account of these organs, in his description of LacinularUi
socialis, " Transact, of Microsc. Soc," Ser. 2, Vol. i. — Other observers have
supposed that the pyrifomi sacs communicate with the general cavity of the
body; but the Author has much confidence in the correctness of Mr. Huxley's
statements on this point.
f " Philosophical Transactions," 1857, p. 313.
% In Eotifer, &c, " Monthly Microsc. Joum.," Vol. viii. (1872), p. 209.
508 MICROSCOPIC FORMS OF ANIMAL LIFE.
the size of the female ones, and are carried in clusters of often a
score at a time." Dr. Hudson describes and figures the males as
very small in comparison with the females ; and states that
they are very short lived, sometimes dying within an hour. In
Rotifer, however, as in by far the larger proportion of the class, no
males have been discovered ; probably because they are produced
only at certain times. The female organ consists of a single
ovarian sac, which frequently occupies a large part of the cavity of
the body, and opens at its lower end by a narrow orifice into
the cloaca. — Although the number of eggs in these animals is so
small, yet the rapidity with which the whole process of their
development and maturation is accomplished, renders the multipli-
cation of the race very rapid. The egg of the Hydatina is
extruded from the cloaca within a few hours after the first rudi-
ment of it is visible ; and within twelve honrs more the shell
bursts, and the young animal comes forth. In Rotifer and
several other genera, the development of the embryo takes-place
whilst the egg is yet retained within the body of the parent
(Fig. 262, h), and the young are extruded alive ; whilst in some
other instances the eggs, after their extrusion, remain attached to
the posterior extremity of the body (Fig. 261), until the young are
set free. In general it would seem that whether the rupture of the
egg-membrane takes-place before or after the egg has left the body,
the germinal mass within it is developed at once into the form of
the young animal, which -usually resembles that of its parent ; no
preliminary metamorphosis being gone through, nor any parts de-
veloped which are not to be permanent. In Floscularia omata,
however, the young leave the eggs in the shape of little maggots,
from one end of which a tuft of cilia soon appears. The form
changes in a few hours, the ciliated end becoming lobed, and
the body rounded. The foot is developed later.* The transpa-
rence of the egg-membrane, and also of the tissnes, of the parent
Rotifer, allows the process of development to be watched, even
when the egg is retained within the body ; and it is curious to
observe, at a very early period, not merely the red eye-spot of the
embryo, but also a distinct ciliary movement. The multiplication
of Hydatina (in which genus three or four eggs are deposited at
once, and their development completed out of the body) takes
place so rapidly, that, according to the estimate of Prof. Ehreu-
berg, nearly seventeen millions may be produced within twenty-
four days from a single individual.
412. Even in those species which usually hatch their eggs
within their bodies, a different set of Ova is occasionally deve-
loped, which are furnished with a thick glutinous investment ;
these, which are extruded entire, and are laid one upon another,
so as at last to form masses of considerable size in proportion
to the bulk of the animals, seem not to be destined to come so
* See Mr. Slack's "Marvels of Pond Life," 2nd Edit., p. 54.
REPRODUCTION OF ROTIFERA. 509
early to maturity, but very probably remain dormant during the
whole winter season, so as to produce a new brood in the spring.
These ' winter-eggs ' are inferred by Mr. Huxley, from the history
of their development, to be really gemmce produced by a non-
sexual operation ; while the bodies ordinarily known as ova, he
considers to be true generative products. Dr. Cohn, however,
states that he has ascertained, by direct experiment upon those
species in which the sexes are distinct, that the bodies com-
monly termed 'ova' (Figs. 261, 262), are really internal gemmce,
since they are reproduced, through many successions, without any
sexual process, just like the external gemmae of Hydra (§ 471), or
the internal gemma? of Entomostraca (§ 568) and Aphides (§ 603) ;
whilst the ' winter-eggs ' are only produced as the result of a true
generative act.* And this view appears to the Author more
accordant with general physiological analogy than that of Mr.
Huxley ; since, in Botifera, as in the other instances referred to,
the multiplication by gemmation goes-on rapidly so long as food
and warmth are abundantly supplied, but gives place to the
generative process, when the nutritive activity is lowered by
their withdrawal.
413. Certain Botifera, among them the common Wheel-Ani-
malcule, are remarkable for their tenacity of life, even when reduced
to such a state of dryness that they will break in pieces when
touched with the point of a needle (as the Author has himself
ascertained) ; for they can be kept in this condition for any length
of time, and will yet revive very speedily upon being moistened.
Taking advantage of this fact, many microscopists are in the habit
of keeping by them stocks of desiccated Eotifers, which can be
distributed in the condition of dry dusty powder. The desiccating
process has been carried yet farther with the tribe of Tarcligrada
(§ 414, iv.) ; individuals of which have been kept in a vacuum for
thirty days, with sulphuric acid and chloride of calcium, and yet
have not lost their capability of revivification. These facts, taken
in connection with the extraordinary rate of increase mentioned in
the preceding paragraph, remove all difficulty in accounting for the
extent of the diffusion of these animals, and for their occurrence in
incalculable numbers in situations where, a few days previously,
none were known to exist. For their entire bodies may be wafted
in a dry state by the atmosphere from place to place ; and their
return to a state of active life, after a desiccation of unlimited
duration, may take place whenever they meet with the requisite
conditions — moisture, warmth, and food. It is probable that the
Ova are capable of sustaining treatment even more severe than the
fully developed Animals can bear ; and that the race is frequently
continued by them when the latter have perished. — It is not
requisite to suppose, however, that in any of the foregoing cases
* See his important Memoir, 'Ueber die Fortpflanzung der Eaderthiere,' in
" Siebold and Kolliker's Zeitschrift," 1855.
510
MICROSCOPIC FOEMS OF ANIMAL LIFE.
Fig. 264.
^"
the desiccation is complete ; for it appears that Wheel- Animal-
cules, in drying, exude a glutinous matter that forms a sort of
impervious casing, and keeps-in the remaining fluid * When acted
on by heat as well as by drought, Rotifers and Tardigrades lose
their vitality ; yet the former have survived a gradual heating up
to 200° Eahr. _
414. The principles on which the various forms that belong to
this Class should be systematically arranged, have not yet been
satisfactorily determined. By Prof. Ehrenberg, the disposition of
the ciliated lobes or wheel-organs, and the enclosure or non-enclo-
sure of the body in a lorica or case, were taken as the basis of his
classification ; but as his ideas on both these points are incon-
sistent with the actual facts of organization, the arrangement
founded upon them cannot be received. Another division of the
class has been propounded by M. Dujardin,
which is based on the several modes of life of
the most characteristic forms. And in a
third, more recently put forth by Prof. Ley dig,
the general configuration of the body, with
the presence, absence, and conformation of
the foot (or tail) are made to furnish the
characters of the subordinate groups. Either
of the two latter is certainly more natural
than the first, as bringing together for the
most part the forms which most agree in
general organization, and separating those
which differ; and we shall adopt that of
M. Dujardin as most suitable to our present
purpose.
i. The first group includes those that habi-
tually live attached by the foot, which is
prolonged into a pedicle ; and it includes two
families, the Floscularians and the Melicer-
tians, the members of which are commonly
found attached to the stems and leaves of
aquatic plants, by a long pedicle or foot-stalk,
bearing a somewhat bell-shaped body. In one
of the most beautiful species, the Steplianoceros
Eiclwrnii (Eig. 264), this body has five long
tentacles, beset with tufts of cilia, whilst the
body is enclosed in a gelatinous cylindrical
cell. At first sight, the tentacles of this
Rotifer may seem to resemble those of the
Polyzoa ; but, if they are carefully illumi-
nated, the filaments which beset them will
be found to be much larger, to be arranged
differently, and to exhibit only an occasional
* See Davis in "Monthly Microsc. Joum.," Vol. ix. (1863), p. 207 ; also
Slack, at p. 211 of same volume.
Steplianoceros Eichornii.
CLASSIFICATION OF EOTIFEEA. 511
motion, not. at all resembling the regular rhythmical vibrations of
those of Polyzoa.* In fact, they seem rather to deserve the designa-
tion of setce (bristles) ; for " their action is spasmodic, it creates no
vortex, and it is only by actual contact with these setce that floating
particles are whipped within the area enclosed by the lobes, where
by the same whipping action they are twitched from rjoint to point
irregularly downwards, until they come within the range of a
vortex that is due, not to any action of the setce, but to a range of
minute cilia in the funnel."f A careful comparison of Stephano-
ceros with other forms, shows that its tentacles are only extensions
of the ciliated lobes which are common to all the members of these
families ; and the cylindrical ' cell ' which envelopes the body
is formed by a gelatinous secretion from its surface, thrown-off
in rings, the indications of which often remain as a series of con-
strictions. In respect of the length of the filaments projecting
from its lobes, and the breadth of these expansions, Floscularia is
still more aberrant. — The body of Melicerta is protected by a most
curious cylindrical tube, composed of little rounded pellets agglu-
tinated together ; this is obviously an artificial construction, and
Mr. Gosse has been fortunate enough to have an opportunity of
watching the animal whilst engaged in building it up. J Beneath
a projection on its head, which he terms the chin, there is observed
a small disk-like organ, in which, when the wheels are at work, a
movement is seen very much resembling that of a revolving venti-
lator. Towards this disk the greater proportion of the solid
particles that may be drawn from the surrounding liquid into the
vortex of the wheel-organs, are driven by their ciliary movement,
a small part only being taken into the alimentary canal ; and there
they accumulate until the aggregation (probably cemented by a
glutinous secretion furnished by the organ itself) acquires the size
and form of one of the globular pellets of the case ; the time ordi-
narily required being about three minutes. The head of the animal
then bends itself down, the pellet-disk is applied to the edge of the
tube, the newly-formed pellet is left attached there, and, the head
being lifted into its former position, the formation of a new pellet
at once commences.
n. The next of M. Dujardin's primary groups (ranged by him,
however, as the third) consists of the ordinary Rotifer and its
allies, which pass their lives in a state of alternation between
the conditions of those attached by a pedicle, of those which
habitually swim freely through the water, and of those which
* In ordinary drawings, the filaments of the Steplianoceros are represented as
short bristles ; this is an error arising from bad instruments or defective illu-
mination. It requires considerable skill to show these filaments, or those of
the Floscularia, in their true length ; but the beauty of the objects is greatly
increased when this is accomplished.
f See Mr. C. Cubitt's ' Observations on the Economy of Stephanoceros,' in
" Monthly Microsc. Journ.,"' Vol. iii., 1870, p. 242.
t ' On the architectural instincts of Melicerta ringensf in " Trans, of Microsc.
Soc," Vol. iii. (1852), p. 58.
512
MICROSCOPIC FORMS OF ANIMAL LIFE.
creep or crawl over hard surfaces. — As these have already been
fully described, it is not requisite to dwell longer upon them.
in. The next group consists of those Eotifers which seldom or
never attach themselves by the foot, but habitually swim freely
through the water ; and putting aside the peculiar aberrant form
Albertia, which has only been found as a parasite in the intestines
of Worms, it may be divided into two families, the Bracliionians
and the Furcularians. The former are for the most part dis-
tinguished by the short, broad, and flattened form of the body
(Figs. 261, 265) ; which is, moreover, enclosed in a sort of
Fig. 265.
2sloteus quadricornis ; A, dorsal view ; B, side view.
cuirass, formed by the consolidation of the external integument.
This cuirass is often very beautifully marked on its surface, and
may be prolonged into extensions of various forms, which are
sometimes of very considerable length. The latter (corresponding
almost exactly with the Hydatineoe of Prof. Ehrenberg) derive
their name from the bifurcation of the foot into a sort of two-
bladed forceps ; their bodies are ovoidal or cylindrical, and are en-
closed in a flexible integument, which is often seen to wrinkle
itself into longitudinal and transverse folds at equidistant lines.
To this family belongs the Hydatina senta, one of the largest of
the Eotifera, which was employed by Prof. Ehrenberg as the
chief subject of his examination of the internal structure of this
group ; as does also the Asplanclma, the curious condition of whose
digestive apparatus has been already noticed (§ 409).
iv. The fourth of M. Dujardin's primary orders consists of the
very curious tribe, first carefully investigated by M. Doyere, to
which the name of Tardigrada has been given, on account of the
CLASSIFICATION OF EOTIFERA. 513
slowness of their creeping movement. Their relation to the true
Rotifera, however, is not at all clear ; and many naturalists regard
them as altogether distinct. They are found in the same localities
with the Eotifers, and, like them, can be revivified after desiccation
(§ 413) ; but they have a vermiform body, divided transversely into
five segments, of which one constitutes the head, whilst each of
the others bears a pair of little fleshy protuberances, furnished
with four curved hooks, and much resembling the pro-legs of a
Caterpillar. The head is entirely unpossessed of ciliated lobes ; and
It is only in the presence of a pair of jaws somewhat resembling
those of Rotifera, and in the correspondence of their general grade
of organization, that they bear any structural relation to the
class we have now been considering. They may be pretty cer-
tainly regarded as a connecting link between the Eotifera and the
"Worms ; but they should probably be ranked on the worm- side
of the boundary.
415. Notwithstanding that all the best-informed Zoologists are
now agreed in ranking the class of Rotifera in the Articulated
series, yet there is still a considerable discordance of opinion as
to the precise part of that series in which they should stand. For
whilst Prof. Leydig, who has devoted much attention to the
study of the class, regards them as most allied to the Crustacea,
and terms them ' Cilio-crustaceans,' Prof. Huxley, with (as it seems
to the Author) a clearer insight into their real nature, has argued
that they are more connected with the Annelida, through the
resemblance which they bear to the early larval forms of that class
(§ 554). Considered in this light, the Tardigrada might seem to
represent a more advanced phase of the same developmental his-
tory.*
* The following are the Treatises and Memoirs which (in addition to those
already referred to) contain the most valuable information in regard to the
principal forms of Animalcules : — Ehreuberg, "Die Infusionsthierchen," Berlin,
1838; Dujardin, "Histoire Naturelle des Zoophytes Infusoires," Paris, 1841;
and Pritchard, "History of Infusoria," 4th Ed., London, 18(Jl (a comprehensive
repertory of information). For the Khizopoda and Infusoria specially, see
Claparede and Lachmann, "Etudes sur les Infusoires et les Iihi'zopodes,"
Geneva, 1858-1861 ; Cohn, in "Siebold and Kolliker's Zeitschrift," 1851-4, and
1857 ; Lieberkiihn, in " Midler's Archiv," 1856, and " Ann. of Nat. Hist.," 2nd
Ser., Vol. xviii. 1856 ; and the elaborate systematic Treatise of Stein, " Der
Organismus des Infusionsthiere," Leipzig, Erste Abtheilung, 1859, Zweite
Abtheilung, 1867. And for the Eotifera specially, see Leydig, in " Siebold
and Kolliker's Zeitschrift," Bd. vi., 1854 ; Gosse on Melicerta ringens, in " Quart.
Journ. of Microsc. Science," Vol. i., p. 71; Williamson on Melicerta ringens,
" Quart. Journ. of Microsc. Science," Vol. i. (1858), p. 1 ; Huxley on Lacinu-
laria socialis, in " Transact, of Microsc. Soc," Ser. 2, Vol. i. (1853\ p. 1 ; and
Cohn, in " Siebold and Kolliker's Zeitschrift," Bde. vii., ix., 1856, 1858. Mr.
Slack's "Marvels of Pond Life " (2nd Edit., London, 1871) contains many inte-
restiug observations on the habits of Infusoria and Eotifera.
L L
CHAPTER X.
EORAMINIFERA, POLYCYSTIC, AND SPONGES.
416. Returning now to the lowest or ffliizopod type of Animal
life (§ 369), we have to direct our attention to three very remark-
able series of forms, almost exclusively marine, under which that
type manifests itself ; all of them distinguished by skeletons of
greater or less density ; and these skeletons generally so consolidated
by Mineral deposit, as to retain their form and intimate structure
long after the Animals to which they belonged have ceased to live,
even for those undefined periods in which they have been im-
bedded as Fossils in strata of various geological ages. In the first
of these groups, the Foraminifera, the skeleton usually consists of
a calcareous many-chambered Shell, which closely invests the
sarcode-body, and which, in a large proportion of the group, is
perforated with numerous minute apertures ; this shell, however,
is sometimes replaced by a ' test' formed of minute grains of sand
cemented together ; and there are a few cases in which the animal
has no other protection than a membranous envelope. — In the
second group, also, the Folycystina, there is an investing Shell
perforated with apertures ; but this shell is siliceous, and has
usually but one chamber ; and its apertures are often so large and
numerous, that the solid portion of the shell forms little more than
a network, thus indicating a transition to the succeeding group. —
In the group of Porifera or Sponges, the Skeleton is usually
composed o± a network of horny fibres, strengthened either by
calcareous or by siliceous spicules, and having the soft animal
substance, which is composed of an aggregate of Amceba-like
bodies, in its interstices : in this group, moreover, we have a
departure from the Rhizopod type, in the fact that certain parts
of the free surfaces are furnished with cilia, whereby currents
of water are maintained, that serve both for nutrition and for
respiration.
417. Foraminifera. — The animals now known under this de-
signation possess, for the most part, polythalamous or many
chambered shells (Plate XV.), often so strongly resembling those
of Nautilus, Spirula, and other Cephalopod Mollusks, that it is
not surprising that the older Naturalists, to whom the structure of
GENERAL CHARACTERS OF FORAMINIFERA. 515
these animals was entirely unknown, ranked them under that class.
As such they were described by M. D'Orbigny (to whom we owe
much of our knowledge of this group), in all his earlier publications ;
and they were distinguished from the ordinary Cephalopods that
possess a single siphon passing from chamber to chamber, by the
designation Foraminifera, which originally imported that the com-
munications between the chambers are commonly made by several
such apertures, though it is now more commonly understood as
applying to the sieve-like structure often presented by the external
shell. It was by M. Dujardin, in 1835, that the structure of these
animals was first shown to be conformable to the Ehizopod type ;
and notwithstanding the opposition to his views which was set-up
by Prof. Ehrenberg (who associated them with Bryozoa, Chap,
xin.), they have been confirmed by all subsequent observers,
and more especially by the researches of Prof. Schultze,* who
gave admirable descriptions of the animals of several different
kinds of Foraminifera, derived from observation of them during
their living state. The essential conformity of the Foraminifera to
the ordinary Ehizopod type is best seen in such simple forms as
Lagena (Plate XV. fig. y), in which there is no multiplication of
chambers ; for these, which are termed monothalamous or ' single-
chambered,' hold the same place in the Order Beticularia, that
Arcella and TJifflugia (Fig. 253) hold in the Order Lobosa.
418. By far the greater number of Foraminifera are composite
fabrics, evolved by a process of continuous gemmation, each bud
remaining in connection with the body by which it was put forth ;
and according to the plan on which this gemmation takes place,
will be the configuration of the composite body thereby produced.
Thus, if the bud should be put forth from the aperture of Lagena
in the direction of the axis of its body, and a second shell should be
formed around this bud in continuity with the first, and this pro-
cess should be successionally repeated, a straight rod-like shell would
be produced (fig. 10), having many chambers communicating with
each other by the openings that originall}'' constituted their mouths ;
the mouth of the last-formed chamber being the only aperture
through which the sarcode-body, thus composed of a number of
segments connected by a peduncle or ' stolon' of the same material,
could now project itself or draw-in its food. The successive
segments may be all of the same size, or nearly so, in which case
the entire rod will approach the cylindrical form, or will resemble
a line of beads ; but it often happens that each segment is some-
what larger than the preceding (fig. 11), so that the composite shell
has a conical form, the apex of the cone being the original segment,
and its base the one last formed. The method of growth now
described is common to a large number of Foraminifera, chiefly
belonging to the genus Nodosarina ; but even in that genus we
* "Ueber den Organismus der Polythalamien (Foraminiferen)," Leipzig,
1854.
L L 2
516
FOEAMINIFEEA.
have every gradation between the rectilineal (fig. 10), and the spiral
mode of growth (fig. 11) ; whilst in the genns Peneroplis (fig. 5) it
is not at all nncommon for shells which commence in a spiral to
exchange this in a more advanced stage for the rectilineal. When
the snccessive segments are added in a spiral direction, the character
of the spire will depend in great degree npon the enlargement or
non-enlargement of the successively -formed chambers ; for some-
times it opens-ont very rapidly, every whorl being considerably
broader than that which it surrounds, in consequence of the great
Fig. 2C>6.
liotalia ornata, with its pseudopodia extended.
excess of the size of each segment over that of its predecessor, as in
Peneroplis ; but more commonly there is so little difference between
the successive segments, after the spire has made two or three
turns, that the breadth of each whorl scarcely exceeds that of its
predecessor, as is well seen in the section of the Botalia represented
in Fig. 279. An intermediate condition is presented by such a
Botalia as is shown in Fig. 266, which may be taken as a charac-
teristic type of a very large and important group of Forarninifera,
whose general features will be presently described. Again, a spiral
may be either ' nautiloid' or 'turbinoid;' the former designation
PLATE XV.
Various Forms of Foraminifera.
[To face p. 517.
GENERAL CHARACTERS. 517
being applied to that form in which the successive convolutions all
lie in one plane (as they do in the Nautilus), so that the shell is
' equilateral' or similar on its two sides ; whilst the latter is used
to mark that form in which the spire passes obliquely round an
axis, so that the shell becomes ' inequilateral,' having a more or
less conical form, like that of a Snail or a Periwinkle, the first-formed
chamber being at the apex. Of the former we have characteristic
examples in Folystomella (Plate IV., fig. 16) and Nonionina (fig.
19) ; whilst of the latter we find a typical representation in Rotalia
Beccarii (fig. 18). Further, we find among the shells whose increase
takes place upon the spiral plan, a very marked difference as to the
degree in which the earlier convolutions are invested and concealed
by the later. In the great Rot aline group, whose characteristic
form is a turbinoid spiral, all the convolutions are usually visible,
at least on one side (figs. 15, 17, 18) ; but among the nautiloid
tribes it more frequently happens that the last-formed whorl
encloses the preceding to such an extent that they are scarcely, or
not at all, visible externally, as is the case in Cristellaria (fig. 11),
Folystomella (fig. 16), and Nonionina (fig. 19). — The turbinoid spire
may coil so rapidly round an elongated axis, that the number of
chambers in each turn is very small ; thus in Globigerina (fig.
12) there are usually only four ; and in Yalvulina the regular
number is only three. Thus we are led to the liserial arrange-
ment of the chambers which is characteristic of the Textularian
group (fig. 14) ; in which we find the chambers arranged in two
rows, each chamber communicating with that above and that
below it on the opposite side, without any direct communication
with the chambers of its own side, as will be understood by
reference to Fig. 271, a, which shows a 'cast' of the sarcode-body
of the animal. On the other hand, we find in the nautiloid
spire a tendency to pass (by a curious transitional form to be
presently described, § 425) into the cyclical mode of growth ; in
which the original segment, instead of budding-forth on one side
only, developes gemmae all round, so that a ring of small
chambers (or chamberlets) is formed around the primordial
chamber, and this in its turn surrounds itself after the like
fashion with another ring; and by successive repetitions of the
same process the shell comes to have the form of a disk made up
of a great number of concentric rings, as we see in Orbitolites
(Fig. 268) and in Cycloclypeus (Plate XVL, fig. 1).
4H. These and other differences in the flan of growth were made
by M. D'Orbigny the foundation of his Classification of this group,
which, though at one time generally accepted, has now been
abandoned by most of those who have occupied themselves in the
study of the Foraminif era. For it has come to be generally admitted
that ' plan of growth' is a character of very subordinate importance
among the Foraminifera, so that any classification which is primarily
based upon it must necessarily be altogether unnatural ; those
characters being of primary importance which have an immediate
518 FORAMINIFERA.
and direct relation to the Physiological condition of the Animal,
and are thus indicative of the real affinities of the several groups
which they serve to distinguish. The most important of these
characters will now be noticed *
420. Two very distinct types of Shell- structure prevail among
ordinary Foraminifera, — namely, the porcellanous, and the hyaline
or vitreous. The shell of the former, when viewed by reflected light,
presents an opaque-white aspect which bears a strong resemblance
to porcelain ; but when thin natural or artificial laminee of it are
viewed by transmitted light, the opacity gives place to a rich
brown or amber colour, which in a few instances is tinged with
crimson. No structure of any description can be detected in this
kind of shell-substance, which is apparently homogeneous through-
out. Although the shells of this ' porcellanous' type often present
the appearance of being perforated with foramina, yet this appear-
ance is illusory, being due to a mere 'pitting' of the external
surface, which, though often very deep, never extends through
the whole thickness of the shell. Some kind of inequality of that
surface, indeed, is extremely common in the shells of the ' porcel-
lanous' Foraminifera ; one of the most frequent forms of it being
a regular alternation of ridges and furrows, such as is occasionally
seen in Miliola (Plate XV., fig. 3), but which is an almost constant
characteristic of Penerojplis (fig. 5). But no difference of texture
accompanies either this or any other kind of inequality of surface ;
the raised and depressed portions being alike homogeneous. — In the
shells of the vitreous or hyaline type, on the other hand, the proper
shell -sub stance has an almost glassy transparence, which is shown
by it alike in thin natural lamellsB, and in artificially-prepared
specimens of such as are thicker and older. It is usually colour-
less, even when (as in the case with many Eotalince) the substance
of the animal is deeply coloured ; but in certain aberrant Rotalines
the shell is commonly, like the animal body, of a rich crimson hue.
All the shells of this type are beset more or less closely with
tubular 'perforations, which pass directly, and (in general) without
any subdivision, from one surface to the other. These tubuli are
in some instances sufficiently coarse for their orifices to be distin-
guished as punctations on the surface of the shell with a low
magnifying power, as is shown in Fig. 266 ; whilst in other cases
they are so minute as only to be discernible in thin sections seen
by transmitted light under a higher magnifying power, as is shown
in Figs. 282, 283. When they are very numerous and closely set,
the shell derives from their presence that kind of opacity which is
* This subject will be found amply discussed in the Author's "Introduction
to the Study of the Foraminifera," published by the Eay Society; to which
work he would refer such of his readers as may desire more detailed informa-
tion in regard to it. It was with great satisfaction that he found his own
views on this subject to be in essential accordance with those of the late Prof.
Keuss of Vienna, who ranked as the highest Continental authority upon this
group.
TUBULAR SHELL-STRUCTURE. 519
characteristic of all minutely-tubular textures, whose tubuli are
occupied either by air or by any substance having a refractive
power different from that of the intertubular substance, however
perfect may be the transparence of the latter. The straightness,
parallelism, and isolation of these tubuli are well seen in vertical
sections of the thick shells of the largest examples of the group,
such as Nummulina (Fig. 282). It often happens, however, that
certain parts of the shell are left unchannelled by these tubuli ;
and such are readily distinguished, even under a low magnifying
power; by the readiness with which they allow transmitted light
to pass through them, and by the peculiar vitreous lustre they
exhibit when light is thrown obliquely on their surface. In shells
formed upon this type, we frequently find that the surface presents
either bands or spots which are so distinguished ; the non-tubular
hands usually marking the position of the septa, and being some-
times raised into ridges, though in other instances they are either
level or somewhat depressed ; whilst the non-tubular spots may
occur on any part of the surface, and are most commonly raised
into tubercles, which sometimes attain a size and number that give
a very distinctive aspect to the shells that bear them.
421. Now between the comparatively coarse perforations which
are common in the Botaliae type, and the minute tubuli which are
characteristic of the Nwnmuline, there is such a continuous grada-
tion as indicates that their mode of formation, and probably their
uses, are essentially the same. In the former it has been demon-
strated by actual observation that they allow the passage of
pseudopodial extensions of the sarcode-body through every part of
the external wall of the chambers occupied by it (Fig. 266) ; and
there is nothing to oppose the idea that they answer the same
purpose in the latter, since, minute as they are, their diameter is
not too small to enable them to be traversed by the finest of the
threads into which the branching pseudopodia of Foraminifera are
known to subdivide themselves. Moreover, the close approximation
of the tubuli in the most finely-perforated Nummulines, makes
their collective area fully equal to that of the larger but more
scattered pores of the most c oar sely-perf orated Eotalines. Hence
it is obvious that the tub illation or non-tnbulation of Foramini-
feral shells is the key to a very important Physiological difference
between the Animal inhabitants of the two kinds respectively ;
for whilst every segment of the sarcode-body in the former case
gives off pseudopodia, which pass at once into the surrounding
medium, and contribute by their action to the nutrition of the
segment from which they proceed, these pseudopodia are limited
in the latter case to the final segment, issuing forth only through
the aperture of the last chamber, so that all the nutrient material
which they draw in must be first received into the last segment,
and be transmitted thence from one segment to another until it
reaches the earliest. With this difference in the physiological con-
dition of the Animal of these two types, is usually associated a
520 FOEAMINIFEEA.
further very important difference in the conformation of the Shell —
viz., that whilst the aperture of communication between the
chambers, and between the last chamber and the exterior, is usually
very small in the ' vitreous' shells, serving merely to give passage
to a slender stolon or thread of sarcode from which the succeeding
segment may be budded-off, it is much wider in the ' porcellanous'
shells, so as to give passage to a ' stolon' that may not only bud-
off new segments, but may serve as the medium for transmitting
nutrient material from the outer to the inner chambers. There is
no reason to believe, however, that anything like an alimentary
canal exists among Foraminifera ; the nutrition of the entire body
being doubtless effected by that interchange and circulation of
particles, which (as we have already seen, § 369) is continually
going-on throughout its soft sarcodic substance in this form of the
Ehizopod type.
422. Between the highest types of the porcellanous and the
vitreous series respectively, which frequently bear a close resem-
blance to each other in form, there are certain other well-marked
differences in structure, which clearly indicate their essential dis-
similarity. Thus, for example, if we compare Orbitolites (Fig. 268)
with Cycloclypeus (Plate XVI., fig. 1), we recognise the same plan
of growth in each, the chamberlets being arranged in concen-
tric rings around the primordial chamber; and to a superficial
observer there would appear little difference between them. But
a minuter examination shows that not only is the texture of the
shell ' porcellanous ' and non-tubular in Orbitolites, whilst it is
' vitreous ' and minutely tubular in Cycloclypeus ; but that the par-
titions between the chamberlets are single in the former, whilst
they are double in the latter, each segment of the sarcode-body
having its own proper shelly investment. Moreover, between
these double partitions an additional deposit of calcareous sub-
stance is very commonly found, constituting what may be termed
the ' intermediate' or supplemental skeleton ; and this is traversed
by a peculiar system of inosculating canals, which pass around
the chamberlets in interspaces left between the two laminge of
their partitions, and which seem to convey through its substance
extensions of the sarcode-body whose segments occupy the cham-
berlets. We occasionally find this ' intermediate skeleton' extend-
ing itself into peculiar outgrovjths, which have no direct relation
to the chambered shell ; of this we have a very curious example
in Calcarina- (Plate XVI., fig. 3) ; and it is in these that we find
the ' canal-system' attaining its greatest development. Its most
regular distribution, however, is seen in Polystomella and in
Operculina ; and an account of it will be given in the description
of those types.
423. Miliolida. — Commencing, now, with the porcellanous series,
we shall briefly notice some of its most important forms. Its
simplest type is presented by the Cornuspira (Plate XV., fig. 1) of
our own coasts, found attached to Sea-weeds and Zoophytes ; this
PORCELLANOUS SEEIES ; — MILIOLIDA. 521
is a minute spiral shell, of which the interior forms a continuous
tube not divided into chambers ; the latter portion of the spire is
often very much nattened-out, as in Peneroplis (fig. 5), so that
the form of the mouth is changed from a circle to a long narrow
slit. Among the commonest of all Foraminifera, and abounding
near the shores of almost every sea, are some forms of the Milio-
line type, so named from the resemblance of some of their minute
fossilized forms (of which enormous beds of limestone in the neigh-
bourhood of Paris are almost entirely composed) to millet-seeds. The
peculiar mode of growth by which these are characterized, will be best
understood by examining in the first instance the form which has
been designated as Spiroloculfflfia (Plate XV., fig. 2). This shell is
a spiral elongated in the direction of one of its diameters, and
having in each turn a contraction at either end of that diameter,
which partially divides each convolution into two chambers ; the
separation between the consecutive chambers is made more com-
plete by a peculiar projection from the inner side of the cavity,
known as the ' tongue' or ' valve,' which may be considered as an
imperfect sej^tum ; of this a characteristic example is shown in the
upper part of fig. 4. Now it is a very general habit in the Milio-
line type for the chambers of the later convolutions to extend
themselves over those of the earlier, so as to conceal them more or
less completely ; and this they very commonly do somewhat un-
equally, so that more of the earlier chambers are visible on one
side than on the other. MiMolce thus modified (fig. 3) have
received the names of Quinqueloculina and Triloculina according
to the number of chambers visible externally ; but the extreme
inconstancy which is found to mark such distinctions, when the
comparison of specimens has been sufficiently extended, entirely
destroys their value as differential characters. Sometimes the
earlier convolutions are so completely concealed by the later, that
only the two chambers of the last turn are visible externally ; and
in this type, which has been designated Blloculina, there is often
such an increase in the breadth of the chambers as altogether
changes the usual proportions of the shell, which has almost the
shape of an egg when so placed that either the last or the penulti-
mate chamber faces the observer (Plate XY., fig. 4). It is very
common in Milioline shells for the external surface to present a
' pitting,' more or less deep, a ridge-and-f urrow arrangement (fig. 3),
or a honeycomb division ; and these diversities have been used for
the characterization of species. Not only, however, may every
intermediate gradation be met-with between the most strongly
marked forms, but it is not at all uncommon to find the surface
smooth on some parts, whilst other parts of the surface in the same
shell are deeply pitted or strongly ribbed or honeycombed ; so that
here again the inconstancy of these differences deprives them of
all value as distinctive characters.
424. Eeverting again to the primitive type presented in the
simple spiral of Cornuspira, we find the most complete development
522 PORCELLANOUS FOEAMINIFERA.
of it in Peneroplis (Plate XV., fig. 5), a very beautiful form,
which, although very rare on our own coasts, is one of the
commonest of all Foraminifera in the shore- sands and shallow
water dredgings of the warmer regions of every part of the globe.
This is a nautiloid shell, of which the spire flattens itself out as it
advances in growth ; it is marked externally by a series of trans-
verse bands, which indicate the position of the internal septa that
divide the cavity into chambers ; and these chambers communicate
with each other by numerous minute pores traversing each of the
septa, and giving passage to threads of sarcode that connect the
segments of the body. At a is shown the ' septal plane' closing-in
the last-formed chamber, with its single row of pores, through which
the pseudopodial filaments extend themselves into the surrounding
medium. The surface of the shell, which has a peculiarly ' por-
cellanous' aspect, is marked by closely-set strice that cross the
spaces between the successive septal bands ; these markings, how-
ever, do not indicate internal divisions, and are due to a ridge-and-
furrow arrangement of the shelly walls of the chambers. This
type passes into two very curious modifications ; one having a
spire which remains turgid like that of a Nautilus, instead o£
flattening itself out, with a single aperture which sends out fissured
extensions that subdivide like the branches of a tree, suggesting
the name of Dendritlna which has been given to this variety ; the
other having its spire continued in a rectilineal direction so that
the shell takes the form of a crosier, this being distinguished by the
name of Spirolina. A careful examination of intermediate forms,
however, has made it evident that these modifications, though
ranked as of generic value by M. D'Orbigny, are merely varietal;
a continuous gradation being found to exist from the elongated
septal plane of Peneroplis, with its single row of isolated pores, to
the arrow-shaped, oval, or even circular septal plane of Den-
dritina, with all its pores fused together (so to speak) into one
dendritic aperture ; and a like gradation being presented between
the ordinary and the ' spiroline' forms, into which both Peneroplis
and Dendritina tend to elongate themselves under conditions not
yet fully understood.
425. From the ord'nary nautiloid multilocular spiral, we now
pass to a more complex and highly- developed form, which is re-
stricted to tropical regions, but is there very abundant, — -that,
namely, which has received the designation Orbiculina (Plate XV.,
figs. 6, 7, 8). The relation of this to the preceding will be best
understood by an examination of its early stage of growth, repre-
sented in fig. 7 ; for here we see that the shell resembles that of
Peneroplis in its general form, but that its principal chambers are
divided by ' secondary septa' passing at right angles to the primary,
into ' chamberlets' occupied by sub-segments of the sarcode-body.
Each of these secondary septa is perforated by an aperture, so that
a continuous gallery is formed, through which there passes a stolon
that unites together all the sub- segments of each row. The cham-
523
jerlets of successive rows alternate with one another in position ;
and the pores of the principal septa are so disposed, that each
chamberlet of any row normally communicates with two chamber-
lets in each of the adjacent rows. The later turns of the spire
very commonly grow completely over the earlier, and thus the
central portion or ' umbilicus' comes to be protuberant, whilst the
growing edge is thin. The spire also opens-out at its growing
margin, which tends to encircle the first-formed portion, and thus
gives rise to the peculiar shape represented in fig. 8, which is the
common aduncal type of this organism. But sometimes, even at
an early age, the growing margin extends so far round on each
side, that its two extremities meet on the opposite side of the
original spire, which is thus completely enclosed by it ; and its
subsequent growth is no longer spiral but cyclical, a succession of
concentric rings being added, one around the other, as shown
in fig. 6. This change is extremely curious, as demonstrating the
intimate relationship between the spiral and the cyclical plans of
growth, which at first sight appear essentially distinct. In all
but the youngest examples of Orbiculina, the septal plane presents
more than a single row of pores, the number of rows increasing in
the thickest specimens to six or eight. This increase is associated
with a change in the form of the sub-segments of sarcode from
little blocks to columns, and with a greater complexity in the
general arrangement, such as will be more fully described hereafter
in Orbitolites (§ 430). The largest existing examples of this
type are far surpassed in size by those which make up a consider-
able part of a Tertiary Limestone on the Malabar coast of India,
whose diameter reaches 7 or 8 lines.
426. A very curious modification of the same general plan is
shown in Alveolina, a genus of which the largest existing forms
(Fig. 267) do not attain the size of the smallest sugar-plum, but of
Fig. 267.
Alveolina Quaii: — o, o, septal plane, showiDg multiple pores.
which far larger specimens are found in the Tertiary Limestones
of Scinde. Here the spire turns round a very elongated axis, so
that the shell has almost the form of a cylinder drawn to a point
at each extremity. Its surface shows a series of longitudinal lines
which mark the principal septa ; and the bands which intervene
between these are marked transversely by lines which show the
subdivision of the principal chambers into ' chamberlets.' The
chamberlets of each row are connected with each other, as in the
524 PORCELLANOUS FORAMINIFERA.
preceding type, "by a continuous gallery ; and they communicate
with those of the next row by a series of multiple pores in the
principal septa, such as constitute the external orifices of the last-
formed series, seen on its septal plane at a, a.
427. The highest development of that cyclical plan of growth
which we have seen to be sometimes taken-on by Orbiculina, is
found in Orbitolites ; a type which, long known as a very abundant
fossil in the earlier Tertiaries of the Paris basin, has lately proved
to be scarcely less abundant in certain parts of the existing ocean,
whilst it seems to have attained a gigantic development in that
very early period known as the Silurian. The largest recent
specimens of it, sometimes attaining the size of a sixpence,
have hitherto been obtained only from the coast of New Holland
and various parts of the Polynesian Archipelago ; but disks of
comparatively minute size (from the diameter of an ordinary pin's
head to that of a small pea) and of simpler organization, are to be
found in almost all Foraminiferal sands and dredgings from the
shores of the warmer regions of the globe, being especially abundant
in those of some of the Philippine Islands, of the Eed Sea, of the
Mediterranean, and especially of the iEgean. When such disks
are subjected to microscopic examination, they are found (if
uninjured by abrasion) to present the structure represented in
Fig. 268; where we see on the surface (by incident light) a
Fig. 268.
Simple disk of Orbitolites complanatus, laid open to show its
interior structure : — a, central chamber ; 6, circumambient
chamber, surrounded by concentric zones of chamberlets,
connected with each other by annular and radiating passages.
number of rounded elevations, arranged in concentric zones around
a sort of nucleus (which has been laid-open in the figure to show
its internal structure) ; whilst at the margin we observe a row of
MILIOLIDA :— SIMPLE TYPE OF OEBITOLITE. 525
rounded projections, with a single aperture or pore in each of the
intervening depressions. In very thin disks, the structure may
often be brought into view by mounting them in Canada balsam
and transmitting light through them ; but in those which are too
opaque to be thus seen-through, it is sufficient to rub-down one of
the surfaces upon a stone, and then to mount the specimen in
balsam. Each of the superficial elevations will then be found to
be the roof or cover of an ovate cavity or ' chamberlet,' which
communicates by means of a lateral passage with the chamberlet
on either side of it in the same ring ; so that each circular zone of
chamberlets might be described as a continuous annular passage,
dilated into cavities at intervals. On the other hand, each zone
communicates with the zones that are internal and external to it,
by means of passages in a radiating direction ; these passages run,
however, not from the chamberlets of the inner zone to those of the
outer, but from the connecting passages of the former to the cham-
berlets of the latter ; so that the chamberlets of each zone alternate
in position with those of the zones internal and external to it. The
radial passages from the outermost annulus make their way at
once to the margin, where they terminate, forming the ' pores' which
(as already mentioned) are to be seen on its exterior. The central
nucleus, when rendered sufficiently transparent by the means just
adverted-to, is found -to consist of a ' primordial chamber ' (a),
usually somewhat pear-shaped, that communicates by a narrow
passage with a much larger ' circumambient chamber' (b), which
nearly surrounds it, and which sends- off a variable number of
radiating passages towards the chamberlets of the first zone, which
forms a complete ring around the circumambient chamber.*
•428. The idea of the nature of the living occupant of these
cavities which might be suggested by the foregoing account of
their arrangement, is fully borne-out by the results of the exami-
nation of the sarcode-body, which may be obtained by the macera-
tion in dilute acid (so as to remove the shelly investment) of
specimens of Orbitolite that have been gathered fresh from the
sea- weeds whereto in the living state they are found adherent, and
have been kept in spirit. For this body is found to be composed
(Fig. 269) of a multitude of segments of sarcode, presenting not
the least trace of higher organization in any part, and connected
together by ' stolons ' of the like substance. The * primordial' pear-
shaped segment, a, is seen to have budded- off its ' circumambient'
* Although the above may be considered the typical form of the Orbitolite,
yet, in a very large proportion of specimens, the first few zones are not com-
plete circles, the early growth having taken place rather in a spiral than in a
radial direction; between these two plans there is every variety of gradation;
and even where the spiral is most distinctly marked in the first instance, the
additions soon come to be made in concentric zones. A form of Orbitolite has
been brought up from very great depths, in which the ' nucleus ' is formed by
three or four turns of a spiral closely resembling that of a Cornuspira (§ 423),
with an interruption at every half-turn, as in Spiroloculina ; the growth after-
wards becoming purely concentric.
526
P0RCELLAN0U3 FOKAMINIFERA.
segment, h, by a narrow footstalk or stolon ; and this circumambient
segment, after passing almost entirely ronnd the central one, has
budded-off three stolons, which swell into new snb- segments from
which the first ring is formed. Scarcely any two specimens are
precisely alike as to the mode in which the first ring originates
from the ' circumambient segment ;' for sometimes a score or more
of radial passages extend themselves from every part of the margin
Fig. 269.
Composite Animal of Simple type of Orbitolites complanatus :
— a, central mass of sarcode ; 6, circumambient segment,
giving off peduncles, in which originate the concentric zones
of sub-segments connected by annular bands.
of the latter (and this, as corresponding with the plan of growth
afterwards followed, is probably the typical arrangement) ; whilst
in other cases (as in the example before us) the number of these
primary offsets is extremely small. Each Zone is seen to consist
of an assemblage of ovate sub-segments, whose height (which could
not be shown in the figure) corresponds with the thickness of the
disk ; these sub-segments, which are all exactly similar and equal
to one another, are connected by annular stolons ; and each zone
is connected with that on its exterior by radial extensions of those
stolons passing-off between the sub-segments.
429. The radial extensions of the outermost zone issue-forth as
pseudopodia from the marginal pores, searching-for and drawing-in
alimentary materials in the same manner as those of other
Reticularia (§ 370) ; the whole of the soft body, which has no com-
MILIOLIDA :— SIMPLE TYPE OF OEBITOLITE. 527
nmnicatioii whatever with the exterior save through these marginal
pores, being nourished by the transmission of the products of
digestion from zone to zone, through similar bands of protoplasmic
substance. In all cases in which the growth of the disk takes-place
with normal regularity, it is probable that a complete circular zone
is added at once. When the sarcode-body has increased beyond
the capacity of its enveloping disk, it may be presumed that its
pseudopodial extensions, proceeding from the marginal pores,
coalesce, so as to form a complete annulus of sarcode round the
margin of the outermost zone ; and it is probable that it is by a
deposit of calcareous matter in the surface-portion of this annulus,
that the new zone of shell-substance is formed, which constitutes
the walls of the cells and passages occupied by the soft sarcode
body. Thus we find this simple type of organization giving origin
to fabrics of by no means microscopic dimensions, in which, how-
ever, there is no other differentiation of parts than that concerned
in the formation of the shell ; every segment and every stolon (with
the exception of the two forming the ' nucleus*) being, so far as
can be ascertained, a precise repetition of every other, and the
segments of the nucleus differing from the rest in nothing else than
their form. The equality of the endowments of the segments is
shown by the fact, of which accident has repeatedly furnished
proof, — that a small portion of a disk, entirely separated from the
remainder, will not only continue to live, but will so increase as to
form a new disk ; the want of the ' nucleus' not appearing to be of
the slightest consequence, from the time that active life is esta-
blished in the outer zones. In what manner the multiplication and
reproduction of the species are accomplished, we can as yet do little
more than guess ; but from appearances sometimes presented by
the sarcode-body, it seems reasonable to infer that gemmules, cor-
responding with the zoospores of Protophytes (§ 265), are occa-
sionally formed by the breaking-up of the sarcode into globular
masses ; and that these, escaping through the marginal pores, are
sent forth to develope themselves into new fabrics. Of the mode
wherein that sexual operation is performed, however, in which
alone true Generation consists, nothing whatever is known.
430. One of the most curious features in the history of this
animal is its capacity for developing itself into a form which, whilst
fundamentally the same as that previously described, is very much
more complex. In all the larger specimens of Orbitolite we observe
that the marginal pores, instead of constituting but a single row,
form many rows one above another ; and besides this, the chamber-
lets of the two surfaces, instead of being rounded or ovate in form,
are usually oblong and straight- sided, their long diameters lying
in a radial direction, like those of the cyclical type of Orbiculina
(Plate XY., fig. 6). When a vertical section is made through such
a disk, it is found that these oblong chambers constitute two super-
ficial layers, between which are interposed columnar chambers of a
rounded form ; and these last are connected together by a complex
528
POECELLANOUS FOEAMINIFERA.
Fig. 270.
series of passages, the arrangement of which will be best under-
stood from the examination of a part of the sarcode-body that
occupies them (Fig. 270).
Tor the oblong superficial
chambers are occupied by
sub-segments of sarcode, c c,
d d, lying side by side, so
as to form part of an an-
nulus, but each of them
being disconnected from its
neighbours, and communi-
cating only by a double
footstalk with the two an-
nular ' stolons,' a a', h b',
which obviously correspond
with the single stolon of
the Simple type (Fig. 269).
These indirectly connect to-
gether not merely all the
superficial chamberlets of
each zone, but also the
columnar sub- segments of
the intermediate layer ; for
these columns (e e, e' e') ter-
Portion of Composite Animal of Complex minate above and below in
type of Orbitolites complanatus .— a a', bb', the the annular stolons, some-
upper and lower rings of two concentric times passing directly from
zoues; c c, the upper layer of superficial sub- one to fae 0ther, but SOme-
segments, and d d, the lower layer, connected +- . + f ,-,
with the annular bands of both zones ; e e ™e® g0m» °nt 0t . tne
and e' e', vertical sub-segments of the two direct course to coalesce
zones. with another column. The
columns of the successive
zones (two sets of which are shown in the figure) communicate
with each other by threads of sarcode, in such a manner that (as
in the simple type) each column is thus brought into connection
with two columns of the zone next interior, to which it alternates
in position. Similar threads, passing off from the outermost zone,
through the multiple ranges of marginal pores, would doubtless act
as pseudopodia.
431. Now this plan of growth is so different from that previously
described, that there would at first seem ample ground for sepa-
rating the simple and the complex types as distinct species. But
the test furnished by the examination of a large number of specimens,
which ought never to be passed-by when it can possibly be appealed
to, furnishes these very singular results : — 1st. That the two forms
must be considered as specifically identical ; since there is not only
a gradational passage from one to the other, but they are often
combined in the same individual, the inner and first-formed portion
of a large disk frequently presenting the simple type, whilst the
OEBITOLITES:— LITUOLIDA. 529
outer and later-formed part has developed itself upon the complex : —
2nd. That although the last-mentioned circumstance would natu-
rally suggest that the change from the one plan to another may-
be simply a feature of advancing age, yet this cannot be the case ;
since the complex sometimes evolves itself even from the very first
(the ' nucleus,' though resembling that of the simple form, sending
out two or more tiers of radiating threads), whilst, more frequently,
the simple prevails for an indefinite number of zones, and then
changes itself in the course of a few zones into the complex. — A
more striking instance could scarcely be drawn from any depart-
ment of Natural History, of the wide range of variation that may
occur within the limits of one and the same Species ; and the
Microscopist needs to be specially put ou his guard as to this point,
in respect to the lower types of Animal as to those of Vegetable life,
since the determination of form seems to be far less precise among
such, than it is in the higher types.*
432. Lituolida. — In certain forms of the preceding family, and
especially in the genus Miliola,we not unfrequently find the shells
encrusted with particles of sand, which are imbedded in the proper
shell-substance. This incrustation, however, must be looked on as
(so to speak) accidental ; since we find shells that are in every
other respect of the same type, altogether free from it. A similar
accidental incrustation presents itself among certain ' vitreous ' and
tubular shells (§ 445) ; but there, too, it is on a basis of true shell,
and the sandy incrustation is often entirely absent. There is. how-
ever, a group of Foraminifera in which the true shell is constantly
and entirely replaced by a sandy envelope, which is distinguished
as a 'test ;' the arenaceous particles not being imbedded in a shelly
cement, but being held together only by an organic glue. If
the sand be siliceous, the ' test ' of course has that composition ;
and this envelope often bears such a resemblance to a true shell
exuded from the animal, as to have been mistaken for it by some
excellent observers. It is not a little curious that the forms of
these arenaceous ' tests ' should represent those of many different
types among both the ' porcellanous ' and the ' vitreous ' series ;
whilst yet they graduate into one another in such a manner, as to
indicate that all the members of this ' arenaceous' group are closely
related to each other, so as to form a series of their own. And
it is further remarkable, that while the Deep- Sea dredgings
recently carried down to depths of from 1000 to 2500 fathoms,
have brought up few forms of either ' porcellanous ' or ' vitreous '
Foraminifera that were not previously known, they have added
greatly to our knowledge of the ' arenaceous ' types, the number
and variety of which far exceed all previous conception. These
have not yet been systematically described ; but the following
* For a fuller account of the Organization of Orbitolites, and of the various
conditions under which it presents itself, see the Author's Memoir upon that
genus in the " Philosophical Transactions," 1856, and his " Introduction to
the Study of the Foraminifera," published by the Ray Society, 1862.
M 31
530 ARENACEOUS FORAMINIFEKA.
notice of a few of the more remarkable, will give some idea of the
interest attaching to this portion of the new Fauna which has been
brought to light by Deep- Sea exploration.
433. In the midst of the sandy nmd which formed the bottom
where the warm area of the ' Globigerina-mud ' (§ 443) abutted on
that over which a glacial stream flowed, there were found a
number of little pellets, varying in size from a large pin's head to
that of a large pea, formed of an aggregation of sand- grains,
minute Foraminifers, &c, held together by a tenacious proto-
plasmic substance. On tearing these open, the whole interior was
found to have the same composition ; and no trace of any struc-
tural arrangement could be discovered in their mass. Hence they
might be supposed to be mere accidental agglomerations, were it
not for their conformity to the ' monerozoic ' type previously
described (§ 366) ; for just as a simple ' moner,' by a differentiation
of its homogeneous sarcode, becomes an Am&ba, so would one
of these uniform blendings of sand and sarcode, by a separation of
its two components, — the sand forming the investing ' test,' and the
sarcode occupying its interior, — become an arenaceous Astrozliiza.
This type (§ 380), which was very abundant in certain localities,
presents remarkable variations of form ; being sometimes globular,
sometimes stellate, sometimes cervicorn. But the same general
arrangement prevails throughout ; the cavity being occupied by a
dark-green sarcode, whilst the ' test ' is composed of loosely aggre-
gated sand-grains not held together by any recognizable cement, and
having no definite orifice, so that the pseudopodia must issue from
interstices between the sand-grains, which spaces are probably
occupied during life with living protoplasm that continues to hold
together the sand-grains after death. These are by no means
microscopic forms ; the ' stellate ' varieties ranging to 0*3 or even
0'4 inch in diameter, and the ' cervicorn ' to nearly 0*5 inch in
length.
434. From this least differentiated type, we pass to another
(Fig. 271, a), in which the 'test,' cylindrical or nearly so, and still
composed of loosely-aggregated sand-grains, has a definite circular
mouth at one extremity, surrounded by sand-grains very regularly
arranged, and firmly cemented to one another ; these may be con-
sidered as representing the lageniform type in the ' vitreous' series
(§ 442). But just as the single-chambered Lagence, by the process
of continuous gemmation, become many-chambered Nodosariaz, so
do these lageniform Arenacea become nodosarine by the develop-
ment of a succession of chambers in a straight line, the mouth of
each opening into the cavity of the next (Fig. 271, b). Here,
again, the sand-grains which form the mouth of each chamber are
very regularly arranged and firmly cemented to each other. The
sarcode-body is continuous through them all, and sends out its
pseudopodia through the mouth of the last chamber. These
curious tests sometimes attain a length of nearly half an inch.
435. In the greater number of Arenaceous Foraminifera, how-
VARIOUS TYPES OF LITUOLIDA.
531
ever, the sand-grains are very firmly cemented together, so that
the ' test ' is even less fragile than a calcareous shell of the same
Arenaceous Foraniinifera : — o, elongated form composed of
loosely-aggregated sand-grains ; b, the same laid open ; c,
Rhabdammina ; d, section of one of its radiating tubes ; e,
coarse type of Nodosarine Lituola ; j\ moniliform Lituola.
thickness ; and it is not a little cnrious that this cement should be
phosphate of iron. Sometimes the sand-grains are joined to
one another with the least possible quantity of intervening cement,
as in Rhabdammina (Fig. 271, c, d), Saccamina (Fig. 272, a, b, c),
and the Globigerine, Orbuline, and Nodosarine forms of Lituola
(Fig. 273, a, b, c, g, h) ; while in other instances this cement is
worked up with particles of extreme minuteness into a sort of fine
' plaster,' which is sometimes employed alone, as in the tubes of
Trochammina, while it sometimes has coarse sand-grains embedded
in it, as in the larger Lituolae (Fig. 274, a). In all cases, however,
the presence of phosphate of iron is indicated (1) by the ferru-
ginous hue of the ' tests ;' and (2) by the fact that the cement does
not yield to dilute nitric acid, but dissolves in strong.* The genus
Trochammina in its simplest form represents the undivided spiral
Gomuspira among the ' porcellanous,' and Spirillina among the
' vitreous ' Foraminifera ; but besides presenting a number of other
curious varieties of form, it exhibits in some instances such a
• The Author's conclusion on this curious point has been verified by the
analyses kindly made for him by his friend Prof. A. Williamson.
M M 2
532
ARENACEOUS FOEAMINIFEEA.
tendency to the subdivision of its tube into chambers, as to
approach the lower and less regular forms of the rotcdine series in
its plan of growth. The Saccamina (Sars), on the other hand, is a
remarkably regular type, composed of coarse sand-grains firmly
cemented together in a globular form, so as to form a wall
nearly smooth on the outer, though rough on the inner surface, with
a projecting neck surrounding a circular mouth (Fig. 272, a, b, c).
This type, which occurs in extraordinary abundance in certain
localities (as the entrance of the Christiania-fjord), is of peculiar
interest from the fact that it has been discovered in a fossil state
by Mr. H. B. Brady, in a clay seam between two layers of Carboni-
ferous Limestone. Its size is that of very minute seeds. In
Fig. 272.
iS^
-'"'"r-;-,-
rX
^MM:^
■:/■■/:
^W^:m
-■;;:
Arenaceous Foraminifera: — o., Saccamina spheriea; 6, the
same laid open ; c, portion of the test enlarged to show its
component sand-grains : — d, Pilulina Jeffreysii ; e, portion of
the test enlarged, showing the arrangement of the sponge-
spicules.
striking contrast to the preceding is another single-chambered
type, distinguished by the whiteness of its 'test,' to which I propose
to give the name of Pilulina, from its resemblance to a homoeo-
pathic 'globule' (Fig. 272, d, e). The form of this is a very
regular sphere; and its orifice, instead of being circular and
surrounded by a neck, is a slit or fissure with slightly raised lips,
and having a somewhat S- shaped curvature. It is by the
structure of its ' test,' however, that it is especially distinguished ;
VAEIOUS TYPES OF LITUOLIDA.
533
for this is composed of the finest ends of Sponge-spicules, very
regularly ' laid ' so as to form a kind of felt, through the sub-,
stance of which very fine sand-grains are dispersed. This
'felt' is somewhat flexible, and its components do not seem to
be united by any kind of cement, as it is not affected by being
boiled in strong nitric acid ; its tenacity, therefore, seems entirely-
due to the wonderful manner in which the separate siliceous fibres
are 'laid.' — It is not a little curious that these two forms should
present themselves in the same dredging ; and that there should
be no perceptible diiference in the character of their sarcode-
bodies, which, as in the preceding case, have a dark-green hue.
436. From these single-chambered and single-mouthed types, we
may pass to forms in which, without any internal partition, there
are two or more orifices. The first of these, to which Prof. "W. C.
Williamson's designationProteonina maybe given (as resembling one
of the forms described by him under that name) , is somewhat fusiform
in shape (Fig. 273, d), having its two extremities elongated into
Fig. 273.
Arenaceous Foraminifera : — a, b. Globigerine Lituola, ; — c,
Nodosarine £tfwo?a, having a 'test' composed of fine sand-grains ;
— d, Proteonina; e, terminal portion enlarged ;/, middle portion
enlarged ; — g, Orbuline Lituola ; h, portion of inner surface more
highly magnified.
tubes, with a circular orifice at the end of each. The materials of
the • test' differ remarkably according to the nature of the bottom
534 ARENACEOUS FORAMINIFEEA.
whereon they live. "When they come np with ' Globigerina-mud,'
in which sponge- spicules abound, whilst sand-grains are scarce,
they are almost entirely made np of the former, which are laid
in the larger part in a sort of lattice-work, the interspaces being
filled np by fine sand-grains ; bnt when they are brought up from a
bottom on which sand predominates, the larger part of the ' test' is
made up of sand-grains and minute Forannnifera, with here and
there a sponge-spicule (Fig. 273, d,f.) In each case, however, the
tubular extensions (one of which sometimes forms a sort of pro-
boscis, nearly equalling the body itself in length) are entirely made
up of sponge- spicules laid side by side with extraordinary regu-
larity (e). — The genus Rhabdammina (Sars) resembles Saccamina
in the structure of its 'test,' which is composed of sand-grains
very firmly cemented together; but the grains are of smaller
size, and they are so disposed as to present a smooth surface
internally, though the exterior is rough. What is most remark-
able about this, is the geometrical regularity of its form, which
is typically triradiate (Fig. 271, c), the rays diverging at equal
angles from the central cavity, and each being a tube (d)
with an orifice at its extremity. Not unfrequently, however, it
is quadri-radiate, the rays diverging at right angles ; and occa-
sionally a fifth ray presents itself, its radiation, however, being
on a different plane. The three rays are normally of equal
length ; but one of them is sometimes shorter than the other
two ; and when this is the case, the angle between the long rays
increases at the expense of the other two, so that the long rays
lie more nearly in a straight line. Sometimes the place of the
third ray is indicated only by a little knob ; and then the two
long rays have very nearly the same direction. We are thus led to
forms in which there is no vestige of a third ray, but merely a
single straight tube, with an orifice at each end ; and the length of
this, which often exceeds half an inch, taken in connection with the
abundance in which it presented itself in dredgings in which the
triradiate forms were rare, seems to preclude the idea that these
long single rods are broken rays of the latter.
437. The generic designation Lituola is still given to those
many-chambered forms of the Arenaceous type which have been
long recognised as such ; the first that was described having the
form of a spiral partly unrolled, like that of the ' spiroline' Pene-
roplis (§ 424). But it will be necessary to distinguish in it several
very well-marked modifications, which might be ranked as distinct
generic types, if it were not for their tendency to graduate one into
another. Thus we might begin from the simple continuous tubes
with bead-like expansions at irregular intervals, having no internal
partition (Fig. 271,/), which differ from some forms of Trochain-
mince (§ 435), in little else than in having the test composed of
cemented sand-grains, with sponge-spicules worked-in among them.
And from these we might proceed to the nodosarine forms (Fig.
271, e, and Fig. 273, c), in which the chambers are distinct, com-
VAEIOUS TYPES OF LITUOLIDA. 535
municating only by a small circular orifice that resembles the
projecting mouth of the last (largest) chamber. Now, among these
' nodosarine' Lituolce there seem to be two very distinct types ; the
test in one being composed of coarse materials, such as large
sand-grains or small Foraminifera, rudely cemented together
(Fig. 271, e) ; whilst in the others it is made up of fine sand-grains,
most remarkably uniform in size, and cemented with extraordinary
regularity, so as to form a test which is quite smooth alike on its
outer and on its inner surface, and of perfectly uniform thickness, as
in Fig. 2 73, h. But that this difference is not constant, is proved
by the fact that cases occur in which the coarse and the fine ag-
gregations present themselves in different segments of the same
individual ; so that it probably depends, in part at least, on the
nature of the bottom, and the relative abundance of different
materials. The finer texture is universal (so far as the Author's
experience extends) among the ' globigerine' and ' orbuline' Lituolce,
which simulate in a most extraordinary manner the forms of these
two types. The 'globigerine' (Fig. 273, a, b) are larger than
ordinary Globigerinse (§ 443), but resemble them in mode of growth;
there is this important difference, however, that their ' test' is
altogether destitute of pores, whilst the shell of the true Globigerines,
like that of Rotalia (Fig. 266), is perforated with foramina. So in
the ' orbuline' Lituolas (Fig. 273, g, h), the test has not only the
spherical form of the shell of the true Orbidince (§ 443), but it
has also its characteristic large pores (apparently replacing a single
mouth), which are situated on little nipple-shaped projections ; the
minute foramina, however, which the true Orbulina has in common
with Globigerina, are absent. — These mimetic resemblances are
extremely carious, and suggest many interesting questions, on
which we can at present only speculate.
438. The highest development of the Lituola-type at the present
time is shown in the large ' nautiloid' forms (Fig. 274), which
have been brought up in considerable abundance from depths
between 200 and 500 fathoms. The tests of these are sometimes
composed entirely of aggregated sand-grains, firmly cemented
together ; whilst in other instances they are smoothed over
externally with a kind of plaster, in which large glistening sand-
grains are sometimes set at regular intervals, as if for ornament.
On laying open the spire, it is found to be very regularly divided
into chambers by partitions formed of cemented sand-grains (b) ; a
communication between these chambers being left by a fissure at
the inner margin of the spire, as in Operculum (Plate XVI., fig. 3).
One of the most curious features in the structure of this type, is the
extension of the cavity of each chamber into passages excavated in
its thick external wall ; each passage being surrounded by a very
regular arrangement of sand-grains, as shown at c. It not unfre-
quently happens that the outer layer of the test is worn-away,
and the ends of the passages then show themselves as pores upon
its surface ; this appearance, however, is abnormal, the passages
536
ABENACEOUS FOEAMINIFEKA.
simply running from the chamber-cavffyy into the thickness of its
wall, and having (so long as this is complete) no external opening.
This ' labyrinthic ' structure is of great interest, from its relation not
only to the similar structure of the large fossil examples of the same
Fig. 274.
Nautiloid Litnola .-—Showing a, its external aspect ; 6, its
internal structures ; o, a portion of its outer wall more highly
magnified, showing the sand-grains of which it is built up,
and the passages excavated in its substance.
type, but also to that which is presented in the gigantic fossil
arenaceous forms to be presently described. — It is in the Cretaceous
formation that the Lituoline type appears to have attained its
greatest development. The large ' spiroline' forms, which are met
with abundantly in certain beds of Chalk, have their chambers
irregularly subdivided into ' chamberlets ' by secondary septa,
formed, like the primary, of aggregated sand-grains. On the other
hand, the lower forms often present themselves (as they do at the
present time) adherent to shells, corals, stones, &c, on which they
extend themselves irregularly, not unfrequently branching and
spreading themselves out in different directions.
439. Although some of the Nautiloid Lihwlce are among the
largest of existing Foraminifera, having a diameter of 03 inch, they
are mere dwarfs in comparison with two gigantic Fossil forms, of
which the structure has been recently elucidated by Mr. H. B. Brady
and the Author.* Geologists who have worked over the Greensand
of Cambridgeshire have long been familiar with solid spherical
* See their 'Description of Parlceria and Lo/tusia,' in "Philosophical Trans-
actions," 1869, p. :72U
GIGANTIC FOSSIL LITUOLIDA :— PAEKEEIA.
537
bodies which there present themselves not nnfreqnently, varying in
size from that of a pistol-bullet to that of a small cricket-ball ; and
whilst some regarded them as Mineral concretions, others were led
by certain appearances presented by their surfaces, to suppose them
to be fossilized Sponges. A specimen having been fortunately dis-
covered, however, in which the original structure had remained
unconsolidated by mineral infiltration, it was submitted by Prof.
Morris to the Author ; who was at once led by his examination of it
to recognise it as a member of the Arenaceous group of Poraminifera,
to which he gave the designation Parheria, in compliment to his
valued friend and coadjutor, Mr. ~W. K. Parker. A section of the
sphere taken through its centre (Pig. 275) presents an aspect very
General view of the internal structure of Parleria : — In the
horizontal section, l\ Za, fi, Z4, mark the four thick layers ; in the
vertical sections, A marks the internal surface of a layer
separated by concentric fracture ; B, the appearance presented
by a similar fracture passing through the radiating processes ;
c, the result of a tangential section passing through the cancel-
lated substance of a lamella ; D, the appearance presented by
the external surface of a lamella separated by a concentric
fracture which has passed through the radial processes ; E,
aspect of section taken in a radial direction, so as to cross
the solid lamella and their intervening spaces ; c1, c2, c3, c4,
successive chambers of nucleus.
much resembling that of an Orbitolite (§ 427), a series of cham-
berlets being concentrically arranged round a 'nucleus;' and as
the same appearance is presented, whatever be the direction of the
538
AEENACEOUS FOEAMINIFEEA.
Fig. 276.
section, it becomes apparent that these chamberlets, instead of
being arranged in snccessive rings on a single plane, so as to form
a disk, are grouped in concentric spheres, each completely investing
that which preceded it in date of formation. The outer wall of
each chamberlet is itself penetrated by extensions of the cavity
into its substance, as in the Lituola last described; and these
passages are separated by
partitions very regularly built
up of sand-grains, which also
close-in their extremities, as is
shown in Fig. 276. The con-
centric spheres are occasion-
ally separated by walls of more
than ordinary thickness ; and
such a wall is seen in Fig. 275
to close-in the last formed
series of chamberlets. But
these walls have the same
' labyrinthic' structure as the
thinner ones ; and an exami-
nation of numerous specimens
shows that they are not formed
at any regular intervals. The
' nucleus' is always composed
of a single series of chambers,
arranged end to end, some-
times in a straight line, as in Fig. 275, c1, c2, c3, c4, sometimes forming a
spiral, and in one instance returning upon itself. But the outermost
chamber enlarges, and extends itself over the whole ' nucleus,' very
much as the ' circumambient' chamber of the Orbitolite extends itself
round the primordial chamber (§427); and radial prolongations given
off from this in every direction form the first investing sphere, round
which the entire series of concentric spheres are successively formed.
Of the sand of which this remarkable fabric is constructed, about
60 per cent, consists of phosphate of lime, and nearly the whole re-
mainder of carbonate of lime. — Another large Fossil arenaceous
type, constructed upon the same general plan, but growing spirally
round an elongated axis like Alveolina (Fig. 267), and attaining a
length of three inches, has been described by Mr. H. B. Brady (loc.
tit.), under the name Loftusia, after its discoverer, the late Mr.
"W. K. Loftus, who brought it from the Turko-Persian frontier,
where he found it imbedded in " a blue marly limestone " probably
of early Tertiary age.
440. There is nothing, as it seems to the Author, more wonderful
in Nature, than the building-up of these elaborate and symmetrical
structures by mere ' jelly-specks,' presenting no trace whatever of
that definite ' organization' which we are accustomed to regard as
necessary to the manifestations of Conscious Life. Suppose a
Human mason to be put down by the side of a pile of stones of
Portion of one of the lamellee of Par-
keria, showing the sand-grains of which
it is built up, and the passages extending
into its substance.
YITEEOUS FOEAMIXIFEEA :— LAGEXIDA. 539
various stapes and sizes, and to be told to build a dome of these,
smooth on both surfaces, without using more than the least
possible quantity of a very tenacious but very costly cement
in holding the stones together. If he accomplished this well,
he would receive credit for great intelligence and skill. Yet
this is exactly what these little ' jelly-sj)ecks' do on a most minute
scale ; the ' tests' they construct, when highly magnified, bearing
comparison with the most skilful masonry of Man. From tlie
same sandy bottom, one species picks up the coarser quartz -grains,
cements them together with phosphate of iron secreted from its
own substance, and thus constructs a flask-shaped ' test' having
a short neck and a single large orifice. Another picks up the
finer grains, and puts them together with the same cement into
perfectly spherical ' tests' of the most extraordinary finish, per-
forated with numerous small pores, disposed at pretty regular
intervals. Another selects the minutest sand-grains and the
terminal portions of sponge-spicules, and works these up together,
— apparently with no cement at all, but by the mere ' laying' of the
spicules, — into perfect white spheres, like homoeopathic globules,
each having a single fissured orifice. And another, which makes a
straight many-chambered 'test,' the conical mouth of each
chamber projecting into the cavity of the next, while forming the
walls of its chambers of ordinary sand-grains rather loosely held
together, shapes the conical mouths of the successive chambers by
firmly cementing to each other the quartz-grains which border it. —
To give these actions the vague designation 'instinctive,' does not in
the least help us to account for them ; since what we want, is to
discover the mechanism by which they are worked-out; and it is
most difficult to conceive how so artificial a selection can be made
by a creature so simple.
441. "We now return to the Foraminifera which form true shells
by the calcification of the superficial layer of their sarcode-bodies ;
and shall take a similar general survey of the Vitreous series, in
which the shell is perforated by multitudes of minute foramina,
which, when the shell is thick, form tubes that pass usually straight
and parallel from its inner to its outer surface (Fig. 282).
442. Lagenida. — Reverting in the first instance to the simple
monothalamous or single-chambered shells, we find some of them
repeating in a very curious manner the lowest forms already
described. Thus SpirilUna has a minute, spirally convoluted,
undivided tube, resembling that of Cornuspira (Plate XV., fig. 1),
but having its wall somewhat coarsely perforated by numerous
apertures for the emission of pseudopodia. So in Lagena we seem
to have the representative of Gromia ; not only, however, is the
membranous ' test ' of the latter replaced by a minutely-porous
shell, but its wide mouth is narrowed and prolonged into a tubular
neck (fig. 9), giving to the shell the form of a microscopic flask ;
this neck terminates in an everted lip, which is marked with radia-
ting furrows. — A mouth of this kind is a distinctive character of a
540 VITREOUS FOKAMINIFEKA.
large group of polyihdlambus shells, of which, each single chamber
bears a more or less close resemblance to the simple Lagena,
and of which, like it, the external surface generally presents some
kind of ornamentation, which may have the form either of longi-
tudinal ribs or of pointed tubercles. Thus the shell of Nodosaria
(rig. 10) is obviously made up of a succession of lageniform
chambers, the neck of each being- received into the cavity of that
which succeeds it ; whilst in Gristellaria (fig. 11) we have a similar
succession of chambers, presenting the characteristic radiate aper-
ture, and often longitudinally ribbed, disposed in a nautiloid
spiral. Between Nodosaria and Gristellaria, moreover, there is
such a gradational series of connecting forms, as shows that no
essential difference exists between these two types, which must be
combined into one genus Nodosarina ; and it is a fact of no little
interest, that these varietal forms, of which many are to be met
with on our own shores, but which are more abundant on those of
the Mediterranean, and especially of the Adriatic, can be traced
backwards in Geological time even as far as the New Red Sand-
stone period. — In another genus, Pohjmorpliina, we find the shell
to be made up of lageniform chambers arranged in a double series,
alternating with each other on the two sides of a rectilinear axis
(fig. 13); here again, the forms * of the individual chambers, and
the mode in which they are set one upon another, vary in such a
manner as to give rise to very marked differences in the general
configuration of the shell, which are indicated by the name it
bears. — All these Foraminifera, whether simple or composite, whose
shells are made up of lageniform chambers, may be very naturally
associated under one Family, Lagenida : notwithstanding that
they were distributed by D'Orbigny (according to the differences of
their plans of growth) under four different Orders.
443. Globigerinida. — Returning once again to the simple ' mono-
thalamous' condition, we have in Orbulina — a minute spherical
shell that presents itself in greater or less abundance in DeejD-Sea
dredgings from almost every region of the globe — a globular
chamber with porous walls, and a simple circular aperture that is
frequently replaced by a number of large pores scattered through-
out the wall of the sphere. It is maintained by some that Orbulina
is really a detached generative segment of Globigerina, with which it
is generally found associated. — The shell of Globigerina consists of an
assemblage of nearly spherical chambers (fig. 12), having coarsely
porous walls like those ofBotalia (Fig. 266), and cohering externally
into a more or less regular turbinoid spire, each turn of which
consists of four chambers progressively increasing in size. These
chambers, whose total number seldom exceeds twelve, do not com-
municate directly with each other, but open separately into a common
' vestibule' which occupies the centre of the under side of the spire.
This type has recently attracted great attention, from the extra-
ordinary abundance in which it occurs at great depths over
large areas of the Ocean-bottom. Thus its minute shells have been
GLOBIGEEIXIDA : — CAEPEXTEEIA ; TEXTULAEIA. 541
found to constitute no less than 97 per cent, of the ' ooze' brought
np from depths of from 1260 to 2000 fathoms in the middle
and northern parts of the Atlantic Ocean. The surface-layer of
this ooze consists of living Globigerinse ; whilst its deeper layers are
almost entirely composed of dead shells of the same type. And it
is probable that these Globigerinae form an important article of
sustenance to the higher forms of Animals which have been brought
up alive from the same Ocean-depths.
444. A very remarkable type has recently been discovered ad-
herent to shells and corals brought from tropical seas, to which
the name Garpenteria has been given ; this may be regarded as a
highly developed form of Globigerina, its first-formed portion
having all the essential characters of that genus. It grows attached
by the apex of its spire ; and its later chambers increase rapidly in
size, and are piled on the earlier in such a manner as to form a
depressed cone with an irregular spreading base. The essential
character of Globigerina — the separate orifice of each of its
chambers — is here retained with a curious modification ; for the
central vestibule, into which they all open, forms a sort of vent
whose orifice is at the apex of the cone, and is sometimes prolonged
into a tube that proceeds from it ; and the external wall of this
cone is so marked-out by septal bands, that it comes to bear a
strong resemblance to a minute Balanus (acorn-shell) for which
this type was at first mistaken. The principal chambers are partly
divided into chamberlets by incomplete partitions, as we shall
find them to be in Eozoon (§ 457) ; and the whole assemblage of
cavities is occupied in the living state by a Spongeous substance
beset with siliceous spicules ; but this may perhaps be parasitic*
445. A less aberrant modification of the Globigerine type, how-
ever, is presented in the two great series which may be designated
(after the leading forms of each) as the Textularian and theRotalicm.
For notwithstanding the marked difference in their respective plans
of growth, the characters of the individual chambers are the same ;
their walls being coarsely-porous, and their apertures being oval,
semi-oval, or crescent-shaped, sometimes merely fissured. In
Textularia (Plate XV., fig. 14) the chambers are arranged biserially
along a straight axis, the position of those on the two sides of it
being alternate, and each chamber opening into those above and
below it on the opposite side by a narrow fissure ; as is well shown
in such 'internal casts' (Fig. 277, a) as exhibit the forms and con-
nections of the segments of sarcode by which the chambers are
occupied during life. In the genus Bulimina the chambers are so
arranged as to form a spire like that of a Bulimus, and the aperture
is a curved fissure whose direction is nearly transverse to that of
the fissure of Textularia ; but in this, as in the preceding type,
there is an extraordinary variety in the disposition of the chambers.
In both, moreover, the shell is often covered by a sandy incrusta-
* See the Author's Memoir in "Philos." Transact." for 1860 : and his "Intro
duction to the Study of the Foraminifera," published by the Hay Society.
542 VITREOUS FORAMINIFERA.
tion, so that its perforations are completely hidden, and can only
be made visible by the removal of the adherent crust.
Fig. 277.
Internal siliceous Casts, representing the forms of the segments
of the animals, of A, Textularia, B, liotalia.
446. In the Rotalian series, the chambers are disposed in a tur-
binoid spire, opening one into another by an aperture situated on
the lower and inner side of the spire, as shown in Plate XV., fig. 18 ;
the forms and connections of the segments of their sarcode-bodies
being shown in such 'internal casts' as are represented in Fig. 277, b.
One of the lowest and simplest forms of this type is that very
common one now distinguished as Discorbina, of which a character-
istic example is represented in Plate XV., fig. 15. The early form
of Planorbulina is a rotaline spire, very much resembling that of
Discorbina ; but this afterwards gives place to a cyclical plan of
growth (fig. 17) ; and in those most developed forms of this type
which occur in warmer seas, the earlier chambers are completely
overgrown by the latter, which are often piled-up in an irregular
'acervuline' manner, spreading over the surfaces of shells, or
clustering round the stems of zoophytes. — In the genus Tinojporus
there is a more regular growth of this kind, the chambers being
piled successively on the two sides of the original median plane,
and those of adjacent piles communicating with each other obliquely
(like those of Textularia) by large apertures, whilst they communi-
cate with those directly above and below by the ordinary pores of
the shell. The simple or smooth form of this genus presents great
diversities of shape, with great constancy in its internal structure ;
being sometimes spherical, sometimes resembling a minute sugar-
loaf, and sometimes being irregularly flattened-out. A peculiar
form of this type (Fig. 278), in which the walls of the piles are
thickened at their meeting-angles into solid columns that appear
on the surface as tubercles, and are sometimes prolonged into
EOTALIN.E : — T1N0P0EUS ; POLYTEEMA ; EOTALIA. 543
spinous out-growths that radiate from the central mass, is of very
common occurrence in shore-sands and shallow-water dredgings
on some parts of the Australian coast and among the Polynesian
islands. — To the simple form of this genus we are probably to refer
a large part of the fossils of the
early tertiary period that have been pIG- 278.
described under the name Orbitolina,
some of which attain a very large
size. Globular Orbitolince, which ap-
pear to have been artificially perfo-
rated and strung as beads, are not
unfrequently found associated with
the " flint-implements" of gravel-
beds. — Another very curio as modifi-
cation of the Rotaline type is pre-
sented by Polytrema, which so much
resembles a Zoophyte as to have been
taken for a minute Millepore ; but
which is made up of an aggregation
of ' globigerine' chambers communi-
cating with each other like those of
Tinoporus, and differs from that Tinqporus bactdatus.
genus in nothing else than its erect
and usually branching manner of growth, and the freer communi-
cation between its chambers. This, again, is of special interest in
relation to Eozoon ;
showing that an inde-
finite zoophytic mode of
growth is perfectly com-
patible with truly Fora-
miniferal structure.
447. In Rotalia, pro-
perly so called, we find
a marked advance to-
wards the highest type
of Foraminiferal struc-
ture ; the partitions that
divide the chambers
being composed of two
lamina?, and spacesbeing
left between them which
give passage to a system
of canals, whose general
distribution is shown in
Fig. 279. The proper
walls of the chambers,
moreover, are thickened
by an extraneous de-
posit, or 'intermediate
Fig. 279.
Section of BotaUa ScJiroetteriana near its base
and parallel to it : — showing, a, a, the radiating
interseptal canals ; 6, their internal bifurcations ; c,
a transverse branch ; d: tubular wall of the chambers.
5U VITREOUS FOEAMINIFEEA,
skeleton,' which sometimes forms radiating outgrowths ; bnt
this peculiarity of conformation is carried much further in
the genus which has been designated Galcarina from its resem-
blance to a spur-rowel (Plate XVI., fig. 3). The solid club-
shaped appendages with which this shell is provided, entirely belong
to the ' intermediate skeleton ' b, which is quite independent of the
chambered structure a ; and this is nourished by a set of canals
containing prolongations of the sarcode-body, which not only furrow
the surface of these appendages, but are seen to traverse their inte-
rior when this is laid open by section, as shown at c. In no other
recent Foraminifer does the ' canal system ' attain a like develop-
ment ; and its distribution in this minute shell, which has been
made out by careful microscopic study, affords a valuable clue to
its meaning in the gigantic fossil organism Eozoon Ganaclense
(§ 457). The resemblance which Galcarina bears to the radiate
forms of Tinoporus (Fig. 278) which are often fonnd with them in
the same dredgings, is frequently extremely striking ; and in their
early growth the two can scarcely be distinguished, since both
commence in a ' rotaline' spire with radiating appendages ; but whilst
the successive chambers of Oalcarina continue to be added on the
same plan, those of Tinoporus are heaped-up in less regular piles.
448. Certain beds of Carboniferous Limestone in Eussia are
entirely made-up, like the more modern ISTummulitic Limestone
(§ 452), of an aggregation of the remains of a peculiar type of
Foraminifera, to which the name Fusidina (indicative of its fusi-
form or spindle- shape) has been given. In general aspect and plan
of growth it so much resembles Alveolma, that its relationship to
that type would scarcely be questioned by the superficial observer.
But when its mouth is examined, it is found to consist of a single
slit in the middle of the lip ; and the interior, instead of being
minutely divided into chamberlets, is found to consist of a regular
series of simple chambers ; while from each of these proceeds a |3air
of elongated extensions, which correspond to the ' alar prolonga-
tions' of other spirally-growing Foraminifera (§ 451), but which,
instead of wrapping round the preceding whorls, are prolonged in
the direction of the axis of the spire, those of each whorl projecting
beyond those of the preceding, so that the shell is elongated with
every increase in its diameter. Thus it appears that in its general
plan of growth Fusidina bears much the same relation to a
symmetrical rotaline or nummuline shell, that Alveolina bears to
Orbicidina ; and this view of its affinities is fully confirmed by the
Author's microscopic examination of the structure of its shell.
For although the Fusulina-limestone of Eussia has undergone a
degree of metamorphism, which so far obscures this character that
he could not speak confidently of the shells of which it is composed,
yet the appearances he could distinguish were decidedly in its
favour. And having since received specimens from the Upper
Coal Measures of Iowa, U.S., which are in a much more perfect
state of preservation, he is able to state with certainty, not only
NUMMULINIDA:— AMPHISTEGINA; POLYSTOMELLA. 545
that Fusulina is tubular, but that its tubulation is of the large
coarse nature that marks its affinity rather to the Eotaline than to
the Nummuline series. — This type is of peculiar interest as having
long been regarded as the oldest form of Foraminifera, which was
known to have occurred in sufficient abundance to form Eocks by
the aggregation of its individuals. It will be presently shown,
however, that in point both of antiquity and of importance, it is
far surpassed by another (§ 456).
449. Nummulinida. — All the most elaborately constructed, and
the greater part of the largest, of the ' vitreous' Foraminifera belong
to the group of which the well-known Nummulite may be taken as
the representative.' Various plans of growth prevail in the family ;
but its distinguishing characters consist in the completeness of the
wall that surrounds each segment of the body (the septa being
double instead of single as elsewhere), the density and fine porosity
of the shell-substance, and the presence of an 'intermediate
skeleton,' with a ' canal-system' for its nutrition. It is true that
these characters are also exhibited in the highest of the Eotaline
series (§ 447), whilst they are deficient in the genus Amphistegina,
which connects the Nummuline series with the Eotaline ; but the
occurrence of such modifications in their border-forms is common
to other truly Natural groups. With the exception of Amphis-
tegina, all the genera of this family are symmetrical in form ; the
spire being nautiloid in such as follow that plan of growth, whilst
in those which follow the cyclical plan there is a constant equality
on the two sides of the median plane : but in Amphistegina there
is a reversion to the rotalian type in the turbinoid form of its spire,
as in the characters already specified, whilst its general conformity
to the jSTummuline type is such as to leave no reasonable doubt as
to its title to be placed in this family. Notwithstanding the want
of symmetry of its spire, it accords with Operculina and Nummu-
lina in having its chambers extended by ' alar prolongations' over
each surface of the previous whorl ; but on the under side these
prolongations are almost entirely cut off from the principal
chambers, and are so displaced as apparently to alternate with
them in position ; so that M. D'Orbigny, supposing them to
constitute a distinct series of chambers, described its plan of
growth as a biserial spiral, and made this the character of a
separate Order*
450. The existing Numnmlinida are almost entirely restricted
to tropical climates ; but a beautiful little form, the Polystomella
crisp a (Plate XV., fig. 16), the representative of a genus that
presents the most regular and complete development of the ' canal
system' anywhere to be met with, is common on our own coasts.
The peculiar surface-marking shown in the figure consists in a
* For an account of this curious modification of the Nummuline plan of
growth, the real nature of which was first elucidated by Messrs. Parker and
Rupert Jones, see the Author's ' Introduction to the Study of the Foraminifera '
(published by the Pay Society).
N N
546 VITEEOUS FORAMINIEERA. .
strongly marked ridge-and-furrow plication of the shelly wall of
each segment along its posterior margin ; the furrows being some-
times so deep as to resemble fissures opening into the cavity of the
chamber beneath. JN"o such openings, however, exist ; the only
communication which the sarcode-body of any segment has with
the exterior, being either through the fine tubuli of its shelly walls,
or through the row of pores that are seen in front view along the
inner margin of the septal plane, collectively representing a fissured
Pig. 280.
Internal Oast of Polystomella cratkulata: — a, retral pro-
cesses, proceeding from the posterior margin of one of the
segments ; b, ft1, smooth anterior margin of the same segment ;
c, c1, stolons connecting successive segments, and uniting
themselves with the diverging branches of the meridional
canals ; d, dl, d2, three turns of one of the spiral canals ; e, e\ e2,
three of the meridional canals; /, /], f2, their diverging
branches.
aperture divided by minute bridges of shell. The meaning of the
plication of the shelly wall comes to be understood, when we examine
the conformation of the segments of the sarcode-body, which may
be seen in the common Polystomella crisjpa by dissolving away the
shell of fresh specimens by the action of dilute acid, but which
may be better studied in such internal casts (Fig. 280) of the sarcode-
body and canal-system of the large P. craticulata of the Australian
coast, as may sometimes be obtained by the same means from
dead shells which have undergone infiltration with ferruginous
silicates* Here we see that the segments of the sarcode-body are
* It was by Prof. Phrenberg that the existence of such ' casts' in the Green
Sands of various Geological periods (from the Silurian to the Tertiary) was
first pointed out, in his Memoir ' Ueber der Griinsand und seine Pinlauterung
des organischen Lebens,' in " Abhandlungen der Konigl. Akad. der Wissen-
schaften," Berlin, 1855. It was soon afterwards shown by the late Prof. Bailey
(" Quart. Jourm of Microsc. Science," Vol. v. 1857, p. 83) that the like infiltra-
tion occasionally takes place in recent Foraminifera, enabling similar ' casts' to
be obtained frcm them by the solution of their shells in dilute acid. And,
NUMMULINIDA : — POLYSTOMELLA ; NONIONINA. 547
smooth along their anterior edge b. bl, but that along their poste-
rior edge, a, they are prolonged backwards into a set of ' retral
processes ;' and these processes lie under the ridges of the shell,
whilst the shelly wall dips down into the spaces between them, so
as to form the furrows seen on the surface. The connections of the
segments of stolons, c, c,1 passing through the pores at the inner
margin of each septum, are also admirably displayed in such
' casts.' But what they serve most beautifully to demonstrate is
the canal-system, of which the distribution is here most remark-
ably complete and symmetrical. At d, dx, d2, are seen three
turns of a spiral canal which passes along one end of all the seg-
ments of the like number of convolutions, whilst a corresponding
canal is found on the side which in the figure is undermost ; these
two spires are connected by a set of meridional canals, e, e1, er,
which pass down between the two layers of the septa that divide
the segments ; whilst from each of these there passes-off towards
the surface a set of pairs of diverging branches, /, f1, f2, which
open upon the surface along the two sides of each septal band,
the external openings of those on its anterior margin being in the
furrows between the retral processes of the next segment. These
canals appear to be occupied in the living state by prolongations
of the sarcode-body ; and the diverging branches of those of each
convolution unite themselves, when this is enclosed by another
convolution, with the stolon-processes connecting the successive
segments of the latter, as seen at c1. There can be little doubt
that this remarkable development of the canal-system has refe-
rence to the unusual amount of shell-substance which is deposited
as an ' intermediate skeleton' upon the layer that forms the proper
walls of the chambers, and which fills-up with a solid ' boss' what
would otherwise be the depression at the umbilicus of the spire.
The substance of this 'boss' is traversed by a set of straight
canals, which pass directly from the spinal canal beneath towards
the external surface, where they open in little pits, as is shown in
PL XV., fig. 16 ; the umbilical boss in this species, however, being
much smaller in proportion than it is in P. craticulata. — There is
a group of Foraminifera to which the term Nonionina is properly
applicable, that is probably to be considered as a sub-genus of
Polystomella ; agreeing with it in its general conformation, and
especially in the distribution of its canal-system ; but differing in
its aperture, which is here a single fissure at the inner edge of the
septal plane (Plate XY., fig. 19), and in the absence of the 'retral
processes' of the segments of the sarcode-body, the external walls
of the chambers being smooth. This form constitutes a transition
to the ordinary IS ummuline type, of which Polystomella is a more
aberrant modification.
451. The JNummuline type is most characteristically represented
acting npon this hint, Messrs. Parker and Eupert Jones succeeded in obtaining
from what had been put aside as the refuse of Mr. Jukes's Australian dredgings,
a number of casts of Polystomella, Alreolina, Amplristegina, and other types, of
most wonderful completeness.
X N 2
548 VITREOUS FORAMINIFEEA.
at tlie present time by the genus Operculina ; which is so intimately
united to the true Nummulite by intermediate forms, that it is not
easy to separate the two, notwithstanding that their typical
examples are widely dissimilar. The former genus (Plate XYL,
fig. 2) is represented on our own coast by very small and feeble
forms ; but it attains a much higher development in Tropical seas,
where its diameter sometimes reaches l-4th of an inch. The shell
is a flattened nautiloid spire, the breadth of whose earlier convolu-
tions increases in a regular progression, but of which the last con-
volution (in full-grown specimens) usually flattens itself out like
that of Peneroplis, so as to be very much broader than the preced-
ing. The external walls of the chambers, arching over the spaces
between the septa, are seen at h, b ; and these are bounded at the
outer edge of each convolution by a peculiar band a, termed the
' marginal cord.' This cord, instead of being perforated by minute
tubuli like those which pass from the inner to the outer surface of
the chamber-walls without division or inosculation, is traversed by
a system of comparatively large inosculating passages seen in cross
section at a! ; and these form part of the canal-system to be pre-
sently described. The principal cavities of the chambers are seen
at c, c ; while the ' alar prolongations ' of those cavities over the
surface of the preceding whorl are shown at c', c'. The chambers
are separated by the septa d, d, d, formed of two laminae of shell,
one belonging to each chamber, and having spaces between them
in which lie the ' interseptal canals,' whose general distribution is
seen in the septa marked e, e, and whose smaller branches are seen
irregularly divided in the septa d', d', whilst in the septum d" one
of the principal trunks is laid open through its whole length. At
the approach of each septum to the marginal cord of the preceding,
is seen the narrow fissure which constitutes the principal aper-
ture of communication between the chambers ; in most of the
septa, however, there are also ■ some isolated pores (to which the
lines point that radiate from e, e) varying both in number and
position. The interseptal canals of each septum take their
departure at its inner extremity from a pair of spiral canals, of
which one passes along each side of the marginal cord ; and they
communicate at their outer extremity with the canal-system of the
' marginal cord,' as shown in Fig. 284. The external walls of the
chambers are composed of the same finely -tubular shell-substance
that forms them in the ISTummulite ; but, as in that genus, not only
are the septa themselves composed of vitreous non-tubular sub-
stance, but that which lies over them, continuing them to the sur-
face of the shell, has the same character ; showing itself exter-
nally in the form sometimes of continuous ridges, sometimes of
rows of tubercles, which mark the position of the septa beneath.
These non- tubular plates or columns are often traversed by
branches of the canal-system, as seen at g, g. Similar columns of
non-tubular substance, of which the summits show themselves as
tubercles on the surface, are not unfrequently seen between the
PLATE XVI.
Fig. 1.
J
i
^^S^fet^^^pg^j
Fie. 2.
Fig. 3.
Various Fohms of Fobaminifeka.
[To face p. 545.
NTTMMULINIDA: — OPERCULINA ; NUMMULINA. 549
septal bands, giving a variation to the surface-marking, which,
taken in conjunction with variations in general conformation, might
be fairly held sufficient to characterize distinct species, were it
not that, on a comparison of a great number of specimens, these .
variations are found to be so gradational, that no distinct line of
demarcation can be drawn between the individuals which present
them.
452. The Genus Nwmmulina, of which the fossil forms are com-
monly known as Nummulites, though represented at the present
time by small and comparatively infrequent examples, was for-
merly developed to a vast extent; the Nummulitic Limestone
chiefly made-up by the aggregation of its remains (the material of
which the Pyramids are built) forming a band, often 1800 miles in
breadth and frequently of enormous thickness, that may be traced
from the Atlantic shores of Europe and Africa, through Western
Asia to Northern India and China, and likewise over vast areas
of North America. The diameter of a large proportion of fossil
Nummulites ranges between half an inch and an inch ; but there are
some whose diameter does not exceed l-16th of an inch, whilst others
attain the gigantic diameter of 4^ inches. Their typical form is
that of a double-convex lens ; but sometimes it much more nearly
approaches the globular shape, whilst in other cases it is very
much flattened ; and great differences exist in this respect among
individuals of what must be accounted one and the same species.
Although there are some Nummulites which closely approximate
Operculinoe in their mode of growth, yet the typical forms of this
genus present certain well-marked distinctive peculiarities. Each
convolution is so completely invested by that which succeeds it,
and the external wall or spiral lamina of the new convolution is so
completely separated from that of the convolution it encloses by
the ' alar prolongations ' of its own chambers (the peculiar arrange-
ment of which will be presently described), that the spire is
scarcely if at all visible on the external surface. It is brought into
view, however, by splitting the Nummulite through the median
plane, which may often be accomplished simply by striking it on
one edge with a hammer, the opposite edge being placed on a firm
support ; or, if this method should not succeed, by heating it in the
flame of a spirit-lamp, and then throwing it into cold water or
striking it edgeways. Nummulites usually show many more turns,
and a more gradual rate of increase in the breadth of the spire,
than Eoraminif era generally ; this will be apparent from an exami-
nation of the vertical section shown in Fig. 281, which is taken
from one of the commonest and most characteristic fossil examples
of the genus, and which shows no fewer than ten convolutions in a
fragment that does not by any means extend to the centre of the
spire. This section also shows the complete enclosure of the
older convolutions by the newer, and the interposition of the alar
prolongations of the chambers between the successive layers of the
spiral lamina. These prolongations are variously arranged in
550
VITREOUS FORAMINIFEKA.
different examples of the genus ; thus in some, as N~. distans, they
keep their own separate course, all tending radially towards the
centre ; in others, as N. laevigata, their partitions inosculate with
each other, so as to divide the space intervening between each
layer and the next into an irregular network, presenting in
vertical section the appearance shown in Fig. 281 ; whilst in
Fig. 281
Vertical Section of portion of Nurnmulina Iceviyata: — a,
margin of external whorl ; 6, one o f the outer row of chambers ;
c, c, whorl invested by a ; rf,one of the chambers of the fourth
whorl from the margin ; e, e, marginal portions of the enclosed
whorls ; f, investing portion of outer wuorl ; g, g, spaces left
between the investing portions of successive whorls ; A, h,
sections of the partitions dividing these.
N. garansensis they are broken up into a number of chamberlets,
having little or no direct communication with each other.
453. Notwithstanding that the inner chambers are thus so deeply
Fig. 282.
Portion of a thin Section of Nurnmulina Icevigata, taken in
the direction of the preceding, highly magnified to show the
minute structure of the shell : — a, a, portions of the ordinary
shell-substance traversed by parallel tubuli ; 6, 6, portions
forming the marginal cord, traversed by diverging and larger
tubuli ; c, one of the chambers laid open ; d, d, d, pillars of
solid substance not perforated by tubuli.
NUMMULINIDA :— STEUCTUEE OF NUMMULITES.
551
Fig. 283.
Portion of Horizontal Section of NiemmuKte,
showing the structure of the walls and of the
septa of the chambers : — a, a, a, portion of the
wall covering three chambers, the punctations
of which are the orifices of tubuli ; b, 6, septa
between these chambers, containing canals
which send out lateral branches, c, c, entering
the chambers by larger orifices, one of which
is seen at d.
buried in the mass of investing whorls, yet there is evidence that
the segments of sarcode which they contained were not cut off
from communication with
the exterior, but that they
may have retained their vi-
tality to the last. The
shell itself is almost every-
where minutely porous,
being penetrated by pa-
rallel tubuli which pass
directly from one surface
to the other. These tubes
are shown, as divided
lengthways by a vertical
section, in Fig. 282, a, a;
whilst the appearance they
present when cut across
in a horizontal section is
shown in Fig. 283, the trans-
parent shell - substance
a, a, a, bemg closely dotted
with minute punctations
which mark their orifices.
In that portion of the shell,
however, which forms the
margin of each whorl (Fig. 282, b, b), the tubes are larger, and diverge
from each other at greater intervals ; and it is shown by horizontal
sections that they communicate freely with each other laterally, so
as to form a network such as is shown at b, b, Fig. 284. At certain
other points, d, d, d (Fig. 282), the shell- substance is not perforated
by tubes, but is peculiarly dense in its texture, forming solid pillars
which seem to strengthen the other parts ; and in Nummulites
whose surfaces have been much exposed to attrition, it commonly
happens that the pillars of the superficial layer, being harder than
the ordinary shell- sub stance, and being consequently less worn
down, are left as prominences, the presence of which has often
been accounted (but erroneously) as a specific character. The suc-
cessive chambers of the same whorl communicate with each other
by a passage left between the inner edge of the partition that sepa-
rates them and the ' marginal cord' of the preceding whorl ; this
passage is sometimes a single large broad aperture, but is more
commonly formed by the more or less complete coalescence of
several separate perforations, as is seen in Fig. 281, b. There is
also, as in Operculum, a variable number of isolated pores in most
of the septa, forming a secondary means of communication
between the chambers. — The Canal-system of Nummulina seems to
be distributed upon essentially the same plan as in Operculina ; its
passages, however, are usually more or less obscured by fossilizing
material. A careful examination will generally disclose traces of
552
VITREOUS FORAMINIFERA.
them in the middle of the partitions that divide the chambers
(Fig. 283, b, b), while from these may be seen to proceed the lateral
branches (o, c), which, after bur-
Fig. 284. rowing (so to speak) in the walls
of the chambers, enter them by
3p large orifices (d). The inter -
pa septal canals, and their commu-
j^^^SSr§S2aB5o;=««
^p^mmmrsir^^
vSH
ji
=^s<iCi5g.
nication with the inoscnlating
system of passages excavated in
the marginal cord, are extremely
well seen in the ' internal cast '
represented in Fig. 284.
454. A very interesting modi-
fication of the Nummuline type
is presented in the geims Hete-
rostegina (Fig. 285), which bears
a very strong resemblance to
Orbiculina in its plan of growth,
whilst in every other respect it
is essentially different. If the
principal chambers of an Oper-
culina were divided into cham-
berlets by secondary partitions
in a direction transverse to that
of the principal septa, it would
be converted into a Heteroste-
gina ; just as a Penercplis
subdivision into an Orbiculina
Moreover, we see in Heterostegina, as in Orbiculina, a
great tendency to the open-
/
Internal cast of two of the cham-
bers, a, a, of Nummulina striata, with
the network of Canals, 6, 6, in the
marginal cord, communicating with
canals passing between the cham-
bers.
would be converted by the like
(§425).
Fig. 285.
Heterostegina.
ing-out of the spire with
the advance of age ; so that,
the apertural margin ex-
tends round a large part
of the shell, which thus
tends to become discoidal.
And it is not a little curious
that we have in this series
another form, Cycloclypeus,
which bears exactly the
same relation to Heteros-
tegina, that Orbitolites does
to Orbiculina ; in being con-
structed upon the cyclical
plan from the commence-
ment, its chamberlets being
arranged in rings around
a central chamber (Plate
XVI., fig. 1). This re-
NUMMULINIDA: — CYCLOCLYPETJS ; OEBITOIDES. 553
Fig. 286.
markable genus, at present only known by specimens dredged
np from considerable depths off the coast of Borneo, is the
largest of existing Foraminifera ; some specimens of its disks in
the British Museum having a diameter of 2 -J inches. Notwith-
standing the difference of its plan of growth, it so precisely
accords with the JSTummuline type in every character which
essentially distinguishes the genus, that there cannot be a doubt of
the intimacy of their relationship. It will be seen from the exami-
nation of that portion of the figure which shows Cyeloelypeus in
vertical section, that the solid layers of shell by which the cham-
bered portion is enclosed are so much thicker, and consist of
so many more lamella?, in the central portion of the disk, than they
do nearer its edge, that new lamella? must be progressively added
to the surfaces of the disk, concurrently with the addition of new
rings of chamberlets to its margin. These lamella?, however, are
closely applied one to the other, without any intervening spaces ;
and they are all traversed by columns of non-tubular substance,
which spring from the septal bands, and gradually increase in
diameter with their approach to the surface, from which they pro-
ject in the central portion of the disk as glistening tubercles.
455. The Nummulitic Limestone of certain localities (as the
South-west of France, JSTorth-eastem India, &c.) contains a vast
abundance of discoidal bodies
termed Orbitoides, which are
so similar to ISTummulites as to
have been taken for thern, but
which bear a much closer resem-
blance to Cyeloelypeus. These
are only known in the fossil
state; and their structure can
only be ascertained by the exa-
mination of sections thin enough
to be translucent. When one of
these disks (which vary in size,
in different species, from that
of a fourpenny-piece to that of
half-a-crown) is rubbed-down so
as to display its internal or-
ganization, two different kinds
of structure are usually seen in
it ; one being composed of cham-
berlets of very definite form,
quadrangular in some species,
circular in others, arranged with
a general but not constant re-
gularity in concentric circles
(Figs. 286, 287, b, b) ; the other,
less transparent, being formed
of minuter chamberlets which layer.
Section of Orbitoides ForUsU, parallel
to the surface ; traversing, at a, a, the
superficial layer, and at b, b, the median
554
VITEEOUS FOEAMINIFERA.
have no such constancy of form, but which might almost be taken
for the pieces of a dissected map (a, a). In the upper and lower
Fig. 287.
&&
Portions of the Section of Orbitoides Fortisii shown in Fig. 286, more
highly magnified ; — a, superficial layer; 6, median layer.
walls of these last, minute punctations may be observed, which
seem to be the orifices of connecting tubes whereby they are per-
forated. The relations of these two kinds of structure to each
other are made evident by the examination of a vertical section
Fig. 288.
Fig. 289.
Vertical Section of Orbitoides Fortisii, showing the large
central chamber at a, and the median layer surrounding it,
covered above and below by the superficial layers.
(Fig. 288) : which shows that the portion a, Figs. 286, 287, forms
the median plane, its concentric circles of chamberlets being
arranged round a large central cham-
ber a, as in Cycloclypeus ; whilst the
chamberlets of the portion b are ir-
regularly superposed one upon the
other, so as to form several layers
which are most numerous towards
the centre of the disk, and thin-away
gradually towards its margin. The
disposition and connections of the
chamberlets of the median layer in
Orbitoides seem to correspond very
Internal Cast of portion of me- closely with those which have been
dian plane of Orbitoides Fortisii, already described as prevailing in
showing at a a, a' a' a!' a", six CyclocVupeus ; the most satisfactory
chambers of each of three zones, • "V ■• , , i • «. , i •■ £ J
with their mutual communical indications to this effect being fur-
tions ; and at b b, b' V, b" b", por- nished by the siliceous internal
tions of three annular canals. casts ' to be met with in certain Green
Sands, which afford a model of the
NUMMULINIDA :— EOZOON CANAEENSE. 555
sarcode-body of tlie animal. In snch a fragment (Fig. 289) we
recognise the chamberlets of three successive zones, a, a', a", each
of which seems normally to communicate by one or two passages
with the chamberlets of the zone internal and external to its
own ; whilst between the chamberlets of the same zone there seems
to be no direct connection. They are brought into relation, how-
ever, by means of annular canals, which seem to represent the
spiral canals of the Numnralite, and of which the ' internal casts '
are seen at b b, V b', b" b".
456. A most remarkable Fossil, referable to the Foraminiferal
type, has been recently discovered in strata much older than the
very earliest that were previously known to contain Organic
remains ; and the determination of its real character may be
regarded as one of the most interesting results of Microscopic
research. This fossil, which has received the name Eozobn Cana-
dense, is found in beds of Serpentine Limestone that occur near
the base of the Laurentian Formation* of Canada, which has its
parallel in Europe in the ' fundamental gneiss ' of Bohemia and
Bavaria, and in the very earliest stratified rocks of Scandinavia
and Scotland. These beds are found in many parts to contain
masses of considerable size, but usually of indeterminate form,
disposed after the manner of an ancient Coral Beef, and consisting
of alternating layers— ^frequently numbering more than fifty— of.
Carbonate of Lime and Serpentine (Silicate of Magnesia). The
regularity of this alternation, and the fact that it presents itself
also between other Calcareous and Siliceous minerals, having led
to a suspicion that it had its origin in Organic structure, thin sec-
tions of well-preserved specimens were submitted to microscopic
examination by Dr. Dawson of Montreal, who at once recognised
its Foraminiferal nature :f the calcareous layers presenting the
characteristic appearances of true shell, so disposed as to form an
irregularly chambered structure, and frequently traversed by
systems of ramifying canals corresponding to those of Calcarina
(§ 447) ; whilst the serpentinous or other siliceous layers were
regarded by him as having been formed by the infiltration of sili-
cates in solution iuto the cavities originally occupied by the
sarcode-body of the animal, — a process of whose occurrence at
various Geological periods, and also at the present time, abundant
evidence has already been adduced. Although this determination
has been called in question, on the ground that some resemblance
* This Laurentian Formation was first identified as a regular series of stra-
tified rocks, underlying the equivalents not merely of the Silurian, but also of
the Upper and Lower Cambrian systems of this country, by Sir William Logan,
the former able Director of the Geological Survey of Canada.
t This recognition was due, as Dr. Dawson has explicitly stated in his
original Memoir (u Quarterly Journal of the Geological Society," Vol. xxi.,
p. 54), to his acquaintance not merely with the Author's previous researches
on the minute structure of the Foraminifera, but with the special characters
presented by thin sections of Calcarina, which had been transmitted to him
by the author.
556 VITREOUS FOEAMINIFERA.
to the supposed organic structure of Eozoon is presented by bodies
of purely Mineral origin,* yet, as it has not only been accepted by
all those whose knowledge of Foraminiferal structure gives weight
to their judgment, but has been fully confirmed by subsequent dis-
coveries^ the Author feels justified in here describing Eozoon as he
believes it to have existed when it originally extended itself as a,n
animal growth over vast areas of the sea-bottom in the Lauren-
tian epoch. X
457. Whilst essentially belonging to the Nummuline group, in
virtue of the fine tubulation of the shelly layers forming the
' proper wall' of its chambers, Eozoon is related to various types of
recent Foraminifera in its other characters. For in its indeter-
minate zoophytic mode of growth it agrees with Polytrema (§ 446) ;
in the incomplete separation of its chambers it has its parallel in
Carpentaria (§ 444) ; whilst in the high development of its ' inter-
mediate skeleton' and of the ' canal-system' by which this is formed
and nourished, it finds its nearest representative in C alcanna
(§ 447). Its calcareous layers were so superposed, one upon another,
as to include between them a succession of ' storeys' of chambers
(Plate XVII., fig. 1, a1, a1, a2, a2) ; the chambers of each ' storey'
usually opening one into another, as at a, a, like apartments
en suite; but being occasionally divided by complete septa, as at
b, b. These septa are traversed by passages of communication
between the chambers which they separate ; resembling those which,
in existing types, are occupied by stolons connecting together the
segments of the sarcode-body. Each layer of shell consists of two
finely-tubulated or ' nummuline' lamellaa, b, b, which form the
boundaries of the chambers beneath and above, serving (so to speak)
as the ceiling of the former, and as the floor of the latter ; and of
an intervening deposit of homogeneous shell-substance c, c, which
constitutes the ' intermediate skeleton.' The tubuli of this ' num-
muline layer' (Fig. 290) are usually filled-up (as in theNummulites
of the ' nummulitic limestone') by mineral infiltration, so as in
transparent sections to present a fibrous appearance ; but it fortu-
nately happens that through their having in some cases escaped
infiltration, the tubulation is as distinct as it is even in recent
Nummuline shells (Fig. 282), bearing a singular resemblance in its
occasional waviness to that of the Crab's claw (§ 573). JN"o one
familiar with the Microscopic appearances of tubular structure
can entertain the least doubt of the organic nature of this lamella.
The thickness of this interposed layer varies considerably in diffe-
* See the Memoirs of Profs. King and Rowney, in " Quart. Joum. of Geol.
Soc." Vol. xxii., p. 185 ; and " Ann. of Nat. Hist.," May, 1874.
f See Dr. Dawson's account of a specimen of Eozoon discovered in a homo-
geneous Limestone, in '' Quart. Journ. of Geol. Soc," Vol. xxiii., p. 257.
% For a fuller account of the results of the Author's own Study of Eozoon,
and of the basis on which the above reconstruction is founded, see his Papers
in "Quart. Journ. of Geol. Soc," Vol. xxi., p. 59, and Vol. xxii., p. 219, and in
the "Intellectual Observer," Vol. vii. (1865), p. 278 ; and his 'Further Re-
searches,' in " Ann. of Nat. Hist.," June, 1874.
NUMMULIXID A ; — EOZOON C ANADENSE.
557
rent parts of the same mass ; being in general greatest near its
base, and progressively diminishing towards its npper snrface.
The ' intermediate skeleton' is occasionally traversed by large
(d), which seem to establish a connection between the
Fig. 290.
Vertical Section of a portion of one of the Calcareous
lamellae of Eoznm Canadense : — a «, Xunmiuline layer, per-
forated by parallel tubuli, which show a flexure along the
line a' a'; beneath this is seen the intermediate skeleton, c, c,
traversed by the large canals, b &, and by oblique cleavage
planes, which extend also into the nummulme layer.
snccessive layers of chambers ; and it is penetrated by arborescent
systems of canals (e, e), which are often distributed both so ex-
tensively and so minutely through its substance, as to leave very
little of it without a branch. These canals take their origin, not
directly from the chambers, but from irregular lacunce or inter-
spaces between the outside of the proper chamber -walls and the
' intermediate skeleton,' exactly as in Calcarina (§ 447) ; the exten-
sions of the sarcode-body which occupied them having apparently
been formed by the coalescence of the pseudopodial filaments that
passed through the tubulated lamellae.
458. In the fossilized condition in which Eozoon is most com-
monly found, not only the cavities of the chambers, but the canal-
systems to their smallest ramifications, are filled up by the siliceous
infiltration which has taken the place of the original sarcode-body,
as in the cases already cited (§ 450, note) ; and thus when a piece
of this fossil is subjected to the action of dilute acid, by which its
558 VITREOUS FOEAMINIFEEA.
calcareous portion is dissolved-away, we obtain an internal cast of
its chambers and canal-system (Plate XVII., fig. 2), which, though,
altogether dissimilar in arrangement, is essentially analogous in
character to the ' internal casts' represented in Figs. 280, 284.
This cast presents us, therefore, with a model in hard Serpentine of
the soft sarcode-body which originally occupied the chambers, and
extended itself into the ramifying canals, of the calcareous shell ;
and, like that of Polystomella (§ 450), it affords an even more satis-
factory elucidation of the relations of these parts, than we could
have gained from the study of the living organism. We see that
each of the layers of serpentine, forming the lower part of such a
specimen, is made up of a number of coherent segments, which
have only undergone a partial separation ; these appear to have
extended themselves horizontally without any definite limit ; but
have here and there developed new segments in a vertical direction,
so as to give origin to new layers. In the spaces between these
successive layers, which were originally occupied by the calcareous
shell, we see the ' internal casts' of the branching canal-system ;
which give us the exact models of the extensions of the sarcode-
body that originally passed into them. — But this is uot all. In
specimens in which the nummuline layer constituting the ' proper
wall' of the chambers was originally well preserved, and in which
the decalcifying process has been carefully managed (so as not, by
too rapid an evolution of carbonic acid gas, to disturb the arrange-
ment of the serpentinous residuum), that layer is represented by
a thin white film covering the exposed surfaces of the segments ;
the superficial aspect of which, as well as its sectional view, are
shown in fig. 2. And when this layer is examined with a suffi-
cient magnifying power, it is found to consist of extremely minute
needle-like fibres of Serpentine, which sometimes stand upright,
parallel, and almost in contact with each other, like the fibres of
asbestos* (so that the film which they form has been termed the
' asbestiform layer'), but which are frequently grouped in converg-
ing brush-like bundles, so as to be very close to each other in
certain spots at the surface of the film, whilst widely separated in
others. JSTow these fibres, which are less than l-10,000th of an
inch in diameter, are the ' internal casts' of the tubuli of the
Nammuline layer (a precise parallel to them being presented in
* It would seem to be from having confined their studies to decalcified
specimens, and from never having seen the true ' nummuline layer' shown in
Fig. 290, that Profs. King and Bowney have fallen into the mistake of repre-
senting the 'asbestiform layer ' as merely the superficial lamella of the supposed
' chamber-cast ' in which the serpentine has split up into chrysotile fibres.
The incorrectness of this representation is proved, not merely by the perfectly
distinct line of demarcation which (in transparent sections) separates the 'num-
muline layer ' from the surface of the ' chamber-cast,' but also by the fact that
it is not until after decalcification that this layer presents itself in the form of
separate fibres, the serpentinous aciculee having been previously held together
by the calcareous matrix wherein they are imbedded, into which matrix the
cleavage-planes of the intermediate skeleton extend, as shown in Fig. 290.
PLATE XVII.
Fig. 1.
Fig. 2.
Stsuctube of Eozoojf Canadense.
[To face p. 553.
NUMMULINIDA;— EOZOON CANADENSE. 559
the 'internal cast' of a recent Amphistegina in the Authors
possession) ; and their arrangement presents all the varieties
which have been mentioned (§ 451) as existing in the shells of
Operculina. — Thus these delicate and beautiful siliceous fibres
represent those pseudopodial threads of sarcode, which originally
traversed the minutely -tubular walls of the chambers ; and a
precise model of the most ancient animal of which we have any
knowledge, notwithstanding the extreme softness and tenuity of
its substance, is thus presented to us with a completeness that is
scarcely even approached in any later fossil.
459. In the upper part of the ' decalcified' specimen shown in
Plate XVII., fig. 2, it is to be observed that the segments are
confusedly heaped together, instead of being regularly arranged in
layers ; the lamellated mode of growth having given place to the
acervuline. This change is by no means uncommon among Fora-
minif era ; an irregular piling-together of the chambers being
frequently met-with in the later growth of types, whose earlier
increase takes place upon some much more definite plan. After
what fashion the earliest development of Eozoon took place, we
have at present no knowledge whatever ; but in a young specimen
which has been recently discovered, it is obvious that each successive
' storey' of chambers was limited by the closing-in of the shelly
layer at its edges, so as to give to the entire fabric a definite form
closely resembling that of a straightened Peneroplis (Plate XV.,
fig. 5). Thus it is obvious that the chief peculiarity of Eozoon lay
in its capacity for indefinite extension; so that the product of a
single germ might attain a size comparable to that of a massive
Coral. — Now this, it will be observed, is simply due to the fact that
its increase by gemmation takes place continuously ; the new
segments successively budded-off remaining in connection with
the original stock, instead of detaching themselves from it, as in
Foraminii'era generally. Thus the little Globigerina forms a shell
of which the number of chambers does not usually seem to increase
beyond tiuelve, any additional segments detaching themselves so as
to form separate shells ; but by the repetition of this multiplication,
the sea-bottom of large areas of the Atlantic Ocean at the present
time has come to be covered with accumulations of Globigerince,
which, if fossilized, would form beds of Limestone not less massive
than those which have had their origin in the growth of Eozoon. —
The difference between the two modes of increase may be compared
to the difference between a Plant and a Tree. For in the Plant
the individual organism never attains any considerable size, its
extension by gemmation being limited ; though the aggregation of
individuals produced by the detachment of its buds (as in a Potato-
field) may give rise to a mass of vegetation as great as that
formed in the largest Tree by the continuous putting-forth of new
buds.
460. It has been hitherto only in the Laurentian Serpentine-
Limestone of Canada, that Eozoon has presented itself in such
560 VITREOUS FORAMINIFERA.
a state of preservation as fully to justify the assumption of its
Organic nature. But from the greater or less resemblance which
is presented to this by Se^entine-Limestones occurring in various
localities* among strata that seem the Geological equivalents of
the Canadian Laurentians, it seems a justifiable conclusion that
this type was very generally diffused in the earlier ages of the
Earth's history ; and that it had a large (and probably the chief)
share in the production of the most ancient Calcareous strata,
separating Carbonate of Lime from its solution in Ocean-water, in
the same manner as do the Polypes by whose growth Coral-reefs
and islands are being upraised at the present time.
461. Collection and Selection of 'Foraminifera. — Many of the
Foraminifera attach themselves in the living state to Sea-weeds,
Zoophytes, &c. ; and they should, therefore, be carefully looked-for
on such bodies, especially when it is desired to observe their in-
ternal organization and their habits of life. They are often to be
collected in much larger numbers, however, from the sand or mud
dredged-up from the sea-bottom, or even from that taken from
between the tide-marks. In a paper containing some valuable
hints on this subject,f Mr. Legg mentions that, in walking over
the Small-Mouth Sand, which is situated on the north- side of Port-
land Bay, he observed the sand to be distinctly marked with white
ridges, many yards in length, running parallel with the edge of
the water ; and upon examining portions of these, he found Fora-
minifera in considerable abundance. One of the most fertile sources
of supply that our own coasts afford, is the ooze of the Oyster-beds,
in which large numbers of living specimens will be found ; the
variety of specific forms, however, is usually not very great. In
separating these bodies from the particles of sand, mud, &c, with
which they are mixed, various methods may be adopted, in order
* The Author has satisfied himself of this fact, in regard to various speci-
mens of OpMcalcite obtained from various depths in the great fundamental
Gneiss of Central Europe, the thickness of which formation is estimated by
Sir Roderick Murchison at 90,000 feet ; and the form of Eozoon which there
presents itself has been elaborately studied by Prof. Gunibel. (See his Memoir
• Ueber das Vorkommen von Eozoon im ostbayerischen Urgeberge,' in the
" Sitzungsberichte der Konigl. Acad, der Wissenschaften in Miinchen," 1866,
i. 1.) He has also examined with the same result specimens of Serpentine-
Limestone, obligingly sent to him by Prof. Loven, of Stockholm, from the
Laurentians of Scandinavia. In the case of these, however, as in that of the
Connemara Marble, it is obvious that the rock has undergone very considerable
. metamorphic action ; so that its originally Organic structure has in great degree
given place to a purely mineral arrangement, as has occurred in numberless
other cases. And he believes tbat the objections taken by Profs. King and
Eowney to the doctrine of the Foraminiferal character of Eozoon have been
mainly suggested by their having especially studied one of its most altered and
least characteristic forms ; and by their having had comparatively little oppor-
tunity of examining the Canadian specimens in which the evidences of organic
structure are most unmistakable, and of comparing their characters with those
of other fossil as well as recent Foraminifera.
t " Transactions of Microscopical Society," 2nd Series, Vol. ii. (1854), p. 19.
COLLECTION OF FOKAMIXIFEEA. 561
to shorten the tedious labour of picking them out, one by one,
under the Simple Microscope ; and the choice to be made among
these will mainly depend upon the condition of the Foraminifera,
the importance (or otherwise) of obtaining them alive, and the
nature of the substances with which they are mingled. — Thus, if
it be desired to obtain living specimens from the Oyster-ooze,
for the examination of their soft parts, or for preservation in an
Aquarium, much time will be saved by stirring the mud (which
should be taken from the surface only of the deposit) in a jar with
water, and then allowing it to stand for a few moments ; for the
finer particles will remain diffused through the liquid, while the
heavier will subside ; and as the Foraminifera (in the present case)
belong to the latter category, they will be found at the bottom of
the vessel, almost entirely free from extraneous matter, after this
operation has been repeated two or three times. It would always
be well to examine the first deposit let fall by the water that has
been poured-away ; as this may contain the smaller and lighter
forms of Foraminifera. — But supposing that it be only desired to
obtain the dead shells from a mass of sand brought -up by the
dredge, a very different method should be adopted. The whole
mass should be exposed for some hours to the heat of an oven, and
be turned-over several times, until it is found to have been
thoroughly dried throughout; and then, after being allowed to
cool, it should be stirred in a large vessel of water. The chambers
of their shells being now occupied by air alone (for the bodies of
such as were alive will have shrunk up almost to nothing), the
Foraminifera will be the lightest portion of the mass ; and they
will be found floating on the water, while the particles of sand, &c,
subside. — Another method, devised by Mr. Legg, consists in taking
advantage of the relative sizes of different kinds of Foraminifera
and of the substances that accompany them. This, which is
especially applicable to the sand and rubbish obtainable from
Sponges (which may be got in large quantity from the sponge-
merchants), consists in sifting the whole aggregate through succes-
sive sieves of wire-gauze, commencing with one of 10 wires to the
inch, which will separate large extraneous particles, and proceeding
to those of 20, 40, 70, and 100 wires to the inch, each (especially
that of 70) retaining a much larger proportion of Foraminiferal
shells than of the accompanying particles ; so that a large portion
of the extraneous matter being thus got rid of, the final selection
becomes comparatively easy. — Certain forms of Foraminifera are
found attached to Shells, especially bivalves (such as the Chamacece)
with foliated surfaces ; and a careful examination of those of tropical
seas, when brought home ' in the rough,' is almost sure to yield
most valuable results. — The final selection of specimens for mount-
ing should always be made under some appropriate form of Single
Microscope (§§ 39-41) ; a fine camel-hair j)encil, with the point
wetted between the lips, being the instrument which may be most
conveniently and safely employed, even for the most delicate
o o
562 FOKAMINIFEKA AND POLYCYSTINA.
specimens. In mounting Foraminifera as Microscopic objects, the
method to be adopted mnst entirely depend npon whether they
are to be viewed by transmitted or by reflected light. In the former
case they should be mounted in Canada-balsam ; the various pre-
cautions to prevent the retention of air-bubbles, which have been
already described (§ 176), being carefully observed. In the latter
no plan is so simple, easy, and effectual, as the attaching them
with a little gum to wooden slides (§171). They should be fixed
in various positions, so as to present all the different aspects of the
shell, particular care being taken that its mouth is clearly dis-
played ; and this may often be most readily managed by attaching
the specimens sideways to the wall of the circular depression of
the slide. Or the specimens may be attached to disks fitted for
being held in Morris's Disk-holder (§ 106) ; whilst for the ex-
amination of specimens in every variety of position, Mr. E. Beck's
Disk-holder (Fig. 83) will be found extremely convenient. Where,
as will often happen, the several individuals differ considerably
from one another, special care should be taken to arrange them
in series illustrative of their range of variation and of the mutual
connections of even the most diverse forms. — For the display of
the internal structure of Foraminifera, it will often be necessary
to make extremely thin sections, in the manner already described
(§§ 155-157) ; and much time will be saved by attaching a number
of specimens to the glass slide at once, and by grinding them down
together. For the preparation of sections, however, of the extreme
thinness that is often required, those which have been thus reduced
should be transferred to separate slides, and finished-off each one
by itself.
462. Polyctstina. — These are minute Siliceous shells, possessing
wonderful beauty and variety of form and structure, and containing
in the living state an olive-brown ' sarcode,' which extends itself
into pseudopodial prolongations (resembling those of the Actinoplirys,
§ 373), that pass through the large apertures by which the shells
are perforated (Plate XYIII.,figs. 3, 4). The sarcode-body does not
always fill the shell ; often occupying only its upper part or vault,
and showing a regular division into four lobes. The shells are
in some instances most perfect spheres (Plate XIX., fig. 1) ; and
occasionally we find a whole series of such spheres arranged con-
centrically one within another, and connected by radiating rods
(fig. 2). They are often prolonged into spines or other projections,
which sometimes branch in a very remarkable manner (figs. 4, 5).
The range of variation among Polycystina seems to be not at all
less remarkable than it is in Foraminifera (§ 431), In the former,
as in the latter, well-marked diversities of configuration present
themselves between forms that resemble each other in general plan
of structure ; such as, on a cursory examination, would seem to
justify the establishment of a great number of distinct species, if
not of distinct genera. Such a series of specimens is represented
in Fig. 291, in which it is obvious that the diversity existing
PLATE XVIII.
Fig. 1.
Fig. 2.
Fig. 3.
Fig. 4.
Various Foeiis of Polycystina.
[To face p. 562.
VARIABILITY OF POLYCYSTIXA.
563
amongst the seven specimens is due, on the one hand, to the
presence of only four rays in d, e and g, whilst there are five in
a, e, c, e ; and, on the other, to the degree in which the spaces
between the rays are filled up by siliceous network. Now, in these
Fio. 291.
Varietal modifications of Astromma.
low types of Animal life, as in the discoidal Diatoms (§ 251). it
may be pretty certainly affirmed that the mere number of rays —
the structure of each individual ray being the same — does not con-
stitute a valid specific character ; whilst, on the other hand, when
a large number of examples of this type are passed under review,
it becomes obvious that its diversities of detail are so gradational
as to prevent any line of division from being drawn among them, so
that they must all be accounted as varieties of a single species.*
It seems probable that these creatures are almost as widely diffused
at the present time as are the Foraminifera, although from their
greater minuteness they have not been so often recognised. For
having been first discovered by Prof. Ehrenberg at Cuxhaven on
* The general Plan of Structure of the Pohjcystina, and the signification of
their immense variety of forms, are ably discussed by Dr. Wailich, in the
u Trans, of the Microsc. Society," N.S., Vol. xiii., p/75; but no system of
Classification can at present, in the Author's opinion, be regarded as otherwise
than provisional.
0 0 2
564:
POLYCYSTINA.
the Xorth Sea, they were afterwards found "by him in collections
made in the Antarctic Seas, and have since been recognized
as presenting themselves (with Forammifera and Diatomaceee) in
Fig. 292.
Fig. 293.
Haliomma Humboldtii.
Perichlamy ilium prcetecctum.
Fig. 294.
Fig. 295.
Stylodyctya gracilis.
Astromma Aristotelis.
the deposits bronght-np by the sonnding-lead from the bottom
of the Atlantic, at depths of from 1000 to 3000 fathoms. They have
also been studied by Prof. Miiller* in the Mediterranean ; and an
* 'Ueber die Thallassicollen, Polycystinen, und Acanthometren des Mittel-
meeres,' in ''Abhandlungen der Konigl. Akad. der Wissensch. zu Berlin,"
1858, and separately published ; also ' Ueber die ini Hafen von Messina beo-
bachteten Polycystinen,' in the " Monatsberichte " of the Berlin Academy for
1855, pp. 671-676.
FOSSIL POLYCYSTINA.
56i
immense variety of forms occurring in the Adriatic has been
described in the magnificent work of Prof. Haeckel ;* whilst Dr.
Wallich has met with this type abundantly in the Indian Ocean.
463. The Polycystina appear to have been yet more abundant
during the later Geological periods than they are at present ; for
not only have certain forms (among them Haliomma, Fig. 292)
been detected by Prof. Ehrenberg in the Chalks and Marls of
Sicily and Greece, and of Oran in Africa, and also in the Dia-
Fig. 296
Fossil Polycystina, &c, from Barbadoes : — a, Podocyrtis
mitra ; 6, Rhabdolithus sceptrum ; c, Lychnocaniuni falcife-
rum; rf, Eucyrtidiuni tubulus ; e, Flustrella concentrica ; /,
Lychnocanium. lucerna ; g, Eucyrtidium elegans ; h, Dicty-
ospyris clathrus ; ?, Eucyrtidium Mougolfieri ; A-, Stephano-
litbis spinescens; Z, S. nodosa; m, Litbocyclia ocellus; n,
Cephalolitbis sylvina ; o, Podocyrtis cothumata ; p, Ehabdo-
lithus pipa.
tomaceous deposits of Bermuda and Eichmond (Virginia) ; but a
large proportion of the rock that prevails through an extensive
district in the island of Barbadoes has been found by him to be
composed of Polycystina, mingled with Diatomaceae, with a few
* "Die Badiolarien (Ehizopoda Piadiaria)," Berlin, 1862.
566 POLYCYSTIC AND ACANTHOMETRINA.
calcareous Foraminifera, and with calcareous earth which was
probably derived from the decomposition of Corals, &c. Few
Microscopic objects are more beautiful than an assemblage of
the most remarkable forms of the Barbadian Polycystina (Fig. 296),.
especially when seen brightly illuminated upon a black ground ;.
since (for the reason formerly explained, § 95) their solid forms
then become much more apparent than they are when these objects
are examined by light transmitted through them. And when they
are mounted in Canada-balsam, the Black-ground illumination,
either by the Webster-condenser (§ 89), the Spot-lens (§ 93), or
the Paraboloid (§ 94), is much to be preferred for the purpose of
display, although minute details of structure can be better made
out when they are viewed as transparent objects with higher
powers. Many of the more solid forms, when exposed to a high
temperature on a slip of platinum foil, undergo a change in aspect
which renders them peculiarly beautiful as opaque objects ; their
glassy transparence giving place to an enamel-like opacity. They
may then be mounted on a black ground, and illuminated either
with a Side-condenser, or with the Parabolic S]3eculum (§ 101). —
~No class of objects is more suitable than these to the Binocular
Microscope ; its stereoscopic projection causing them to be pre-
sented to the mind's eye in complete relief, so as to bring-out
with the most marvellous and beautiful effect all their delicate
sculpture.*
464. Acanthometeina. — In this little group, which seems to
form a connecting link between Polycystina and Sponges, the
animal is not enclosed within a shell, but is furnished with a very
regular skeleton composed of elongated spines, which radiate in all
directions from a common centre (Plate XIX., fig. 3). The soft
sarcode-body is spherical in form, and occupies the spaces left
between the bases of these spines, which are sometimes partly
enclosed (as in the species represented) by transverse projections.
The ' ectosarc' seems to have a more definitely membranous con-
sistence than in Actinophrys ; but it is pierced by the pseudopodia,
whose convergence may be traced from without inwards, after
passing through it; and it is itself enveloped in a layer of less
tenacious protoplasm, resembling that of which the pseudopodia
are composed. The ' endosarc' contains a number of yellow cell-like
globules, resembling those of ThalassicollaB (§ 384). — One species,
the Acantlwmetra echlnoides, which presents itself to the naked eye
as a crimson-red point, the diameter of the central part of its body
being about 6-10<J0ths of an inch, is very common on some parts of
the coast of Norway, especially during the prevalence of westerly
* For a fuller description of the Fossil forms of this group, see Prof. Ehren-
berg's Memoirs in the " Monatsberichte " of the Berlin Academy for 1846, 1847,
and 1850 ; also his 'Microgeologie,' 1854; and " Ann. of Nat. Hist.," Vol. xx.
(1847).— The best method of separating the Polycystina from the Barbadoes
sandstone is described by Mr. Furlong in the " Quart. Journ. of Microsc.
Science," ISiew Ser., Vol. i. (18G1), p. 64.
PLATE XIX.
2 ,
~ ■-;.
< -it ■
? i
\im^
VaBIOUS FoKMS OF EaDIOLABIA.
[To face p. 566.
ANIMAL NATURE OF SPONGES.
567
winds ; and the Author has himself met with it abundantly near
Shetland, in the floating brown masses termed madre by the fisher-
men, who believe them to furnish food to the herring, these
masses consisting mainly of this Acanthometra mingled with
Entomostraca.
465. Pokifera. — The determination of the real character of the
animals of this Class, which are commonly known as Sponges, has
been entirely effected by the microscopic examination of their minute
structure ; for until this came to be properly understood, not only
was the general nature of these organisms entirely misapprehended,
but they were regarded by many naturalists as having no certain
claim to a place in the Animal Kingdom. The skeleton of the
living Sponge, usually composed of a fibrous network strengthened
by spicules of Mineral matter — generally siliceous, but sometimes
caltareous — is clothed with a soft flesh ; and this flesh consists of an
aggregation of amceba-like bodies (Fig. 297, b), some of which are
furnished with one or more long cilia, closely resembling those of
Vo'ivox (Plate IX., fig. 9), by the agency of which a current of
wa:er is kept-up through the passages and canals excavated in the
Fig. 297.
Structure of Grantia compressar— A, portion moderately
magnified, showing general arrangement of triradiate spicules
and intervening tissue ; — B, small portion highly magnified,
showing ciliated cells.
substance of the mass. And from the observations of Mr. Carter*
upon the early development of Sponges, it appears that they begin
life as solitary Amcebce ; and that it is only in the midst of aggre-
gations formed by the multiplication of these, that the charac-
teristic sponge- structure makes its appearance, the_ formation of
spicules being the first indication of such organization. The
* " Annals of Natural History," Second Series, Vol. iv. (1849), p. 81.
568
POEIFEEA, OE SPONGES.
ciliated cells seem usually to form the walls of special chambers
lying at some distance beneath the surface ; and these communi-
cate with a system of canals, by which the whole fabric of the
Sponge is traversed. These canals, which are very irregular in
their distribution, may be said to commence in the small pores of
the surface, and to terminate in the large vents ; and a current of
wafer, maintained by the action of the cilia lining the chambers, is
continually entering at the former, and passing forth from the
latter, during the whole life of the Sponge, bringing in alimentary
particles and oxygen, and carrying out exerementitious matter. In
an American species of the fresh-water genus Sjpongilla — whose
green colour is due (like that of Plants) to the formation of chloro-
phyll under the influence of light — it has been shown by the lecent
inquiries of Prof. H. James Clark (Kentucky) that ciliated monads,
resembling the flagellate Infusoria, are arranged round circular
chambers, with their ciliated ends pointing towards the centre,
each chamber having a small aperture which perforates the invest-
ing membrane.*
466. The Skeleton which gives shape and substance to the mass
of sarcode-particles that constitutes the living animal, is composed,
in the Sponges with which we are most familiar, of an irregular
reticulation of horny fibres. The arrangement of these may be
best made out by cutting
Fig. 298. thin slices of a piece of
Sponge submitted to firm
compression, and viewing
these slices, mounted upon
a dark ground, with a low
magnifying power, under
incident light. Such sec-
tions, thus illuminated, are
not merely striking objects,
but serve to show, very
characteristically, the gene-
ral disposition of the larger
canals and of the smaller
areolas with which they com-
municate. In the ordinary
Sponge, the fibrous skeleton
is almost entirely destitute
of spicules ; the absence of
which, in fact, is one impor-
tant condition of that flexi-
Portion of Halichdndrm (?) from Matfa- bility and compressibility on
IS £LT UlCS Pr°,eCtmg fr°m the wMd; it, nses depend, men
spicules exist in connection
* See his Memoir in "Silliman's American Journal," Dec, 1870 ; and the
abstract of it in the " Monthly Microscopical Journal," March, 1872.
SILICEOUS SPONGES: — EUPLECTELLA. 569
with such a skeleton, they are usually either altogether imbedded
in the fibres, or they are implanted into them at their bases, as
shown in Fig. 298.
467. There is an extremely interesting group of Sponges, in
which the horny skeleton is entirely replaced by & siliceous frame-
work of great firmness and of singular beauty of construction.
This framework may be regarded as fundamentally consisting of
an arrangement of six-rayed spicules, the extensions of which come
to be, as it were, soldered to one another ; and hence the group is
distinguished as hexiradiate. Of this type the beautiful Euplectella
of the Manilla Seas — which was for a long time one of the greatest
of zoological rarities, but which now, under the name of ' Venus's
flower-basket,' is a common ornament of our drawing-rooms — is
one of the most characteristic examples. This has the form of a
cornucopia, composed of an exquisitely beautiful network of sili-
ceous fibres, looking like spun-glass, while its expanded top is closed
in, when the organism has come to its full growth, by a lid of
similar structure ; while round its base is a sort of ruff of long
separate fibres, which served to anchor it on the sea bottom. The
framework is clothed, in the living state, by a soft flesh ; but this
does not fill up the larger areolations of the network, so that water
can pass freely through these from the exterior to the interior. —
Another exampleof this type is presented bythe Holtenia Carpenteri,
of which four specimens, dredged up from a depth of 530 fathoms
between the Faroe Islands and the ISTorth of Scotland, was one of
the most valuable of the ' treasures of the deep ' obtained during
the first Deep-sea Exploration (1868), carried on by Prof. Wyville
Thomson and the Author. This is a turnip-shaped body, with a
cavity in its interior, the circular mouth of which is surrounded
with a fringe of elongated siliceous spicules ; whilst from its base
there hangs a sort of beard of siliceous threads, that extend them-
selves, sometimes to a length of several feet, into the Atlantic mud
(§ 443), in which these bodies are found. The framework is much
more massive than that of Euplectella, but it is not so exclusively
mineral ; for if it be boiled in nitric acid it is resolved into separate
spicules, these being not soldered together by siliceous continuity,
but being held together by animal matter. Besides the regular
hexiradiate spicules, there is a remarkable variety of other forms,
which have been fully described and figured by Prof. Wyville Thom-
son.* One of the greatest features of interest in this Holtenia, is its
singular resemblance to the Ventriculites of the Cretaceous formation
(Chap.XTX). Subsequent investigations have shown that it is very
widely diffused, and that it is only one of several Deep-sea forms,
including several of singularly beautiful structure, which represent
the old Ventriculite type at the present time. One of these was
previously known, from being occasionally cast up on the shore of
* See his elaborate Memoir in "Philos. Transact.," 1870 ; and his "Depths
of the Sea " (1872), p. 71.
570 POEIFERA, OE SPONGES.
Barbadoes after a storm. This Bictyocalyx pumiceus* has the
shape of a mushroom, the diameter of its disk sometimes ranging
to a foot. A small portion of its skeleton is a singularly beautiful
object when viewed with incident light under a low magnifying
power. — Another extraordinary production, which is referrible to
the same type, is the Hyalonema, originally brought from the
Japan seas, but since found upon the coast of Portugal and else-
where. This consists of " a bundle of from 2- to 300 threads of
transparent silica, glistening with a satiny lustre, like the most
brilliant spun-glass, each thread about eighteen niches long; in the
middle, of the thickness of a knitting-needle, and gradually taper-
ing towards either end to a fine point ; the whole bundle coiled
like a strand of rope into a lengthened spiral, the threads of the
middle and lower portions remaining compactly coiled by perma-
nent twist of the individual threads ; the upper portions of the
coil frayed out, so that the glassy threads stand separate from
one another, like the bristles of a glittering brush ; the lower
extremity of the coil imbedded perpendicularly in the middle of a
hemispherical or conical undoubted Sponge, and usually part of
the exposed portion of the siliceous coil and part of the sponge
covered with a brown leathery coating, whose surface is studded
with Polypes of an equally undoubted Zoantharian Zoophyte."f —
Sponge-spicules are much more frequently siliceous than calca-
reous ; and the variety of forms presented by the siliceous spicules
is much greater than that which we find in the comparatively
small division in which they are composed of Carbonate of Lime.
The long needle-like spicules (Fig. 299), which are extremely
abundant in several Sponges, lying close together in bundles, are
sometimes straight, sometimes slightly curved ; they are sometimes
pointed at both ends, sometimes at one only ; one or both ends
may be furnished with a head like that of a pin, or may carry
three or more diverging points which sometimes curve back so as
to form hooks (Fig. 433, h). When the spicules project from the
horny framework, they are somewhat conical in form, and their
surface is often beset with little spines, arranged at regular inter-
vals, giving them a jointed appearance (Fig. 298). Sponge-
* By some mistake the name Dactylocalix, which is altogether inappropriate,
has come to be substituted for the appropriate name originally conferred on
this Sponge by Mr. S. Stutchbury.
t See Prof. Wyville Thomson, in the " Intellectual Observer," Vol. xi.,
p. 81. — The nature of this organism has been the subject of much controversy,
of. which a resume is given in the Paper just referred to. There can no longer
be any doubt that the elongated threads forming the 'rope ' are true Sponge-
spicules, which extend themselves — as in Euplectella and Holtenia — from the
siliceous framework of the Sponge that bears them, and serve to anchor it on
the soft sea-bottom whereon it lives. The organism was first found alive in
the deep Dredging which yielded the earliest specimens of the Holtenia; and
the parasitic nature of the Palithoa which invested the flint-rope, was proved
by its occurrence on a Sertularian stem which was brought up at the same
time.
SPICULES OF SPONGES.
571
Siliceous Spicules of Pachymatisma.
spicules frequently occur, however, under forms very different from
the preceding ; some being short and many-branched, and the
branches being themselves very commonly stunted into mere
tubercles (some examples
of which type are pre- Fig. 299.
sented in Fig. 433, a, c) ;
whilst others are stellate,
having a central body with
conical spines projecting
from it in all directions
(as at d of the same figure).
Great varieties present
themselves in the stellate
form, according to the re-
lative predominance of the
body and of the rays : in
those represented in Fig.
299, the rays, though very
numerous, are extremely short ; in other instances the rays
are much longer, and scarcely any central nucleus can be said
to exist. The varieties in the form of Sponge-spicules are, in
fact, almost endless ; and a single sponge often presents two or
more (as shown in Fig. 299), the stellate spicules usually occur-
ring either in the interspaces between the elongated kinds, or in
the external crust.* There are many Sponges in which no fibrous
network can be discerned, the spicules lying imbedded in the midst
of the sarcode-mass ; such is the case in Grantia (Fig. 297, a), whose
triradiate spicules are composed of Carbonate of Lime. In one
curious Sponge, described by Mr. Bowerbank (the Dusideia fra-
gilis), the spicules are for the most part replaced by particles of
sand, of very uniform size, which are found imbedded in the horny
fibre. — The spicules of Sponges cannot be considered, like theraphicles
of Plants (§ 328), simply as deposits of Mineral matter in a
crystalline state ; for the forms of many of them are such as no
mere crystallization can produce ; they generally (at least, in the
earlier stage of their formation) possess internal cavities, which
contain organic matter; and the calcareous spicules, whose
mineral matter can be readily dissolved away by an acid, ^are
found to have a distinct animal basis. Hence it seems probable
that each spicule was originally a segment of sarcode, which has
undergone either calcification or silicification, and by the self-
shaping power of which the form of the spicule is mainly
determined.
* A minute account of the various forms of spicules contained in Sponges is
given by Mr. Bowerbank in his First Memoir ' On the Anatomy and Physi-
ology of the Spongiadas,' in " Philos. Transact.,'' 1858, pp. 279-332 ; and in
his " Monograph of the British Spongiadge" published by the Eay Society. —
The Calcareous Sponges have been lately made by Prof. Haeckel the subject of
an elaborate Monograph, " Die Kalkschwamme," Berlin, 1872.
572 POEIFEEA, OE SPONGES.
468. Of the Keproductive process in Sponges, much has yet to
be learned :— the following is perhaps the most probable account of
it. Multiplication by Gemmation is effected by the detachment of
minute globular particles of sarcode from the interior of the
canals, where they sprout-forth as little protuberances, whose foot-
stalks gradually become narrower and narrower until they give
way altogether; these gernmules, like the zoospores of Algae,
possess cilia, and issuing-forth from the vents, transport them-
selves to distant localities, where they may lay the foundation of
new fabrics. — But according to the observations of Mr. Huxley on
the marine genus Tethya* a time sexual Generation also takes-
place ; both ova and sperm-cells being found imbedded in the sub-
stance of the sponge. The bodies distinguished as capsules, which
are larger than the gernmules, and which usually have their invest-
ment strengthened with siliceous spicules very regularly disposed,
are probably the products of this operation. They contain nume-
rous globular particles of sarcode, every one of which, when set
free by the rupture of its envelope, becomes an independent amcebi-
form body, and may develope itself into a complete sponge. The
phenomena of Sexual generation and development have since
been more particularly studied in the Spongilla or Fresh-water
sponge, especially by Carter f and Lieberkiihn ; J and in the Calca-
reous sponges by Haeckel (op. cit.), whose researches have thrown
great light on the embryology, not only of Sponges, but of the
whole Animal Kingdom. By the repeated ' segmentation' of the
ovum, as in other instances (§ 540), a mulberry mass, or "morula
is first produced ; and this next becomes converted, by the forma-
tion of a gastric cavity opening externally by a mouth, into a
gastrula or primitive stomach. || The wall of this stomach is formed
* ' On the Anatomy of the genus Tethya,' in " Ann. of Nat. Hist.," 2nd Ser.,
Vol. vii. (1851), p. 370.
f See his Memoirs ' On Zoosperms in Spongilla,' in " Ann. of Nat. Hist.,"
2nd Ser., Vol. xiv. (1854), p. 334, and ' On the Ultimate Structure of Spongilla,'
in " Ann. of Nat. Hist,," 2nd Ser., Vol. xx. (1857), p. 21.
X See the Memoirs of Lieberkiihn, ' On the Development of the Spongillce^ in
"Miiller's Archiv " for 1856, and his 'New Kesearches on the Anatomy of
Sponges,' in "Eeichert's und Du Bois Eeymond's Archiv" for 1859. Abstracts
of the former are contained in the " Ann. of Nat. Hist.," 2nd Ser., Vol. xvii.
(18*3), p. 403, and in the "Quart. Journ. of Microsc. Science," Vol. v. (1857),
p. 212. See also the Monograph of Oscar Schmidt on the Sponges of the
Adriatic, and the Article 'Spongiadse,' in the Supplemental Volume of the
" English Cyclopaedia."
|| The mode in which the morula comes to be converted into the gastrula,
does not appear to be always the same ; the gastric cavity being sometimes
f ormed by an inflexion or invagination of the surface-layer, and sometimes by
the hollo wing-out of the interior of the morula, and the breaking down of the
wall of the cavity so as to form a mouth. — See Prof. Haeckel's Memoir on
' The Gastraea Theory,' translated by Dr. Perceval Wright, in " Quart. Journ.
of Microsc. Science," April and July, 1874 ; also Kay Lankester, ' On the Pri-
mitive Cell-layers of the Embryo, as the basis of the Genealogical Classifica-
tion of Animals,' in " Ann. Nat. Hist.," June, 1873, and ' On the Development
of Limnams stagnalis, and on the early stages of other Mollusca,' in " Quart.
Journ. of Microsc. Sci .," Oct., 1874.
PREPARATION OF THEIR SPICULES. 573
of two cellular lamellas, the ectoderm or outer, and the endoderm
or inner ; the former consists of large nearly-globular cells, differing
little from those of the morula ; whilst the cells of the latter are
small and nearly cylindrical, each carrying a long cilium. The
subsequent development of this gastrula into a Sponge mainly
consists (1) in the extension and ramification of the gastric cavity,
and (2) in the production of the skeleton and of other intermediate
tissue between the two original lamellae, which continue to retain
their distinctive characters. — This gastrula seems to represent the
primitive embryonic type of all animals from Sponges to Vertebrata ;
the ' ectoderm' always remaining as the tegumentary layer, and
the ' endoderm' as the lining of the digestive cavity and its glandular
extensions, whilst intermediate lamella?, developed from one or
other of these, give origin to all the other organs.
469. With the exception of those that belong to the genus Spon-
gttla, all known Sponges are marine ; but they differ very much in
habit of growth. For whilst some can only be obtained by dredging
at considerable depths, others live near the surface, whilst others
attach themselves to the surfaces of rocks, shells, &c, between the
tide-marks. The various species of Grantia, in which, of all
the marine Sponges, the ciliary movement can most readily be
observed, belong to this last category. They have a peculiarly
simple structure, each being a sort of bag whose wall is so thin
that no system of canals is required ; the water absorbed by the
outer surface passing directly towards the inner, and being expelled
by the mouth of the bag. The cilia may be plainly distinguished
with a l-8th inch objective, on some of the cells of the gelatinous
substance scraped from the interior of the bag ; or they may be
seen in situ, by making very thin transverse sections of the
substance of the sponge. It is by such sections alone that the
internal structure of Sponges, and the relation of their spicular
and horny skeletons to their fleshy substance, can be demonstrated.
— In order to obtain the spicules in an isolated condition, however,
the animal matter must be got-rid-of, either by incineration, or by
chemical reagents. The latter method is preferable, as it is diffi-
cult to free the mineral residue from carbonaceous particles by heat
alone. If (as is commonly the case) the spicules are siliceous, the
Sponge may be treated with strong nitric or nitro-muriatic acid,
until its animal substance is dissolved away ; if, on the other hand,
they be calcareous, a strong solution of potass must be employed
instead of the acid. The operation is more rapidly accomplished
by the aid of heat ; but if the saving of time be not of importance,
it is preferable on several accounts to dispense with it. The
spicules, when obtained in a separate state, should be mounted in
Canada-balsam. — Sponge-tissue may often be distinctly recognised
in sections of Agate, Chalcedony, and other siliceous accretions, as
will hereafter be stated in more detail (Chap. XIX.).
CHAPTER XI.
ZOOPHYTES.
470. Under the general designation Zoo-pliytes it will be still con-
venient to group those animals which form composite skeletons
or ' polyparies' of a more or less Plant-like character ; associating
with them the Acalephs, which are now known to be the ' sexual
zooids' of Polypes (§ 480) ; but excluding the Poly zoa (Chap. XIII) on
account of their truly Molluscan structure, notwithstanding their
Zoophytic forms and habits of life. The animals belonging to this
group may be considered as formed upon the primitive gastrula
type (§ 468) ; their gastric cavity (though sometimes extending
itself almost indefinitely) being lined by the original endoderm, and
their surface being covered by the original ectoderm ; and these
two lamellae not being separated by the interposition of any body-
cavity or ccelom.* This great division includes the two principal
groups, the Hydrozoa and the Actinozoa ; the former comprehend-
ing the Polypes, and the latter the Anemonies. In the Hydrozoa
there is no separation between the digestive cavity and the external
body -wall ; and the reproductive organs are external. In the Acti-
nozoa the wall of the digestive sac is separated from the external
body -wall by an intervening space, which communicates with it,
and must be regarded as an extension of it ; and this is subdivided
into chambers by a series of vertical partitions, to which the re-
productive organs are attached. — As most of the Hydrozoa or
Hydroid Polypes are essentially Microscopic animals, they need to
be described with some minuteness ; whilst in regard to the Actinozoa
these points only can be dwelt-on which are of special interest
to the Microscopist.
471. Hydrozoa. — The type of this group is the Hydra or Fresh-
water polype, a very common inhabitant of pools and ditches, where
* Agreeing with those eminent Naturalists who regard the chambers sur-
rounding the stomach in Actinozoa as extensions of the gastric cavity, and not
as in any sense representing the perigastric cavity of higher animals, the
Author has never been able to accept the term Cozlenterata as applicable to this
group in the sense intended by Prof. Leuckart, its proposer ; and he entirely
accords in the idea of the Morphology of Zoophytes expressed by Prof. Haeckel,
in his inrporta t Memoir ' On the Gastrsea-Theory,' already referred to.
HYDKOZQA:— STEUCTUEE OF HYDE A.
575
Fig. 300.
it is most commonly to be found attached to the leaves or stems of
aquatic plants, floating pieces of stick, &c. Two species are common
in this country, the H. viridis or green Polype, and the H. vulgaris,
which is usually orange-brown, but sometimes yellowish or red (its
colour being liable to some variation according to the nature of the
food on which it has been subsisting) ; a third less common species,
the H.fusca, is distinguished from both the j^receding by the length
of its tentacles, which in the former are scarcely as long as the
body, whilst in the latter
they are, when fully ex-
tended, many times longer
(Fig. 300). The body of the
Hydra consists of a simple
bag or sac, which may be
regarded as a stomach, and
is capable of varying its shape
and dimensions in a very re-
markable degree ; sometimes
extending itself in a straight
line so as to form a long
narrow cylinder, at other
timesbeing seen(when empty)
as a minute contracted globe,
whilst, if distended with food,
it may present the form of
an inverted flask or bottle,
or even of a button. At the
upper end of this sac is a
central opening, the mouth;
and this is surrounded by a
circle of tentacles or ' arms,'
usually from six to ten in
number, which are arranged
with great regularity around
the orifice. The body is pro-
longed at its lower end into
a narrow base, which is fur-
nished with a suctorial disk ;
and the Hydra usually at-
taches itself by this, while it
allows its tendril-like tenta-
cles to float freely in the
water. The wall of the body
is composed of cells im-
bedded in sarcode-substance ;
and between its two laj-ers
there is a space chiefly occupied by undifferentiated sarcode,
having many ' vacuoles' or ' lacunae" (which often seem to commu-
Me
Hydra /wen, with a young bud at b, and a
more advanced bud at c.
576 HYDEOID ZOOPHYTES.
nicate witli one another) excavated in its snbstance. The arms
are made-np of the same materials as the body ; bnt their surface
is beset with little wart-like prominences, which, when carefully
examined, are found to be composed of clusters of ' thread-cells,'
having a single large cell with a long spiculum in the centre of
each. The structure of these thread-cells or ' urticating organs'
will be described hereafter (§ 486) ; at present it will be enough to
point-out that this apparatus, repeated many times on each tentacle,
is doubtless intended to give to the organ a great prehensile power ;
the minute filaments forming a rough surface adapted to prevent
the object from readily slipping out of the grasp of the arm, whilst
the central spicule or ' dart' is projected into its substance, probably
conveying into it a poisonous fluid secreted by a vesicle at its base.
The latter inference is founded upon the oft-repeated observation,
that if the living prey seized by the tentacles have a body destitute
of hard integument, as is the case with the minute aquatic Worms
which constitute a large part of its aliment, this speedily dies,
even if, instead of being swallowed, it escapes from their grasp ;
whilst, on the other hand, minute Entomostraca, Insects, and other
animals with hard envelopes, may escape without injury, even
after having been detained for some time in the polype's embrace.
The contractility of the tentacles (the interior of which is traversed
by a canal that communicates with the cavity of the stomach)
is very remarkable, especially in the Hydra fusca; whose arms
when extended in search of prey, are not less than seven or eight
inches in length ; whilst they are sometimes so contracted, when the
stomach is filled with food, as to appear only like little tuber-
cles around its entrance. By means of these instruments the Hydra
is enabled to derive its substance from animals whose activity, as
compared with its own slight powers of locomotion, might have been
supposed to remove them altogether from its reach ; for when, in
its movements through the water, a minute Worm or a Water-flea
happens to touch one of the tentacles of the Polype, spread-out
as these are in readiness for prey, it is immediately seized by this,
other arms are soon coiled around it, and the unfortunate victim is
speedily conveyed to the stomach, within which it may frequently
be seen to continue moving for some little time. Soon, however,
its struggles cease, and its outline is obscured by a turbid film,
which gradually thickens, so that at last its form is wholly lost.
The soft parts are soon completely dissolved, and the harder indi-
gestible portions are rejected through the mouth. A second orifice
has been observed at the lower extremity of the stomach ; but this
would not seem to be properly regarded as anal, since it is not used
for the discharge of such exuviae ; it is probably rather to be con-
sidered as representing, in the Hydra, the entrance to that ra-
mifying cavity, which, in the Compound Hydrozoa, brings into
connection the lower extremities of the stomachs of all the indi-
vidual polypes (Plate XX). A striking proof of the simplicity
of the structure of the Hydra, is the fact that it may be turned
GEMMATION AND GENERATION OF HYDE A.
577
inside out like a glove ; that which was before its external
tegument becoming the lining of its stomach, and vice versa.
472. The ordinary mode of reprodnction in this animal is by a
' gemmation' resembling that of Plants. Little bud- like processes
(Eig. 300, b, c) developed from its external surface gradually come
to resemble the parent in character, and to possess a digestive sac,
mouth, and tentacles ; for a
long time, however, their ca-
vity is connected with that of
the parent, but at last the
communication is cut-off by
the closure of the canal of the
foot-stalk, and the young po-
lype quits its attachment and
goes in quest of its own
maintenance. A second ge-
neration of buds is sometimes
observed on the young po-
lype before quitting its pa-
rent ; and as many as nine-
teen young Hyclrce in different
stages of development have
been thus connected with a
single original stock (Fig. 301) . ,
This process takes place ;
most rapidly under the in- ^_.
fluence of warmth and abun- Tfc^
dant food ; it is usually sus-
pended in winter, but may be
made to continue by keeping
the polypes in a warm situa-
tion and well supplied with
food. Another very curious
endowment seems to depend on
the same condition, — the extra-
ordinary power which one por-
tion possesses of reproducing
the rest. Into whatever number of parts a Hydra may be divided, each
may retain its vitality, and give origin to a new and entire fabric ;
so that thirty or forty individuals may be formed by the section
of one. The Hydra also propagates itself, however, by a truly
Sexual process ; the fecundating apparatus, or vesicle producing
* sperm-cells,' and the ovum (containing the ' germ-cell,' imbedded
in a store of nutriment adapted for its early development) being
both evolved in the substance of the walls ' of the stomach,— the
male apparatus forming a conical projection just beneath the arms,
while the female ovary, or portion of the body-substance in which
the ovum is generated, has the form of a knob protruding from
the middle of its length. It would appear that sometimes one
p p
Hydra fusca in gemmation ; «, mouth ; 6,
base ; c, origin of one of the buds.
578 HYDKOID ZOOPHYTES.
individual Hydra developes only the male cysts or sperm-
cells, while another developes only the female cysts or ovisacs ;
but the general rule seems to be that the same individual
forms both organs. The fertilization of the ova, however, cannot
take-place until after the rupture of the spermatic cyst and of
the ovisac, by which the contents of both are set entirely free from
the body of the parent. — The autumn is the chief time for the
development of the sexual organs ; but they also present them-
selves in the earlier part of the year, chiefly between April and
July. According to Ecker, the eggs of H. viriclis produced early
in the season, run their course in the summer of the same year ;
while those produced in the autumn, pass the winter without
change. When the ovum is nearly ripe for fecundation, the ovary
bursts its ectodermal covering, and remains attached by a kind of
pedicle. It seems to be at this stage that the act of fecundation
occurs ; a very strong elastic shell or capsule then forms round
the ovum, the surface of which is in some cases studded with spine-
like points, in others tuberculated, the divisions between the tubercles
being polygonal. The ovum finally drops from its pedicle, and
attaches itself by means of a mucous secretion, till the hatching of
the young Hydra, which comes forth provided with four rudimen-
tary tentacles like buds. — The Hydra possesses the power of free
locomotion, being able to remove from the spot to which it has
attached itself, to any other that may be more suitable to its wants ;
its changes of place, however, seem rather to be performed under
the influence of light, towards which the Hydra seeks to move itself,
than with reference to the search after food.*
473. The Compound Hydroids may be likened to a Hydra whose
gemmae, instead of becoming detached, remain permanently con-
nected with, the parent ; and as these in their turn may develope
gemm.33 from their own bodies, a structure of more or less
arborescent' character, termed a polypary, may be produced. The
form which this will present, and the relation of the component
polypes to each other, will depend upon the mode in which the
gemmation takes-place ; in all instances, however, the entire cluster
is produced by continuous growth from a single individual ; and the
stomachs of the several polypes are united by tubes, which proceed
from the base of each, along the stalk and branches, to communicate
with the cavity of the central stem. Whatever may be the form
taken by the stem and branches constituting the polypary of a
Hydroid colony, they will be found to be, or to contain, fleshy tubes
having two distinct layers ; the inner (encloderm) having nu-
tritive functions ; the outer (ectoderm) usually secreting a hard
outer layer, and thus giving rise to fabrics of various forms.
Between these a muscular coat is sometimes noticed. The fleshy
* A very full account of the structure and development of Hydra has
recently been published by Kleinenbeig ; of whose admirable Monograph a
summary is given by Prof. Allman, with valuable remarks of his own, in
" Quart. Journ. of Microsc. Science,'7 N.S., Vol. xiv., p. 1.
COMPOUND HYDEOZOA. 579
tube, whether single or componnd, is called a ccenosarc ; and through
it the nutrient matter circulates. The ' zooids,' or individual mem-
bers of the colony, are of two kinds : one the poZypite, or alimentary
zopid, resembling the Hydra in essential structure, and more or
less in aspect ; the other, the gonozooid or sexual zooid, developed
at certain seasons only, in buds of particular shape.
474. The simplest division of the Hydroida is that adopted by
Hincks,* who groups them under the sub-order Atliecata and
Thecata, the latter being again divided into the Thecaphora and the
Gymnochroa. In the first, neither the ' polypites' nor the sexual
zooids bear true protective cases ; in the second, the polypites are
lodged in cells, or, as Mr. Hincks prefers to call them, cabjcles,
many of which resemble exquisitely formed crystal cups, variously
ornamented, and sometimes furnished with lids or opercula ; in the
third, which contains the Hydras, there is no polypary, and the
reproductive zooids (gonozooids) are always fixed and developed in
the body-walls. According to Mr. Hincks, the two sexes are some-
time borne on the same colony, but more commonly the zoophyte
is dioecious. The cases, however, are much less rare than has
been supposed, in which both male and female are mingled on the
same shoots. The sexual zooids either remain attached, and dis-
charge their contents at maturity, or become free and enter upon
an independent existence. The free forms nearly always take the
shape ofMedusai (jelly-fish), swimming by rhythmical contractions
of their bell or umbrella. The digestive cavity is in the handle
(manubrium) of the bell ; and the generative elements (sperm-cells
or ova) are developed either between the membranes of the manu-
brium, or in special sacs in the canals radiating from it. The ova,
when fertilized by the spermatozoa, undergo ' segmentation'
according to the ordinary type (§ 540), the whole yolk-mass sub-
dividing successively into 2, 4, 8, 16, 32 or more parts, until a
' mulberry mass' is formed ; this then begins to elongate itself, the
surface becoming smooth, and showing a transparent margin ; and
this surf ace becomes covered with cilia, by whose agency these little
bodies, closely resembling ciliated Infusoria, first move-about
within the capsule, and then swim forth freely when liberated by
the opening of its mouth. At this period the embryo can be made
out to consist of an outer and an inner layer of cells, with a hollow
interior ; after some little time the cilia disappear, and one ex-
tremity becomes expanded into a kind of disk by which it attaches
itself to some fixed object ; a mouth is formed, and tentacles sprout
forth around it ; and the body increases in length and thickness, so
as gradually to acquire the likeness of one of the parent polypes,
after which the ' polypary' characteristic of the genus is gradually
evolved by the successive development of polype-buds from the first-
formed polype and its subsequent offsets. — The Medusas of these
polypes (Fig. 304) belong to the division called ' naked-eyed,' on
account of the (supposed) eye- spots usually seen surrounding the
margin of the bell at the base of the tentacles.
* " History of British Hydroid Zoophytes," 1868.
P p 2
580
HYDEOID ZOOPHYTES.
475. A characteristic example of this production of Medusa-like
' gonozooids' is presented by the form termed Syncoryne Sarsii
(Fig. 302) belonging to the sub-order Athecata. At a is shown the
alimentary zooid, or polypite,
with its tentacles, and at B
the successive stages a, b, c,
of the sexual zooids, or me-
dusa-buds. When sufficiently
developed, the medusa swims
away, and as it grows to
maturity enlarges its manu-
brium, so that it hangs below
the bell. The medusaB of the
genus Syncoryne (as now re-
stricted) have the form named
Sarsia in honour of the Swe-
dish naturalist Sars. Their
normal character is that of
free swimmers ; but Agassiz
ascertained that in some cases,
towards the end of the breed-
ing season, the sexual zooids
remain fixed, and mature their
products while attached to the
zoophyte.* This condition of
the sexual zooids is very com-
mon amongst the Hydroida ;
and various intermediate
stages may be traced in dif-
ferent genera, between the
mode in which the gonozooids
are produced in the common
Hydra, as already described,
and that of Syncoryne. In
Tubularia the gonozooids,
Development of Medusa-buds in Syn-
coryne Sarsii: — A, an ordinary polype,
with its club-shaped body covered with though permanently attached,
are furnished with swimming
bells, having four tubercles
representing marginal tenta-
cles. A common and interest-
ing species Tubularia indivisa
receives its specific name from
the infrequency with which branches are given-off from the stems,
these for the most part standing erect and parallel, like the stalks
of corn, upon the base to which they are attached. This beautiful
Zoophyte, which sometimes grows between the tide-marks, but is
more abundantly obtained by dredging in deep water, often attains
tentacles : — B, a polype putting forth
Medusan gemmse ; a, a very young bud ;
6, a bud more advanced, the quadrangular
form of which, with the four nuclei
whence the cirrhi afterwards spring, is
shown at d; c, a bud still more advanced.
* Hincks, op, cit., p. 49.
PLATE XX.
?^c
CaMPANTJLARIA GELATIiTOSA.
[To face p. 531.
TUBULAEIDiE ; CAMPANULAEIDJS ; SEETULAEIM. 5S1
a size which renders it scarcely a microscopic object ; its stems
being sometimes no less than a foot in height and a line in diameter.
Several cnrions phenomena, however, are brought into view by
Microscopic examination. The Polype-stomach is connected with
the cavity of the stem by a circular opening, which is surrounded
by a sphincter ; and an alternate movement of dilatation and
contraction takes-place in it, fluid being apparently forced-up from
below, and then expelled again, after which the sphincter closes in
preparation for a recurrence of the operation ; this, as observed by
Mr. Lister, being repeated at intervals of eighty seconds. Besides
the foregoing movement, a regular flow of fluid, carrying with it
solid particles of various sizes, may be observed along the whole
length of the stem, passing in a somewhat spiral direction. — It is
worthy of mention here, that when a Tubularia is kept in con-
finement, the polype-heads almost always drop-off after a few days,
but are soon renewed again by a new growth from the stem
beneath; and this exuviation and regeneration may take place
niany times in the same individual.
476. It is in the Families Camjmnidaridce and Sertidaridce
(whose polyparies are commonly known as 'corallines'), that the
horny branching fabric attains its completest development ; not only
affording an investment to the stem, but forming cups or cells for
the protection of the polypites, as well as capsules for the repro-
ductive gonozooids. Both these families thus belong to the Sub-
order Thecata. . In the Carupanidaridce the polype-cells are cam-
panulate or bell-shaped, and are borne at the extremities of
ringed-stalks (Plate XX., c) ; in the Sertidaridce, on the other
hand, the polype-cells lie along the stem and branches, attached
either to one side only, or to both sides (Fig. 303). In both, the
general structure of the individual polypes (Plate XX., d) closely
corresponds with that of the Hydra ; and the mode in which they
obtain their food is essentially the same. Of the products of
digestion, however, a portion finds its way down into the tubular
stem, for the nourishment of the general fabric ; and very much
the same kind of circulatory movement can be seen in Campanu-
laria as in Tubularia, the circulation being most vigorous in the
neighbourhood of growing parts. It is from the 'ccenosarc' (/) con-
tained in the stem and branches, that new polype-buds (b) are
evolved ; these carry before them (so to speak) a portion of the
horny integument, which at first completely invests the bud ; but
as the latter acquires the organization of a polype, the case thins-
away at its most prominent part, and an opening is formed
through which the young polype protrudes itself.
477. The origin of the reproductive capsules ' gonothecse ' (e) is
exactly similar ; but their destination is very different. Within
them are evolved, by a budding process, the generative organs of
the Zoophyte : and these in the C amp anular idee may either
develope themselves into the form of independent Medusoids,
which completely detach themselves from the stock that bore
582
HYDEOID ZOOPHYTES.
them, make their way out of the capsule, and swim-forth freely,
to mature their sexual products (some developing sperm -cells, and
others ova), and give origin to a new generation of polypes ; or, in
cases in which the medusan structure is less distinctly pronounced,
may not completely detach themselves, but (like the flower-buds of
a Plant) expand one after another at the mouth of the capsule,
withering and dropping-off after they have matured their genera-
tive products. In the
Yig. 303. Sertularidce, on the
other hand, the Medu-
san conformation is
wanting as the gono-
zooids are always fixed ;
the reproductive cells
(Fig. 303), which were
shown by Prof. Edward
Forbes to be really me-
tamorphosed branches,
developing in their in-
terior certain bodies
which were formerly
supposed to be ova, but
which are now known
to be 'medusoids' re-
duced to their most ru-
dimentary condition.
Within these are de-
veloped,— in separate
gonothecae, sometimes
perhaps on distinct po-
ly paries, — spermatozoa
and ova ; and the latter
are fertilized by the en-
trance of the former
whilst still contained
within their capsules.
b, portion magnified. The fertilized ova, whe-
ther produced in free or
in attached medusoids, develope themselves in the first instance into
ciliated ' gemmules,' which soon evolve themselves into true polypes,
from every one of which a new composite polypary may spring.
478. There are few parts of our coasts which will not supply
some or other of the beautiful and interesting forms of Zoophytic
life which have been thus briefly noticed, without any more
trouble in searching for them than that of examining the surfaces
of rocks, stones, sea-weeds, and dead shells between the tide-
marks. Many of them habitually live in that situation ; and
others are frequently cast-up by the waves from the deeper waters,
especially after a storm. Many kinds, however, can only be
Sertularia
natural size
PREPARING AND MOUNTING ZOOPHYTES. 5S3
obtained by means of the dredge. For observing them during
their living state, no means is so convenient as the Zoophyte-
trough (§ 110), devised for that express purpose by Mr. Lister, to
whom we owe not only many improvements in the Microscope and
its appurtenances, but also some of the earliest and best observa-
tions upon this class of Zoophytes which the application of the
achromatic principle permitted.* In mounting Compound Hydro-
zoa, as well as Polyzoa, it will be found of great advantage to place
the specimens alive in the cells they are permanently to occupy,
and to then add Alcohol drop by drop to the sea-water ; this has the
effect of causing the protrusion of the animals, and of rendering
their tentacles rigid. The alcoholized liquid may be withdrawn,
and replaced by Goadby's solution, Deane's Gelatine, Glycerine
jelly, weak Spirit, diluted Glycerine, a mixture of Spirit and Gly-
cerine with sea-water, or any other menstruum, by means of the
Syringe ; and it is well to mount specimens in several different
menstrua, marking the nature and strength of each, as some forms
are better preserved by one and some by another. f The size of the
cell must of course be proportioned to that of the object ; and if it
be desired to mount such a specimen as may serve for a characteristic
illustration of the mode of growth of the species it represents, the
large shallow cells, whose walls are made by cementing four strips
of glass to the plate that forms the bottom (§ 188), will generally
be found preferable.
479. The horny polyparies of the SerttdaridoB, when mounted
in Canada balsam, are beautiful objects for the Polariscope ; but
in order to prepare them successfully, some nicety of management
is required. The following are the outlines of the method recom-
mended by Dr. Golding Bird, who very successfully practised it.
The specimens selected, which should not exceed two inches in
length, are first to be submitted, while immersed in water of 120°,
to the vacuum of an air-pump. The ebullition which will take-
place within the cavities, will have the effect of freeing the poly-
paries from dead polypes and other animal matter ; and this
cleansing process should be repeated several times. The specimens
are then to be dried, by first draining them for a few seconds on
bibulous paper, and then by submitting them to the vacuum of an
air-pump, within a thick earthenware ointment-pot fitted with a
cover, which has been previously heated to about 200° ; by this
means the specimens are very quickly and completely dried, the
water being evaporated so quickly that the cells and tubes hardly
collapse or wrinkle. The specimens are then placed in camphine,
and again subjected to the exhausting process, for the displace-
ment of the air by that liquid ; and when they have been thoroughly
saturated, they should be mounted in Canada balsam in the usual
* See his Memoir in the " Philosophical Transactions " for 1834.
+ See Mr. J. "W. Morris in "Quart. Journ. of Microsc. Science," N.S., Vol.
ii. (1862), p. 116.
584
HYDROID ZOOPHYTES.
Fig. 304.
mode. When thus prepared, they become very beautiful trans-
parent objects for low magnifying powers ; and they present a
gorgeous display of colours when examined by Polarized light,
with the interposition of a plate of Selenite. These objects are
peculiarly fitted for the use of the Polarizing apparatus in com-
bination with the Spot-lens (§ 98) ; as they then exhibit all the
richness of coloration which the former developes, with the pecu-
liar solidity or appearance of projection which they derive from
the use of the latter.
480. ISTo result of Microscopic research was more unexpected,
than the discovery of the close relationship subsisting between
the Hydroid Zoophytes and the Medusoid Acalephce (or 'jelly-
fish'). We now know that the
small free-swimming Medusoids be-
longing to the ' naked-eyed' group, of
which Thaumantias (Fig. 304) may
be taken as a representative, are
really to be considered as the de-
tached sexual apparatus of the
Zoophytes from which they have
been budded-off, endowed with in-
dependent organs of nutrition and
locomotion, whereby they become
capable of maintaining their own
existence and of developing their
sexual products. The general con-
formation of these organs will be
understood from the accompanying
figure. Many of this group are very
beautiful objects for Microscopic
examination, being small enough to
be viewed entire in the Zoophyte-trough. There are few parts
of the coast on which they may not be found, especially on a calm
warm day, by skimming the surface of the sea with the Tow-net
(§ 195) ; and they are capable of being well preserved in Goadby's
solution.
481. When we turn from these small and simple forms to the
large and highly-developed Medusce or Acalepil-e (' sea-nettles,'
so-named on account of their stinging powers), which are commonly
known as ' jelly-fish,' we find that their history is essentially similar ;
for their progeny have been ascertained to develope themselves in
the first instance under the Polype-form, and to lead a life which
in all essential respects is zoophytic ; their development into Medusas
taking-place only in the closing phase of their existence, and then
rather by gemmation from the original Polype, than by a metamor-
phosis of its own fabric. The huge Rhizostoma found commonly
swimming round our coasts, and the beautiful Chrysaora remarkable
for its long ' furbelows ' which act as organs of prehension, are Oceanic
Acalephs developed from very small polypites, which fix themselves
Thaumantias pilosella, one of the
'naked-eyed' Medusas : — a «, oral
tentacles ; 6, stomach ; c, gastro-
vascular canals, having the ovaries,
d d, on either side, and terminating
in the marginal canal, e e.
ZOOPHYTIC ORIGIN OF MEDUSA.
5S5
Fig. 305.
by a basal cup or disk. The embryo emerges from the cavity of its
parent, within which the first stages of its development have taken
place, in the condition of a ciliated ' gemmule,' of rather oblong
form, very closely resembling an Infnsory Animalcule, but desti-
tute of a mouth. One end soon contracts and attaches itself, how-
ever, so as to form a foot ; the other enlarges and opens to form a
mouth, four tubercles sprouting around it, which grow into
tentacles ; whilst the central cells melt-down to form the cavity of
the stomach. Thus a Hydra-like polype is formed, which soon
acquires many additional
tentacles ; and this, accord-
ing to the observations of
Sir J. G. Dalyell, on the
Hydra tuba, which is the
polype-stage of the Chrysa-
ora, leads in every impor-
tant particular the life of
a Hydra; propagates like
it by repeated gemmation,
so that whole colonies are
formed as offsets from a
single stock ; and can be
multiplied like it by arti-
ficial division, each segment
developing itself into a
perfect Hydra. There
seems to be no definite
limit to its continuance in
this state, or to its power
of giving origin to new
polype-buds ; but when the
time comes for the deve-
lopment of its sexual or-
gans, the polype, from its
original condition of a
minute bell with slender
tentacles (Fig. 305, c, a),
assumes a cylindrical form,
and elongates itself consi-
derably; a constriction or
indentation is then seen
around it, just below the
ring which encircles the
mouth and gives origin to
the tentacles ; and similar
constrictions are soon re-
peated round the lower
parts of the cylinder, so as to give to the whole body somewhat the
appearance of a rouleau of coins ; a sort of fleshy bulb, somewhat of
«N
Successive stages of development of Chry-
saora: — a, elongated and constricted Polype-
body; b, its original circle of tentacles ; c, its
secondary circle of tentacles ; d, proboscis of
most advanced Medusa-disk; e, polype-bud
from side of polype-body.
586 HYDEOID ZOOPHYTES.
the form of the original polype, being still left at the attached ex-
tremity (Fig. 305, a). The number of circles is indefinite, and all are
not formed at once, new constrictions appearing below, after the
npper portions have been detached ; as many as 30 or even 40 have
thus been produced in one specimen. The constrictions then gra-
dually deepen, so as to divide the cylinder into a pile of saucer-like
bodies ; the division being most complete above, and the upper disks
usually presenting some increase in diameter : and whilst this is
taking place, the edges of the disks become divided into lobes (Breach
lobe soon presenting the cleft with the supposed rudimentary eye
at the bottom of it, which is to be plainly geen in the detached
Medusas (Fig. 306, c). Up to this period, the tentacles of the
original polype surmount the highest of the disks ; but before the
detachment of the topmost disk, this circle disappears, and a new
one is developed at the summit of the bulb which remains at the
base of the pile (c, c). At last the topmost and largest disk begins
to exhibit a sort of convulsive struggle ; it becomes detached, and
swims freely away ; and the same series of changes takes-place
from above downwards, until the whole pile of disks is detached
and converted into free-swimming Medusas. But the original poly-
poid body still remains, and may return to its polype-like and
orginal mode of gemmation (d, e) ; becoming the progenitor of a
new colony, every member of which may in its turn bud-off a pile
of Medusa-disks.
482. The bodies thus detached have all the essential characters
of the adult Medusce. Each consists of an umbrella-like disk,
divided at its edge into a variable number of lobes, usually eight ;
and of a stomach, which occupies a considerable proportion of the
disk, and projects downwards in the form of a proboscis, in the
centre of which is the quadrangular mouth (Fig. 306, a, b). As
the animal advances towards maturity, the intervals between the
segments of the border of the disk gradually fill-up, so that the
divisions are obliterated; tubular prolongations of the stomach
extend themselves over the disk ; and from its borders there
sprout forth tendril-like filaments, which hang down like a fringe
around its margin. From the four angles of the mouth, which,
even in the youngest detached animal, admits of being greatly
extended and protruded, prolongations are put forth, which form
the four large tentacles of the adult. The young Medusas are very
voracious, and grow rapidly, so as to attain a very large size. The
Cyanceoe and Chrysaorm, which are common all round our coasts,
often have a diameter of from 6 to 15 inches ; while the Bhizostoma
sometimes reaches a diameter of from two to three feet. The
quantity of solid matter, however, which their fabrics contain is
extremely small. It is not until adult age has been attained, that
the generative organs make their appearance, in four chambers
disposed around the stomach, which are occupied by plaited mem-
branous ribands containing sperm-cells in the male and ova in the
female ; and the embryoes evolved from the latter, when they have
ZOOPHYTIC ORIGIN OF MEDUSiE.
587
been fertilized by the agency of the former, repeat the extraordi-
nary cycle of phenomena which has been now described, develop-
ing themselves in the first instance into Hydroid Polypes, from
which Medusoids are subsequently bndded-off.
Development of CJirysaora from Hydra tuba: — A, detached individual
viewed sideways, and enlarged, showing the proboscis a, and b the
bifid lobes ; B, individual seen from above, showing the bifid lobes of
the margin, and the quadrilateral mouth ; c, one of the bifid lobes still
more enlarged, showing the rudimentary eye (?) at the bottom of the
cleft ; D, group of young Medusa?, as seen swimming in the water, of
the natural size.
483. This cycle of phenomena is one of those to which the term
' alternation of generations' was applied by Steenstrup,* who
brought together under this designation a number of cases in
which generation a does not produce a form resembling itself,
but a different form, b ; whilst generation b gives origin to a form
which does not resemble itself, but returns to the form a, from
which b itself sprang. It was early pointed out, however, by
the Author,f that the term ' alternation of 'generations ' does
not appropriately represent the facts either of this case, or of
any of the other cases grouped under the same category; the
real fact being that the two organisms, a and b, only constitute
* See his Treatise on " The Alternation of Generations," published by the
Kay Society.
f " Brit, and For. Med.-Chir. Review," Vol. i. (1848), p. 192, et seq.
588 HYDKOZOA : — ACTINOZOA.
two stages in the life-history of one generation ; the production
of one form from the other being in only one instance by a
trnly generative or sexnal act, whilst in the other it is by a
process of gemmation or bndding. Thns the Medusae of both
orders (the ' naked-eyed' and the ' covered-eyed' of Forbes) are
detached flower-buds, so to speak, of the Hydroid Zoophytes which
bnd them off ; the Zoophytic phase of life being the most con-
spicnous in the Thecata (of which the Gampanularida and Sertu-
larida are characteristic examples), while their Medusa-buds are
of small size and simple conformation, and not unfrequently do
not detach themselves as independent organisms ; whilst the
Medusan phase of life is the most conspicuous in the ordinary
Acalephs, their Zoophytic stage being passed in such obscurity
as only to be detected by careful research. The Author's views
on this subject, which were at first strongly contested by Prof.
E. Forbes, and other eminent Zoologists, have now come to be
generally adopted.
484. Actixozoa. — The common Sea-Anemonies may be taken as
the typical members of this class ; constituting, with their allies,
the group Zoantharia, which have numerous tentacles disposed
in several rows. Next to them come the Alcyonaria, consisting
of those whose polypes, having only six or eight broad short ten-
tacles, present a star-like aspect when expanded ; as is the case
with various composite Sponge-like bodies, unpossessed of any hard
skeleton, which inhabit our own shores, and also with the Eed
Coral and the Tubiporous Corals of warmer seas, which have a
stony skeleton that is internal in the first case and external in the
second, as also with the Sea-pens, and the Gorgonias or Sea-fans.
A third order, Bugosa, consists of fossil Corals, whose stony poly-
paries are intermediate in character between those of the two pre-
ceding. And lastly, the Ctenopliora, free-swimming gelatinous
animals, many of which are beautiful objects for the Microscope,
are by most Zoologists ranked with the Actinozoa.
485. Of the Zoantharia, the common Actinia or ' sea anemone '
may be taken as the type ; the individual polypites of all the com-
posite fabrics included in the group being constructed upon the
same model. In by far the larger proportion of these Zoophytes,
the bases of the polypites, as well as the soft flesh that con-
nects-together the members of aggregate masses, are consolidated
by calcareous deposit into stony Corals ; and the surfaces of these
are beset with cells, usually of a nearly circular form, each having
numerous vertical plates or lamellae radiating from its centre
towards its circumference, which are formed by the consolidation
of the lower portions of the radiating partitions, that divide the
space intervening between the stomach and the general integu-
ment of the animal into separate chambers. This arrangement
is seen on a large scale in the ~Fnngia or ' mushroom-coral ' of
tropical seas, which is the stony base of a solitary Anemone-like
animal; on a far smaller scale, it is seen in the little Garyo-
THREAD-CELLS OF ACTINOZOA.
589
Fig. 307.
Vj
phijllia, a like solitary Anemone of onr own coasts, which is
scarcely distinguishable from an Actinia by any other character
than the presence of this disk, and also on the surface of many
of those stony corals known as ' madrepores ;' whilst in some of
these the individual polype-cells are so small, that the lamel-
lated arrangement can only
be made-out when they are
considerably magnified. Por-
tions of the surface of such
Corals, or sections taken
at a small depth, are very
beautiful objects for low
powers, the former being
viewed by reflected, and
the latter by transmitted
light. And thin sections
of various fossil Corals of
this group are very striking
objects for the lower powers
of the Oxy-hydrogen Micro-
scope.
486. The chief point of in-
terest to the Microscopist,
however, in the structure of
these animals, lies in the ex-
traordinary abundance and
high development of those
'nliferous capsules,' or 'thread-
cells,' the presence of which
on the tentacles of the Hydroid
polypes has been already no-
ticed (§ 470), and which are
also to be found, sometimes
sparingly, sometimes very
abundantly, in the tentacles
surrounding the mouth of the
Medusae, as well as on other
parts of their bodies. If a
tentacle of any of the
Sea-anemonies so abundant
on our coasts (the smaller
and more transparent kinds
being selected in preference)
be cut-off, and be subjected
to gentle pressure between the
two glasses of the Aquatic-
box or the Compressorium, *lllfer™s Capsules of Actinozoa:— A, b,
™ i+-+ i„n p r,ii i i vi L-orynactis Allmanni ; C, E, F, Caryophyllia
multitudes Of little dart-like SmitMi;Ji G Actinia crasskorrns^Actinm
organs will be seen to pro- Candida.
590 ACTINOZOA : — ALCYONAEIA.
ject themselves from its surface near its tip ; and if the pressure
be gradually augmented, many additional darts will every moment
come into view. Not only do these organs present different
forms in different species, but even in one and the same in-
dividual very strongly marked diversities are shown, of which a
few examples are given in Fig. 307. At a, b, c, d, is shown the
appearance of the ' filif erous capsules,' whilst as yet the thread lies
coiled-up in their interior ; whilst at e, f, g, h, are seen a few of
the most striking forms which they exhibit when the thread or dart
has started-forth. These thread-cells are found not merely in the
tentacles and other parts of the external integument of Actinozoa,
but also in the long filaments which he in coils within the
chambers that surround the stomach, in contact with the sexual
organs which are attached to the lamellae dividing the chambers. The
latter sometimes contain ' sperm-cells ' and sometimes ova, the two
sexes being here divided, not united in the same individual. — What
can be the office of the filif erous filaments thus contained in the
interior of the body, it is difficult to guess-at. They are often
found to protrude from rents in the external tegument, when any
violence has been used in detaching the animal from its base ; and
when there is no external rupture, they are often forced through
the wall of the stomach into its cavity, and may be seen hanging
out of the mouth. The largest of these capsules, in their unpro-
jected state, are about l-300th of an inch in length ; while the
thread or dart, in Corynadis Allmanni, when fully extended,
is not less than l-8th of an inch, or thirty -seven times the length
of its capsule.*
487. Of the Alcyonaria, a characteristic example is found in the
Alcyonium digitatum of our coasts, which is commonly known
under the name of ' dead-man's toes,' or by the more elegant name
of 'mermaids' fingers.' When a specimen of this is first torn
from the rock to which it has attached itself, it contracts into an
unshapely mass, whose surface presents nothing but a series of
slight depressions arranged with a certain regularity. But after
being immersed for a little time in a jar of sea-water, the mass
swells-out again, and from every one of these depressions an
eight-armed polype is protruded, " which resembles a flower of ex-
quisite beauty and perfect symmetry. In specimens recently taken,
each of the petal-like tentacula is seen with a hand-glass to be fur-
nished with a row of delicately- slender pinnce or filaments, fringing
each margin, and arching onwards ; and with a higher power, these
pinnae are seen to be roughened throughout their whole length,
with numerous prickly rings. After a day's captivity, however,
the petals shrink up into short, thick, unshapely masses, rudely
notched at their edges" (G-osse). When a mass of this sort is
cut-into, it is found to be channelled-out, somewhat like a Sponge,
* For the fullest description of these curious bodies, as well as for much
other valuable information upon Zoophytes, see ]VIr. Gosse's "Naturalist's
Rambles on the Devonshire Coast."
ALCYONIUM :— SPICULES OF GOKGONIA.
591
Fig. 308.
by ramifying canals ; the vents of which open into the stomachal
cavities of the polypes, which are thus brought into free communi-
cation with each other, — a character that especially distinguishes
this Order. A movement of fluid is kept-up within these canals (as
may be distinctly seen through their transparent bodies) by
means of cilia lining the internal
surfaces of the polypes ; but no
cilia can be discerned on their ex-
ternal surfaces. The tissue of this
spongy polypidom is strengthened
throughout, like that of Sponges
(§ 467), with mineral spicules (al-
ways, however, calcareous), which
are remarkable for the elegance of
their forms ; these are disposed
with great regularity around the
basis of the polypes, and even ex-
tend part of their length upwards
on their bodies. In the Gorgonia
or Sea-fan, whilst the central part
of the polypidom is consolidated
into a horny axis, the soft flesh
which clothes this axis is so full of
tuberculated spicules', especially in Spicules of Alcyonium and Gorgonia.
its outer layer, that, when this
dries-up, they form a thick yellowish or reddish incrustation upon
the horny stem ; this crust is, however, so friable, that it may be
easily rubbed down between the fingers, and, when examined with
the Microscope, it is found to consist of spicules of different shapes
and sizes, more or less resem-
"'^-';;
bling those shown in Figs.
308, 309, sometimes colour-
less, but sometimes of a beau-
tiful crimson, yellow, or pur-
ple. These spicules are best
seen by the methods of illu-
mination that give a black
ground (§ 93), on which they
stand out with great bril-
liancy, especially when viewed
by the Binocular Microscope.
They are, of course, to be se-
parated from the animal sub-
stance in the same manner
as the calcareous spicules of
Sponges (§ 469) ; and they
should be mounted, like
them, in Canada balsam.—
The spicules always possess
Fig. 309.
A, Spicules of Gorgonia guttata.
B, Spicules of Muricia elongata.
592
ACTINOZOA :— CTENOPHOKA.
an organic basis ; as is proved by the fact, that when their
lime is dissolved by dilute acid, a gelatinous -looking residuum is
left, which preserves the form of the spicule.
488. The Ctenophora, or ' comb-bearers.' are so named from the
comb-like arrangement of the rows of tiny paddles, by the move-
ment of which the bodies of these animals are propelled. A very
beautiful and not uncommon representative of this Order is
furnished by the Cydippe pileus (Fig. 310, a), very commonly
known as the Beroe, which designation, however, properly
appertains to another animal (b) of the same grade of organi-
zation. The body of Gydippe is a nearly-globular mass of soft
jelly, usually about 3-8ths of an inch in diameter; and it may
be observed, even with the naked eye, to be marked by eight
Fig. 310.
A, Cydippe pileus with its tentacles extended : — B, Beroe ForsJcalii, showing the
tubular prolongations of the stomach.
bright bands, which proceed from pole to pole like meridian lines.
These bands are seen with the Microscope to be formed of rows of
flattened paddles, which act quite independently of one another, so
as to give to the body every variety of motion, but sometimes work
all together. If the sun-light should fall upon them when they
are in activity, they display very beautiful iridescent colours.
The mouth of the animal, situated at one of the poles, leads
first to a quadrifid cavity bounded by four folds, which seem
to the Author to represent the oral proboscis of the ordinary
Medusas (Fig. 305) ; and this leads to the true stomach, which
passes towards the opposite pole, near to which it bifurcates,
its branches passing towards the polar surface on either side
of a little body which has every appearance of being a ner-
vous ganglion, and which is surmounted externally by a fringe-
593
like apparatus that seems essentially to consist of sensory ten-
tacles.* From the cavity of the stomach, tubular prolongations
pass-off beneath the ciliated bands, very much as in the true
Beroe (b) ; these may easily be injected with coloured liquids, by
the introduction of the extremity of a fine-pointed glass syringe
(Fig. 96) into the mouth. In addition to the bands of cilia, the
Cyd/ippe is furnished with a pair of locomotive organs of a very
peculiar kind ; these are long tendril-like filaments, arising from
the bottom of a pair of cavities in the posterior part of the body,
and furnished with lateral branches (a) ; within these cavities they
are often doubled-up, so as not to be visible externally ; and when
they are ejected, which often happens quite suddenly, the main
filaments first come-forth, and the lateral tendrils subsequently
uncoil themselves, to be drawn-in again and packed-up within the
cavities, with almost equal suddenness. The liveliness of this
little creature, which may sometimes be collected in large quanti-
ties at once by the Tow-net, renders it a most beautiful subject for
observation when due scope is given to its movements ; but for the
sake of Microscopic examination, it is of course necessary to con-
fine these. — Various species of true Beroe, some of them even
attaining the size of a small lemon, are occasionally to be met
with on our coasts ; in all of which the movements of the body are
effected by the like agency of cilia arranged in meridional bands.
These are splendidly luminous in the dark, and the luminosity is
retained even by fragments of their bodies, being augmented by
agitation of the water containing them. — All the Ctenoplwra are
reproduced from eggs, and are already quite advanced in their
development by the time they are hatched. Long before they
escape, indeed, they swim about with great activity within the
walls of their diminutive prison ; their rows of locomotive paddles
early attaining a large size, although the long flexile tentacles
of Cydeppe are then only short stumpy tentacles. Through the
embryonic forms of the two groups, Prof. Alex. Agassiz considers
the Gtenopliora as related to Echinodermata.f
* It is commonly stated that the two branches of the alimentary canal open
on the surface by two pores situated in the hollow of the fringe, one on either
side of the nervous ganglion. The Author, however, has not been able to satisfy
himself of the existence of such excretory pores in the ordinary Cydippe or Beroe,
although he has repeatedly injected their whole alimentary canal and its exten-
sions, and has attentively Avatched the currents produced by ciliary action in
the interior of the bifurcating prolongations, which currents always appear to
him to return as from csecal extremities. He is himself inclined to believe that
this arrangement has reference solely to the nutrition of the nervous ganglion
and tentacular apparatus, which lies imbedded (so to speak) in the bifurcation
of the alimentary canal, so as to be able to draw its supply of nutriment direct
from that cavity.
f The Ctenophora are specially treated of in vol. iii. of Prof. Agassiz3
" Contributions to the Natural History of the United States." See also Prof.
Alex. Agassiz' "Sea-side Studies in Natural History," and his "Illustrated
Catalogue of the Museum of Comparative Anatomy at Harvard College," Prof.
James Clark in " American Journal of Science," Ser. 2, Vol. xxxv. p. 348, and
Dr. D. Macdonald in " Transact. Eoy. Soc. Edinb.," Vol. xxiii. p. 515.
QQ
594 NOCTILUCA.
489. Very different from any of the creatures now described, is
the structure of another little globular jelly-like animal, the Noc-
tiluca miliaris (Fig. 311), to which the diffused luminosity of the
sea, a beautiful phenomenon that is of very frequent occurrence on
our shores, is chiefly attributable. This animal, much resembling
Fig. 311.
Noctiluca miliaris.
in appearance a grain of boiled sago, is just large enough to be dis-
cerned by the naked eye, when the water in which it may be
swimming is contained in a glass jar exposed to the light ; and a
tail-like appendage, marked with transverse rings, which is em-
ployed by the animal as an instrument of locomotion, both for
swimming and for pushing, may also be observed with a hand-
glass. Near the point of its implantation in the body is a definite
mouth, on one side of which a projecting tooth has been seen by
Prof. Huxley ; and this mouth leads through a sort of oesophagus
into a large irregular cavity, apparently channelled-out in the
jelly-like substance of the body, and therefore regarded by some in
the light of a mere ' vacuole,' though by Prof. Huxley it is consi-
dered to possess regular walls and to be a true stomach ; whilst
from its cavity there passes-forth a prolongation, which leads, in
his belief, to a distinct anal orifice.* The external coat is denser
than the contained sarcode ; and the former sends thread-like pro-
longations through the latter, so as to divide the entire body into
irregular chambers, in some of which 'vacuoles' are frequently
to be seen. It seems to feed on Diatoms, as their loricm may fre-
quently be detected in its interior. This animal appears to mul-
* " Quart. Journ. of Microsc. Science," Vol. iii. (1855), p. 49 ; see also Dr.
Webb, at p. 102, and Dr. Busch, at p. 199 of the same volume ; and Gosse, in
"Eambles on the Devonshire Coast," p. 257.
NOCTILUCA. 595
tiply both "by subdivision and by gemmation ;* but nothing is yet
known of its sexual generation ; and until the mode in which it
performs that important function shall have been made-out, and it
shall have also been determined whether it passes through any
other phase of existence, we are scarcely in a position to speak
positively of its true affinities. So far as its character is at present
known, its place would seem to be rather among the Protozoa, than
in any more elevated group. The nature of its luminosity is found
by Microscopic examination to be very peculiar ; for what appears
to the eye to be a uniform glow, is resolvable under a sufficient
magnifying power into a multitude of evanescent scintillations ;
and these are given-f orth with increased intensity whenever the
body of the animal receives any mechanical shock (such as that
produced by shaking the vessel or pouring out its contents), or is
acted-on by various chemical stimuli, such as dilute acids, which,
however, speedily exhaust the light-producing power, occasioning
disorganization of the body.
* See Brightwell in " Quart. Joum. of Microsc. Science," Vol. v. (1857),
p. 185.
Those who may desire to acquire a more systematic and detailed acquain-
tance with the Zoophyte-group, may be especially referred to the following
Treatises and Memoirs, in addition to those already cited, and to the vaiious
recent systematic Treatises on Zoology : — Dr. Johnston's " History of British
Zoophytes," Prof. Milne-Edwards's "Becherches sur les Polypes," and his
" Histoire des Corallaires " (in the ' Suites a Buff on '), Paris, 1857, Prof. Van
Beneden 'Sur les Tubulaires,' and 'Sur les Campanulaires,' in "Mem. de
l'Acad. Boy. de Bruxelles," Tom. xvii., and his " Becherches sur l'Hist. Nat.
des Polypes qui frequentent les Cotes de Belgique," Op. cit. Tom. xxxvi., Sir J.
G. DalyelTs "Bare and Bemarkable Animals of Scotland," Vol. i., Trembley's
" Mem. pour servir a l'histoire d'un genre de Polype d'Eau douce,'' M. Hollard's
'Monographie du Genre Actinia,' in "Ann. des Sci. Nat.,'' Ser. 3, Tom. xv., Mr.
Mummery, ' On the Development of Tubularia indivisa,'' in " Trans, of Microsc.
Soc," 2nd Ser., Vol. i., p. 28 ; Prof. Max. Schultze, 'On the Male Reproduc-
tive Organs of Campanularia geniculata,'' in "Quart. Journ. of Microsc. Sci.,"
Vol. iii. (1855), p. 59, Prof. Agassiz's beautiful Monograph on American Me-
dusae, forming the third volume of his "Contributions to the Natural History
of the United States of America," Mr. Hincks's " British Hydroid Zoophytes,"
Prof. Allman's admirable Monograph on the British Tubular ida (published by
the Ray Society), Prof. J. R. Greene's "Manual of the Sub-Kingdom
Cadenterata" which contains a Bibliography very complete to the date of its
publication, and the articles ' Actinozoa,' ' Ctenophora,' and 'Hydrozoa,' in the
Supplement to the Natural History Division of the "English Cyclopaedia."
QQ-2
CHAPTEE XII.
ECHINODERMATA.
490. As we ascend the scale of Animal life, we meet with such a
rapid advance in complexity of structure, that it is no longer pos-
sible to acquaint one's-self with any organism by Microscopic exa-
mination of it as a whole ; and the dissection or analysis which
becomes necessary, in order that each separate part may be
studied in detail, belongs rather to the Comparative Anatomist
than to the ordinary Microscopist. This is especially the case
with the Echinus ( Sea-Urchin), Asterias (Star-fish), and other
members of the class Echinodermata, even a general account of
whose complex organization would be quite foreign to the purpose
of this work. Yet there are certain parts of their structure which
furnish Microscopic objects of such beauty and interest that they
cannot by any means be passed by ; besides which, recent observa-
tions on their Embryonic forms have revealed a most unexpected
order of facts, the extension and verification of which will be
of the greatest service to science, — a service that can only be
effectually rendered by well-directed Microscopic research in fitting
localities.
491. It is in the structure of that Calcareous Skeleton which
probably exists under some form in every member of this class,
that the ordinary Microscopist finds most to interest him. This
attains its highest development in the Echinida ; in which it forms
a box-like shell or ' test,' composed of numerous polygonal plates
jointed to each other with great exactness, and beset on its
external surface with ' spines,' which may have the form of prickles
of no great length, or may be stout club-shaped bodies, or, again,
may be very long and slender rods. The intimate structure of the
shell is everywhere the same ; for it is composed of a network,
which consists of Carbonate of Lime with a very small quantity of
animal matter as a basis, and which extends in every direction
(i.e., in thickness as well as in length and breadth), its areolae or
interspaces freely communicating with each other (Figs. 312, 313).
These ' areolae,' and the solid structure which surrounds them, may
bear an extremely variable proportion one to the other ; so that in
two masses of equal size, the one or the other may greatly predo-
CALCAKEOUS SKELETON OF ECHINODEEMS.
597
Section of Shell of Echinus, showing
the calcareous network of which it is
a, portions of a deeper
minate ; and the texture may Lave either a remarkable lightness
and porosity, if the network be a very open one like that of Fig.
313, or may possess a considerable degree of compactness, if the
solid portion be strengthened. Generally speaking, the different
layers of this network, which are connected together by pillars
that pass from one to the other
in a direction perpendicular to Fig. 312.
their plane, are so arranged that
the perforations in one shall
correspond to the intermediate
solid structure in the next ; and
their transparence is such that (^^^3^D/l*i
when we are examining a section (^^^^^^'^^t^^y^V
thin enough to contain only two f^^^i^^\^y^^'
or three such layers, it is easy, ^^Q^y^ f^i^^^^''^s%
by properly focussing the Micro- '^^L^AL^^Qi^
scope, to bring either one of ~jQ^/ (J 0,'r
them into distinct view. From
this very simple but very beau-
tiful arrangement, it comes to
pass that the plates of which
the entire ' test ' is made-up
possess a very considerable de- composed:'
gree of strength, notwith- layer,
standing that their porousness
is such that if a portion of a fractured edge, or any other part
from which the investing membrane has been removed, be laid
upon fluid of almost any description, this will be rapidly sucked
up into its substance. — A very beautiful example of the same kind
of calcareous skeleton, having a
more regular conformation, is Fig. 313.
furnished by the disk or ' ro-
sette ' which is contained in the
tip of every one of the tubular
suckers put forth by the living
Echinus from the ' ambulacral
pores ' that are seen in the rows
of smaller plates interposed be-
tween the larger spine-bearing
plates of its box-like shell. If
the entire disk be cut-off, and
be mounted when dry in Canada
balsam, the calcareous rosette
may be seen sufficiently well ;
but its beautiful structure is open network,
better made-out when the ani-
mal membrane that encloses it has been got rid-of by boiling in
a solution of caustic potass ; and the appearance of one of the
Transverse Section of central portion
of Spine of Acrocladia, showing its more
598 CALCAEEOUS SKELETON OF ECHINODEKMATA.
five segments of which it is composed, when thus prepared, is
shown in Fig. 314,
Fig. 314
One of the segments of the calcareous skeleton of an Ambulacral Disk of
Echinus.
492. The most beautiful display of this reticulated structure,
however, is shown in the structure of the ' spines ' of Echinus,
Oidaris, &c. ; in which it is combined with solid ribs or pillars, dis-
posed in such a manner as to increase the strength of these
organs ; a regular and elaborate pattern being formed by their
intermixture, which shows considerable variety in different species.
— When we make a thin transverse section (Plate II., fig. 1) of
almost any spine belonging to the genus Echinus (the small spines
of our British species, however, being exceptional in this respect)
or to its immediate allies, we are at once made aware of the exis-
tence of a number of concentric layers, arranged in a manner
that strongly reminds us of the concentric rings of an Exo-
genous tree (Fig. 229). The number of these layers is extremely
variable ; depending not merely upon the age of the spine, but (as
will presently appear) upon the part of its length from which the
section happens to be taken. The centre is usually occupied by a
very open network (Fig. 313) ; and this is bounded by a row of
transparent spaces (like those at a a', b V , c c', &c, Fig. 315), which
on a cursory inspection might be supposed to be void, but which on
a closer examination are found to be the sections of solid ribs or
pillars, which run in the direction of the length of the spine, and
form the exterior of every layer. Their solidity becomes very
obvious, when we either examine a section of a spine whose
substance is pervaded (as often happens) with a colouring matter
of some depth, or when we look at a very thin section by the black-
ground illumination. Around the innermost circle of these solid
pillars there is another layer of the calcareous network, which
again is surrounded by another circle of solid pillars ; and this ar-
STRUCTURE OF SPINE OF ECHINUS.
599
rangement may be repeated many times, as shown in Fig. 315, the
ontermost row of pillars forming the projecting ribs that are very
commonly to be distinguished on the surface of the spine. Aronnd
the cnp-shaped base of the spine is a membrane which is con-
tinuous with that covering the surface of the shell, and which
Fig. 315.
Portion of transverse section of Spine of Acrocladia mammillata.
serves not merely to hold-down the cup upon the tubercle over
which it works, but also by its contractility to move the spine in
any required direction. This membrane is probably continued
onwards over the whole surface of the spine, although it cannot be
clearly traced to any distance from the base ; and the new forma-
tions may be presumed to take-place in its substance. Each new
formation completely ensheaths the old ; not merely surrounding
the part previously formed, but also projecting considerably
beyond it ; and thus it happens that the number of layers shown
in a transverse section will depend in part upon the place of that
section. For if it cross near the base, it will traverse every one of
the successive layers from the very commencement ; whilst if
it cross near the apex, it will traverse only the single layer of the
last growth, notwithstanding that, in the club-shaped spines, this
terminal portion may be of considerably larger diameter than the
basal ; and in any intermediate part of the spine, so many layers
will be traversed as have been formed since the spine first attained
that length. The basal portion of the spine is enveloped in a reti-
culation of a very close texture, without concentric layers ; forming
the cup or socket which works over the tubercle of the shell.
493. The combination of elegance of pattern with richness of
colouring renders well-prepared specimens of these spines among
the most beautiful objects that the Microscopist can anywhere
meet- with. The large spines of the various species of the genus
Acrocladia furnish sections most remarkable for size and elabo-
rateness, as well as for depth of colour (in which last point, how-
ever, the deep purple spines of Ecli inus lividus are pre-eminent) ;
but for exquisite neatness of pattern, there are no spines that can
600
CALCAREOUS SKELETON OF ECHINODERMATA.
approach those of Echinometra lieteropora (Plate II., fig. 1) and
E. lucunter. The spines of Heliocidaris variolar is are also re-
markable for their beauty. — ISTo snccession of concentric layers is
seen in the spines of the British Echini, probably becanse (accord-
ing to the opinion of the late Sir J. Gr. Dalyell) these spines are
cast-off and renewed every year ; each new formation thns going
to make an entire spine, instead of making an addition to that pre-
viously existing. — Most cnrions indications are sometimes afforded
by sections of Echinus-spines, of an extraordinary power of Sepa-
ration inherent in these bodies. For irregularities are often seen
in the transverse sections, which can be accounted-for in no other
way than by supposing the spines to have received an injury when
the irregular part was at the exterior, and to have had its loss of
substance supplied by the growth of new tissue, over which the
subsequent layers have been formed as usual. And sometimes a
peculiar ring may be seen upon the surface of a spine, which in-
dicates the place of a complete fracture, all beyond it being a new
growth, whose unconformableness to the older or basal portion is
clearly shown by a longitudinal section.* — The Spines of Cidaris
present a marked departure from the plan of structure exhibited in
Echinus ; for not only are they destitute of concentric layers, but
Fig. 316.
Spines of Spatangus.
the calcareous network which forms their principal substance is
encased in a solid calcareous sheath perforated with tubules, which
seems to take the place of the separate pillars of the Echini. This
is usually found to close-in the spine at its tip also ; and thus it
would appear that the entire spine must be formed at once, since
no addition could be made either to its length or to its diameter,
* See the Author's description of such Separations in the "Monthly Micro-
scopical Journal," Vol. ii. p. 225.
STRUCTURE OF SPINES AND TEETH. 601
save on the outside of the sheath, where it is never to be found.
The sheath itself often rises up in prominent points or ridges on
the surface of these spines ; thus giving theni a character by which
they may be distinguished from those of Echini. — The slender,
almost filamentary spines of Spatangus (Fig. 316), and the in-
numerable minute hair-like processes attached to the shell of
Clypeaster, are composed of the like regularly-reticulated substance ;
and these are very beautiful objects for the lower powers of the
Microscope, when laid upon a black ground and examined by re-
flected light without any further preparation. — It is interesting
also to find that the same structure presents itself in the curious
Pedicellarice (forceps-like bodies mounted on long stalks), which
are found on the surface of many Echinida, and the nature of which
was formerly a source of much perplexity to Naturalists, some
having maintained that they are parasites, whilst others considered
them as proper appendages of the Echinus itself. The complete
conformity which exists between the structure of their skeleton and
that of the animal to which they are attached, removes all doubt of
their being truly appendages to it, as observation of their actions
in the living state would indicate.
494. Another example of the same structure is found in the
peculiar framework of plates which surrounds the interior of the
oral orifice of the shell, and which includes the five teeth that
may often be seen projecting externally through that orifice ;
the whole forming what is known as the ' lantern of Aristotle.'
The texture of the plates or jaws resembles that of the shell in
every respect, save that the network is more open ; but that of
the teeth differs from it so widely, as to have been likened to
that of the bone and dentine of Vertebrate animals. The care-
ful investigations of Mr. James Salter,* however, have fully
demonstrated that the appearances which have suggested this
comparison are to be otherwise explained ; the plan of structure
of the tooth being essentially the same as that of the shell,
although greatly modified in its working-out. The complete
tooth has somewhat the form of that of the front tooth of a
Rodent ; save that its concave side is strengthened by a projecting
' keel,' so that a transverse section of the tooth presents the form
of a J.. This keel is composed of cylindrical rods of carbonate of
lime, having club-shaped extremities lying obliquely to the axis of
the tooth (Fig. 317, a, d) ; these rods do not adhere very firmly
together, so that it is difficult to keep them in their places in making
sections of the part. The convex surface of the tooth (c, c, c) is
covered with a firmer layer, which has received the name of ' enamel;'
this is composed of shorter rods, also obliquely arranged, but
having a much more intimate mutual adhesion than we find among
the rods of the kee]. The principal part of the substance of the
* See his Memoir ' On the Structure and Growth of the Tooth of Echinus,'
in " Philos. Transact." for 1861.
602
CALOAEEOUS SKELETON OF ECHINODEKMATA.
tooth, (a, h) is made-up of what may be called the ' primary plates ;'
these are triangular plates of calcareous shell-substance, arranged
in two series (as shown at b), and constituting a sort of framework
with which the other parts to be presently described become con-
nected. These plates may be seen by examining the growing base
of an adult tooth that has been preserved with its attached soft
Fig. 317.
Structure of the Tooth of Echinus: — A, vertical section,
showing the form of the apex of the tooth as produced by
wear, and retained by the relative hardness of its elementary
parts ; a, the clear condensed axis ; &, the body formed of
plates ; c, the so-called enamel ; d, the keel : — B, commencing
growth of the tooth, as seen at its base, showing its two
systems of plates ; the dark appearance in the central portion
of the upper part is produced by the incipient reticulations of
the flabelliform processes : — c, transverse section of the tooth,
showing at a the ridge of the keel, at b its lateral portion,
resembling the shell in texture ; at c, c, the enamel.
parts in alcohol, or (which is preferable) by examining the base of
the tooth of a fresh specimen, the minuter the better. The lengthen-
ing of the tooth below, as it is worn-away above, is mainly affected
by the successive addition of new 'primary plates.' To the outer
edge of the primary plates, at some little distance from the base,
we find attached a set of lappet-like appendages, which are formed
of similar plates of calcareous shell-substance, and are denominated
by Mr. Salter ' secondary plates.' Another set of appendages
termed ' flabelliform processes' is added at some little distance from
the growing base ; these consist of elaborate reticulations of cal-
TOOTH OF ECHINUS.— ASTEEIADA ; OPHIUEIDA.
603
careous fibres, ending in fan-shaped extremities. And at a point
still further from the base, we find the different components of the
tooth connected together by ' soldering particles,' which are
minute calcareous disks interposed between the previously-formed
structures ; and it is by the increased development of this connective
substance, that the intervening spaces are narrowed into the sem-
blance of tubuli like those of bone or dentine. Thus a vertical
section of the tooth comes to present an appearance very like that
of the bone of a Vertebrate animal, with its lacunae, canaliculi, and
lamellae ; but in a transverse section the body of the tooth bears a
stronger resemblance to dentine ; whilst the keel and enamel-layer
more resemble an oblique section of Pinna than any other form
of shell- structure. — It is interesting to remark that the gradational
transition between the ordinary reticular structure of the Shell,
and the dentine and enamel-like substance of the Tooth, which can
only be traced in the adult tooth of the Echinus by examining it
near its base, is most distinctly presented by the tooth of Ophiocoma ;
which is so minute that it may be mounted in balsam as a trans-
parent object with scarcely any grinding-down, and which then
shows that the basal portion of the tooth is formed upon the open
reticular plan characteristic of the ' shell,' whilst this is so modified
in the older portion by subsequent addition, that the upper part
of the tooth has the bone-like character of that of the tooth of
Echinus.
495. The calcareous plates which form the less compact skele-
tons of the Aster iada (' star-fish' and their allies), and of the
Opliiurida (' sand-stars' and ' brittle-stars'), have the same texture
as those of the shell of Echinus. And this presents itself, too, in
the spines or prickles of their
surface, when these (as in the
great Goniaster equestris) are
large enough to be furnished
with a calcareous framework,
and are not mere projections of
the horny integument. An ex-
ample of this kind, furnished by
the Astrophyton (better known
as the Euryale), is represented
in Fig. 318. The spines with
which the arms of the species n ,
of OpUocoma (< brittle- star') are Calcare011s ^^^ T of ^cphykm
beset, are often remarkable for
their beauty of conformation ; those of 0. rosula, one of the most
common kinds, might serve (as Prof. E. Forbes justly remarked),
in point of lightness and beauty, as models for the spire of a
cathedral. These are seen to the greatest advantage when mounted
in Canada balsam, and viewed by the Binocular Microscope with
black-ground illumination.
496. The calcareous skeleton is very highly developed in the
Fig. 318.
604 CALCAEEOUS SKELETON OF ECHINODEEMATA.
Crinoidea ; their stems and branches being made-up of a calcareous
network closely resembling that of the shell of the Echinus. This
is extremely well seen, not only in the recent Pentacrinus Caput
Medusce, a somewhat rare animal of the "West Indian seas, but also
in a large proportion of the fossil Crinoids, whose remains are so
abundant in many of the older Geological formations ; for notwith-
standing that these bodies have been penetrated in the act of f os-
silization by a Mineral infiltration, which seems to have substituted
itself for the original fabric (a regularly-crystalline cleavage being
commonly found to exist in the fossil stems of Encrinites, &c, as in
the fossil spines of Echinida), yet their organic structure is often
most perfectly preserved.* In the circular stems of Encrinites,
the texture of the calcareous network is uniform, or nearly so,
throughout ; but in the pentangular Pentacrini, a certain figure or
pattern is formed by variations of texture in different parts of the
transverse section.
497. The minute structure of the Shells, Spines, and other solid
parts of the skeleton of Echinodermata can only be displayed
by thin sections made upon the general plan already described
(§§ 154-156). But their peculiar texture requires that certain
precautions should be taken ; in the first place, in order to prevent
the section from breaking whilst being reduced to the desirable
thinness ; and in the second, to prevent the interspaces of the net-
work from being clogged by the particles abraded in the reducing
process. — A section of the Shell, Spine, or other portion of the
skeleton should first be cut with a fine saw, and be rubbed on a
flat file until it is about as thin as an ordinary card, after which it
should be smoothed on one side by friction with water on a Water-
of-Ayr stone. It should then be carefully dried, first on white
blotting-paper, afterwards by exposure for some time to a gentle
heat, so that no water may be retained in the interstices of the
network, which would oppose the complete penetration of the Canada
balsam. Next, it is to be attached to a glass-slip by balsam
hardened in the usual manner ; but particular care should be taken,
first, that the balsam be brought to exactly the right degree of
hardness, and second, that there be enough not merely to attach
the specimen to the glass, but also to saturate its substance
throughout. The right degree of hardness is that at which the
balsam can be with difficulty indented by the thumb-nail ; if it be
made harder than this, it is apt to chip-off the glass in grinding, so
that the specimen also breaks away ; and if it be softer, it holds
* The calcareous skeleton even of living Echinoderms has a crystalline
aggregation, as is very obvious in the more solid spines of Echinometrce, &c. ;
for it is difficult, in sawing these across, to avoid their tendency to cleavage in
the oblique plane of calcite. And the Author is informed by Mr. Sorby, that
the calcareous deposit which fills up the areolae of the fossilized skeleton has
always the same crystalline system with the skeleton itself, as is shown not
merely by the uniformity of their cleavage, but by their similar action on
Polarized light.
GRINDING AND MOUNTING THIN SECTIONS. 605
the abraded particles, so that the openings of the network become
clogged with thern. If, when rubbed- down nearly to the required
thinness, the section appears to be uniform and satisfactory through-
out, the reduction may be completed without displacing it ; but if
(as often happens) some inequality in thickness should be
observable, or some minute air-bubbles should show themselves
between the glass and the under surface, it is desirable to loosen
the specimen by the application of just enough heat to melt the
balsam (special care being taken to avoid the production of fresh
air-bubbles), and to turn it over so as to attach the side last-
polished to the glass, taking care to remove or to break with the
needle-point any air-bubbles that there may be in the balsam cover-
ing the part of the glass on which it is laid. The surface now
brought uppermost is then to be very carefully ground down ;
special care being taken to keep its thickness uniform through
every part (which may be even better judged-of by the touch than
by the eye), and to carry the reducing process far enough, without
carrying it too far. Until practice shall have enabled the operator
to judge of this by passing his finger over the specimen, he must
have continual recourse to the microscope during the later stages of
his work ; and he should bear constantly in mind that, as the
specimen will become much more transparent when mounted in
balsam and covered with glass, than it is when the ground surface
is exposed, he need not carry his reducing process so far as to pro-
duce at once the entire transparence he aims at, the attempt to
accomplish which would involve the risk of the destruction of the
specimen. In ' mounting' the specimen, liquid balsam should be
employed, and only a very gentle heat (not sufficient to produce
air-bubbles, or to loosen the specimen from the glass) should be
applied ; and if after it has been mounted the section should be
found too thick, it will be easy to remove the glass cover and to re-
duce it further, care being taken to harden to the proper degree the
balsam which has been newly laid-on.
498. If a number of sections are to be prepared at once (which
it is often useful to do for the sake of economy of time, or in
order to compare sections taken from different parts of the same
spine), this may be most readily accomplished by laying them
down, when cut-off by the saw, without any preliminary prepara-
tion save the blowing of the calcareous dust from their surfaces,
upon a thick slip of glass well covered with hardened balsam ; a
large proportion of its surface may thus be occupied by the sections
attached to it, the chief precaution required being that all the
sections come into equally close contact with it. Their surfaces
may then be brought to an exact level, by rubbing them down,
first upon a flat piece of grit (which is very suitable for the rough
grinding of such sections), and then upon a large "Water-of-Ayr
stone whose surface is ' true.' When this level has been attained,
the ground surface is to be well washed and dried, and some
balsam previously hardened is to be spread over it, so as to be
606 CALCAREOUS SKELETON OF ECHINODEEMATA.
sucked-in by tlie sections, a moderate neat being at the same time
applied to the glass slide ; and when this has been increased suffi-
ciently to loosen the sections without overheating the balsam, the
sections are to be turned-over, one by one, so that the ground sur-
faces are now to be attached to the glass slip, special care being
taken to press them all into close contact with it. They are then
to be very carefully rubbed- down, until they are nearly reduced to
the required thinness ; and if, on examining them from time to
time, their thinness should be found to be uniform throughout, the
reduction of the entire set may be completed at once ; and when
it has been carried sufficiently far, the sections, loosened by warmth,
are to be taken-up upon a camel-hair brush, dipped in turpentine,
and transferred to separate slips of glass whereon some liquid
balsam, has been previously laid, in which they are to be mounted
in the usual manner. It more frequently happens, however, that,
notwithstanding every care, the sections, when ground in a number
together, are not of uniform thickness, owing to some of them
being underlaid by a thicker stratum of balsam than others are ;
and it is then necessary to transfer them to separate slips before
the reducing process is completed, attaching them with hardened
balsam, and finishing each section separately.
499. Avery curious internal skeleton, formed of detached plates
or spicules, is found in many members of this class ; often forming
an investment like a coat of mail to some of the viscera, especially
to the ovaries. The forms of these plates and spicules are generally
so diverse, even in closely-allied species, as to afford very good
differential characters. This subject is one that has been as yet
but very little studied, Mr. Stewart being the only Microscopist
who has given much attention to it ;* but it is well worthy of much
more extended research.
500. It now remains for us to notice the curious and often very
beautiful structures, which represent, in the order Holothurida,
the solid calcareous skeleton of the orders already noticed. All
the animals belonging to this Order are distinguished by the flexi-
bility and absence of firmness of their envelopes ; and excepting
in the case of certain species which have a set of calcareous plates,
supporting teeth, disposed around the mouth, very much as in the
Echinida, we do not find among them any representation that is
apparent to the unassisted eye, of that skeleton which constitutes
so distinctive a feature of the class generally. But a microscopic
examination of their integumentat once brings to view the existence
of great numbers of minute isolated plates, every one of them pre-
senting the characteristic reticulated structure, which are set with
greater or less closeness in the substance of the skin. Yarious
forms of the plates which thus present themselves in Holothuria
are shown in Fig. 319 ; and at a is seen an oblique view of the
kind marked a, more highly magnified, showing the very peculiar
* See his Memoir in the " Linnsean Transactions," Vol. xxv. p. 365.
HOLOTHUMDA :— ANCHOES OF SYNAPTA.
607
manner wherein one part is superposed on the other, which is not
at all brought into view when it is merely seen-through in the ordi-
Fig. 319.
Calcareous plates in Skin of Holothuria.
nary manner. — In the Synapta, one of the long-bodied forms of this
order, which abounds in the Adriatic Sea, and of which two species
(the 8. digitata and 8. inlicerens) occasionally occur upon our own
coasts,* the calcareous plates of the integument have the regular
form shown at a, Fig. 320 ; and each of these carries the curious
Calcareous Skeleton of Synapta: — A. plate imbedded in
Skin ; B, the same, with its anchor-like spine attacked ; c,
anchor-like spine separated.
anchor-like appendage, c, which is articulated to it by the notched
piece at the foot, in the manner shown (in side view) at b. The
anchor-like appendages project from the surface of the skin, and
may be considered as representing the spines of Echinida. — jSTearly
allied to the Synapta is the Chirodota, the integument of which is
entirely destitute of ' anchors,' but is furnished with very remark-
able wheel-like plates ; those represented in Fig. 321 are found in
the skin of Chirodota violacea, a species inhabiting the Mediter-
ranean. These * wheels' are objects of singular beauty and delicacy,
being especially remarkable for the very minute notching (scarcely
* See Woodward in "Proceedings of Zoological Society,'' July 18, 1858.
608 ECHINODERM ATA :— SKELETON AND LARV.E.
to be discerned in the figures without the aid of a magnifying-glass)
which is traceable round the inner margin of their ' tires.'— There
can be scarcely any reasonable doubt that every member of this
Order has some kind of cal-
Fig. 321. careous skeleton, disposed in
a manner conformable to the
examples now cited ; and it
would be very valuable to de-
termine how far the marked
peculiarities by which they
are respectively distinguished,
are characteristic of genera
and species. The plates may
be obtained separately by the
usual method of treating the
Wheel-like plates from Skin of Chirodota gkm with a solution of potass ;
violacea. an(j they should be mounted in
Canada balsam. But their po-
sition in the skin can only be ascertained by making sections of the
integument, both vertical and parallel to its surface ; and these
sections, when dry, are most advantageously mounted in the same
medium, by which their transparence is greatly increased. All
the objects of this class are most beautifully displayed by the
Black-ground illumination (§§ 93-95) ; and their solid forms
are seen with increased effect under the Binocular. The Black-
ground illumination applied to very thin sections of Echinus
spines brings out some effects of marvellous beauty; and even
in these the solid form of the network connecting the pillars
is better seen with the Binocular than it can be with the ordinary
Microscope.*
501. Echinoderm-Larvce. — We have now to notice that most
remarkable set of objects furnished to the Microscopic inquirer by
the larval states of this class ; for our present knowledge of which,
imperfect as it still is, we are almost entirely indebted to the
painstaking and widely- extended investigations of Prof. J. Miiller.
All that our limits permit is a notice of two of the most curious
forms of these larvae, by way of sample of the wonderful pheno-
mena which his researches brought to light; so as (it may be
hoped) to excite such an interest among those Microscopists in
particular who may have the opportunity of pursuing these in-
quiries, as may induce them to apply themselves perseveringly to
them, and thus to supply the numerous links which are at present
wanting in the chain of developmental history. — The peculiar
* It may be here pointed out that the reticulated appearance is sometimes
deceptive ; what seems to be a solid network being in many instances a hollow
network of passages channelled out in solid calcareous substance. Between
these two conditions, in which the relation between the solid framework and
the intervening space is completely reversed, there is every intermediate
gradation.
LAEYAL ZOOIDS OF ECHIXODEEMS.
609
Fig. 322.
feature by which, the early history of the Echinoderms generally
seems to be distinguished, is this, — that the embryonic mass of
cells is converted, not into a larva which subsequently attains the
adult form by a process of metamorphosis, but into a peculiar
' zooid' or pseud&mbryo, which seems to exist for no other purpose
than to give origin to the Echinoderm by a kind of internal gem-
mation, and to carry it to a distance by its active locomotive
powers, so as to prevent the spots inhabited by the respective
species from being overcrowded by the accumulation of their pro-
geny. The larval zooids are formed upon a type quite different
from that which characterizes the adults ; for instead of a radial
symmetry, they exhibit a bilateral, the two sides being precisely
alike, and each having a ciliated fringe along the greater part or
the whole of its length. The two fringes are united by a superior
and an inferior transverse ciliated band ; and between these two
the mouth of the zooid is always situated. Further, although the
adult Star-fish and Sand-stars have usually neither intestinal tube
nor anal orifice, their larval
zooids, like those of other
Echinoderms, always possess
both. The external forms of
these larvae, however, vary in
a most remarkable degree,
owing to the unequal evo-
lution of their different parts ;
and there is also a consider-
able diversity in the several
Orders, as to the proportion
of the fabric of the larva
which enters into the com-
position of the adult form.
In the fully-developed Star-
fish and Sea-urchin, the only
part retained is a portion of
the stomach and intestine,
which is pinched-off, so to
speak, from that of the larval
zooid.
502. One of the most re-
markable forms of Echino-
derm-larvae is that which has
received the name of Bipin-
naria (Fig-. 322), from the «-.... T t Ci
i • ° t , Bipumaria asterinera. or Larva of fetar-
symmetrical arrangement of fish:_«, mouth; a', oesophagus: 6, intes-
lts natatory organs. The tinal tube and anal orifice; c, furrow in
mouth (a), which opens in which the mouth is situated; d d\ bilobed
the middle of a transverse peduncle ; 1, 2, 3, 4, 5, 6, 7, ciliated arms,
furrow, leads through an
oesophagus a' to a large stomach, around which the body of a
It K,
610 DEVELOPMENT OF ECHINODEEMATA.
Star-fish is developing itself ; and on one side of this month are
observed the intestinal tnbe and anns (&). On either side of the
anterior portion of the body are six or more narrow fin-like appen-
dages, which are fringed with cilia ; and the posterior part of the
body is prolonged into a sort of pedicle, bilobed towards its ex-
tremity, which also is covered with cilia. The organization of this
larva seems completed, and its movements through the water
become very active, before the mass at its anterior extremity pre-
sents- anything of the aspect of the Star-fish; in this respect
corresponding with the movements of the gluteus of the Echinida
(§ 503). The temporary month of the larva does not remain as the
permanent month of the Star -fish; for the oesophagus of the latter
enters on what is to become the dorsal side of its body, and the
true month is subsequently formed by the thinning- away of the
integument on its ventral surface. The young Star-fish is sepa-
rated from the Bipinnarian larva by the forcible contractions of the
connecting stalk, as soon as the calcareous consolidation of its
integument Las taken-place and its true mouth has been formed,
but long before it has attained the adult condition ; and as its ulte-
rior development has not hitherto been observed in any instance,
it is not yet known what are the species in which this mode of
evolution prevails. The larval zooid continues active for several
days after its detachment ; and it is possible, though perhaps
scarcely probable, that it may develope another Asteroid by a
repetition of this process of gemmation.*
503. In the Bipinnaria, as in other larval zooids of the Asteriada,
there is no internal calcareous frame-work ; such a frame-work,
however, is found in the larvas of the Eclvinida and Ophnirida, of
which the form delineated in Fig. 323 is an example. f The
embryo issues from the ovum as soon as it has attained, by repeated
' segmentation' of the yolk (§ 540), the condition of the ' mulberry-
mass;' and the superficial cells of this are covered with cilia, by
whose agency it swims freely through the water. So rapid are the
* See the observations of Koren and Daniellsen (of Bergen) in the "Zoolo-
giske Bidrag," Bergen, 1847 (translated in the "Ann. des Sci. Nat.," Ser. 3, Zool.,
Tom. iii., p. 347) : and the Memoir of Prof. Miiller, 'Ueber die Larven nnd die
Metamorphose der Echinodermen,' in " Abhaldlnngen der Koniglichen Akade-
mie der Wissenschaften zu Berlin," 1848. — Another very dissimilar mode of
development in certain Star-fish was first described by Sars, in his " Fanna
littoralis Norvegiee," 1846, and has been since investigated by Bnsch ("Beo-
bachtungen uber Anatomie nnd Entwickelung einiger Werbellosen Seethiere,"
1851), Prof. Miiller (" Uber den allgemeinen Plan in der Entwickelung der
Echinodermen," 1853), and Prof. Wyville Thomson ('On the Embryology of
Asieracanthion violaceus ') in " Quart. Journ. of Microsc. Science," N.S., Vol. i.
(1861), p. 99.
t See Prof. Miiller, ' Ueber die Larven und die Metamorphose der Ophiuren
nnd Seeigel,' in " Abhaldlungen der Koniglichen Akademie der Wissenschaften
zu Berlin," 1846. See also, for the earlier stages, a Memoir by M. Derbes, in
''Ann. des Sci. Nat.," Ser. 3, Zool., Tom. viii.,p. 80 ; and for the later, Krohn's
" Beitrag zur Entwickelungsgeschichte der Seeigillarven," Heidelberg, 1849,
and his Memoir in " Miiller' s Archiv.,'' 1851.
PLUTEUS-LAKVA OF ECHINUS.
611
early processes of development, that no more than from twelve to
twenty -four hours intervene between fecundation and the emersion
of the embryo ; the division into two, four, or even eight segments
taking-place within three hours after impregnation. Within a few
Fig. 323.
Embryonic development of Echinus : — A, Pluteus-larva at
the time of the first appearance of the disk ; o, mouth in the
midst of the four-pronged proboscis ; 6, stomach ; c, Echinoid
disk ; <Z, d, d, d, four arms of the pluteus-body ; e, calcareous
framework ; /, ciliated lobes ; g, g, g, g, ciliated processes of
the proboscis ; — B, Disk with the first indication of the cirrhi:
c. Disk, with the origin of the spines between the cirrhi : — E,
more advanced disk, with the cirrhi, g, and spines, x, pro-
jecting considerably from the surface. (K.B. — In B, c, and D,
the Pluteus is not represented, its parts having undergone no
change, save in becoming relatively smaller.)
ER2
612 DEVELOPMENT OF ECHINODEEMATA.
hours after its emersion, the embryo changes from the spherical
into a sub -pyramidal form with a flattened base ; and in the centre
of this base is a depression, which gradually deepens, so as to form
a mouth that communicates with a cavity in the interior of the
body, which is surrounded by a portion of the yolk-mass that has
returned to the liquid granular state. Subsequently a short
intestinal tube is found, with an anal orifice opening on one side
of the body. The pyramid is at first triangular, but it afterwards
becomes quadrangular ; and the angles are greatly prolonged
round the mouth (or base), whilst the apex of the pyramid is some-
times much extended in the opposite direction, but is sometimes
rounded- off into a kind of dome (Fig. 323, a). All j>arts of this
curious body, and especially its most projecting portions, are
strengthened by a frame-work of thread-like calcareous rods (e).
In this condition the embryo swims freely through the water, being
propelled by the action of the cilia, which clothe the four angles of
the pyramid and its projecting arms, and which are sometimes
thickly set upon two or four projecting lobes (/) ; and it has
received the designation of pluteus. The mouth is usually sur-
rounded by a sort of proboscis, the angles of which are prolonged
into four slender processes (g, g, g, g), shorter than the four outer
legs, but furnished with a similar calcareous frame-work.
504. The first indication of the production of the young Echinus
from its ' pluteus,' is given by the formation of a circular disk
(Fig. 323, a, c), on one side of the central stomach (b) ; and this
disk soon presents five prominent tubercles (b), which subsequently
become elongated into tubular cirrhi. The disk gradually extends
itself over the stomach, and between its cirrhi the rudiments of
spines are seen to protrude (c) ; these, with the cirrhi, increase in
length, so as to project against the envelope of the pluteus, and
to push themselves through it ; whilst, at the same time, the
original angular appendages of the pluteus diminish in size, the
ciliary movement becomes less active, being superseded by the
action of the cirrhi and spines, and the mouth of the pluteus
closes-up. By the time that the disk has grown over half of the
gastric sphere, very little of the pluteus remains, except some
of the slender calcareous rods ; and the number of cirrhi and
spines rapidly increases. The calcareous frame-work of the shell
at first consists, like that of the Star-fishes, of a series of isolated
networks developed between the cirrhi ; and upon these rest the
first-formed spines (d). But they gradually become more consoli-
dated, and extend themselves over the granular mass, so as to form
the series of plates constituting the shell. The mouth of the
Echinus (which is altogether distinct from that of the pluteus) is
formed at that side of the granular mass over which the shell is
last extended ; and the first indication of it consists in the appear-
ance of the five calcareous concretions, which are the summits of the
five portions of the frame-work of jaws and teeth that surround it.
All traces of the original pluteus are now lost; and the larva,
CEIXOIDEA :— COMATULA,
613
which now presents the general aspect of an Echinoid animal,
gradually augments in size, multiplies the number of its plates,
cirrhi, and spines, evolves itself into its particular generic and
specific type, and undergoes various changes of internal structure,
tending to the development of the complete organism. — In collect-
ing the free-swimming larvas of Echinodermata, the Tow-net should
be carefully employed in the manner already described (§ 195) ;
and the search for them is of course most likely to be successful in
those localities in which the adult forms of the respective species
abound, and on warm calm days, in which they seem to come to
the surface in the greatest numbers.*
505. One of the most interesting to the Microscopist of all Echi-
nodermata is the Antedonf (more generally known as Comatula),
Fig. 324.
'imbi
#»%. mm
%m
€■:
Vv.,
Mil
,Vv~>
Aniedon (Comatula) or Feather-star, seen from its under side.
or ' feather-star' (Fig. 324), which is the commonest existing re-
presentative of the great fossil series of Crinoidea, or ' lily- stars,'
that were among the most abundant types of this class in the
* The development of the Holothurida generally has been studied by Prof.
M tiller (See his Memoir in the "Berlin Transactions " for 1849); and that of
Synapta inhcerens, bv Prof. Wyville Thomson, in "Quart. Joum. of Microsc.
Science," N.S., Vol. iL (1862), p. 105.
t The Author has found himself obliged by the accepted rules of Zoological
Nomenclature, to adopt the designation Antedon, instead of the much better
known and very appropriate name given to this type by Lamarck. See his
' Researches on the Structure, Physiology, and Development of Antedon rosa-
ceus,' in "Philos. Transact.," 1866,"p. 671."
614
ECH1N0DEKMATA : — CKINOIDEA.
earlier epochs of the world's history. Like these, the young of
Antedon is attached by a stalk to a fixed base, as shown in
Fig. 325 ; but when it has arrived at a certain stage of development,
it drops off from this like a fruit from its stalk ; and. the animal is
thenceforth free to move through the ocean-waters it inhabits. It
can swim with considerable activity : but it exerts this power
chiefly to gain a suitable place for attaching itself by means of the
jointed prehensile cirrhi put forth
Fig. 325. from the under side of the central
disk (Fig. 324), so that, notwith-
standing its locomotive power,
it is nearly as stationary in its
free adult condition, as it is in
its earlier Pentacrinoid stage.
The pentacrinoid larva, — first
discovered by Mr. J. Y. Thompson,
of Cork, in 1823, but originally
supposed by him to be a perma-
nently-attached Criuoid, — forms
a most beautiful object for the
lower powers of the Microscope,
when well preserved in fluid, and
viewed by a strong incident light
(Plate XXI., fig. 3) ; and a series
of specimens in different stages
of development shows most cu-
rious modifications in the form
and arrangement of the various
component pieces of its calca-
reous skeleton. In its earliest
stage (Fig. 325, a), the body is
enclosed in a calyx composed of
two circles of plates ; namely,
five basals, forming a sort of py-
ramid whose apex points down-
wards, and is attached to the
highest joint of the stem ; and five
orals superposed on these, forming when closed a like pyramid whose
apex points upwards, but usually separating to give passage to the
tentacles, of which a circlet surrounds the mouth. In this condition
there is no rudiment of arms. In the more advanced stage shown
at b, the arms have begun to make their appearance ; and the
skeleton, when carefully examined, is found to consist of the
following pieces, as shown in Plate XXI., fig. 1 : — b, b, the circlet
of basals supported on the part of the stem ; rl, the circlet of first
radials, now interposed between the basals and the orals, and
alternating with both ; between two of these is interposed the
single anal plate, a ; whilst they support the second and the third
radials (r2, r3), from the latter of which the bifurcating arms
Crinoid Larva of Antedon: — A, B,
successive stages of development.
PLATE XXI.
Pjjntacrinoid Lauva of Antedon (Cobnatula).
I To face p. 6)5.
CEINOIDE A : — COMATULA. 61 5
spring ; finally, between the second radials we see the five oralo,
lifted from the basals on which they originally rested, by the inter-
position of the first radials. In the more advanced stage shown in
Fig. 325, c, and on a larger scale in Plate XXI., figs. 2, 3, we find
the highest joint of the stem beginning to enlarge, to form the
centro-dorsal plate (fig. 2, cd), from which are beginning to spring
the dorsal cirrhi (cir), that serve to anchor the animal when it
drops from the stem ; this supports the basals (b), on which rest
the first radials (r1); whilst the anal plate (a) is now lifted nearly
to the level of the second radials (r2), by the development of the
anal funnel or vent (v) to which it is attached. The oral plates are
not at first apparent, as they no longer occupy their first position ;
but on being carefully looked-for, they are found still to form a
circlet around the mouth (fig. 3, o, o), not having undergone any
increase in size, whilst the visceral disk and the calyx in which it
is lodged have greatly extended. These oral plates finally dis-
appear by absorption ; while the basals are at first concealed by the
great enlargement of the centro-dorsal (which finally extends so far
as to conceal the first radials also), and at last undergo metamor-
phosis into a beautiful ' rosette,' which lies between the cavity of
the centro-dorsal and that of the calyx.— In common with other
members of its Class, the Antedon is represented in its earliest
phase of development by a free-swimming ' larval zooid' or pseudem-
bryo, which was first observed by Busch, but has since been
most carefully studied by Prof. Wyville Thomson. This zooid has
an elongated egg-like form, and is furnished with transverse bands
of cilia, and with a mouth and anus of its own. After a time,
however, rudiments of the calcareous plates forming the stem and
calyx begin to show themselves in its interior; a disk is then
formed at the posterior extremity, by which it attaches itself to a
Sea-weed (very commonly Laminaria), Zoophyte, or Polyzoary;
the calyx, containing the true stomach, with its central mouth
surrounded by tentacles, is gradually evolved ; and the sarcodic
substance of the pseudembryo, by which this calyx and the rudi-
mentary stem were originally invested, gradually shrinks, until the
young Pentacrinoid presents itself in its characteristic form and
proportions.*
* See Prof. Wyville Thomson's Memoir ' On the Development of Antedon
rosaceus' in the " Philos. Transact." for 1865, p. 513. — The Pentacrinoid Larvse
of Antedon have been found abundantly at Millport, on the Clyde, and in
Lamlash Bay, Arran ; in Kirkwall Bay, Orkney; in Lough Strangford, near
Belfast, and in the Bay of Cork ; and at Ilfracombe, and in Salcombe Bay,
Devon.
CHAPTEE XIII.
POLYZOA AND TUNICATA.
506. At the lower extremity of the great series of Molluscous
animals, we find two very remarkable groups, whose mode of life
has much in common with Zoophytes, whilst their type of struc-
ture is conformable in essential particulars to that of the true
Mollusks. These animals are for the most part microscopic in
their dimensions ; and as some members of both these groups are
found on almost every coast, and are most interesting objects for
anatomical examination as well as for observation in the living
state, a brief general account of them will be here appropriate.
507. Polyzoa. — The group which is known under this name to
British naturalists, corresponds with that which by Continental
Zoologists is designated Bryozoa: the former name (though first
used in the singular instead of the plural number), as having been
introduced by Mr. J. Y. Thompson in a memoir published in 1830,
seems to have precedence in point of time over the latter, which
was conferred by Prof. Ehrenberg in 1831 on a most heterogeneous
group, wherein the Bryozoa, as now limited, were combined with
the Foraminifera. It has been entirely by Microscopic research
that the Polyzoa have been raised from the class of Zoophytes (in
which they were formerly ranked, for the most part in apposition
with the Hydrozoa), to the Molluscan sub-kingdom; whilst the
Foraminifera have been remitted, by the more careful study of
their living forms, to the very lowest division of the Animal
kingdom. — The animals of the Polyzoa, in consequence of their
universal tendency to multiplication by gemmation, are seldom or
never found solitary, but form clusters or colonies of various kinds ;
and as each is enclosed in either a horny or a calcareous sheath or
' cell,' a composite structure is formed, closely corresponding with
the 'polypidom' of a Zoophyte, which has been appropriately
designated the polyzoary. The individual cells of the polyzoary
are sometimes only connected with each other by their common
relation to a creeping stem or stolon, as in Laguncula (Plate XXII.) ;
but more frequently they bud-forth directly, one from another, and
extend themselves in different directions over plane surfaces, as
is the case with Flustrce, Lepralice, &c. (Fig. 326) ; whilst not
unfrequently the polyzoary developes itself into an arborescent
STRUCTURE OF POLYZOA.
617
structure (Fig. 327), which may even present somewhat of the
density and massiveness of the Stony Corals. Each individuals
designated as a polypide or polype-like animal, is composed ex-
ternally of a sort of sac, of which the outer or tegumentary layer is
Fig. 326.
w
Cells of LepraUce: — A, L. Hyndmanni ; B, L.figulariSi c, L. verrucosa.
either simply membranous, or is horny, or in some instances
calcified, so as to form the cell ; this investing sac is lined by a
more delicate membrane, which closes its orifice, and which then
becomes continuous with the wall of the alimentary canal ; this
lies freely in the visceral sac, floating (as it were) in the liquid
which it contains.
508. The principal features in the structure of this group will
be best understood from the examination of a characteristic ex-
ample, such as the Laguncula repens ; which is shown in the state
of expansion at a, Plate XXII., and in the state of contraction at
b and c. The mouth is surrounded by a circle of tubular tentacles,
which are clothed with vibratile cilia ; these tentacles, in the species
we are considering, vary from ten to twelve in number, but in some
other instances they are more numerous. By the ciliary invest-
ment of the tentacles, the Polyzoa are at once distinguishable from
those Hydroid polypes to which they bear a superficial resemblance,
and with which they were at one time confounded ; and accordingly,
whilst still ranked among the Zoophytes, they were characterized
as ciUobracliiate. The tentacula are seated upon an annular disk,
which is termed the lophopliore, and which forms the roof of the
618 STRUCTURE OF POLYZOA.
visceral or perigastric cavity ; and this cavity extends itself into
the interior of the tentacula, through perforations in the lopho-
phore, as is shown at d, Plate XXIL, representing a portion of the
tentacular circle on a larger scale, a a being the tentacula, b b their
internal canals, c the muscles of the tentacula, d the lophophore,
and e its retractile muscles. The mouth, situated in the centre of
the lophophore, as shown at a, leads to a funnel-shaped cavity or
pharynx, b, which is separated from the oesophagus, d, by a valve
at c ; and this oesophagus opens into the stomach, e, which occupies
a considerable part of the visceral cavity. (In the Bowerbankia,
and some other Polyzoa, a muscular -stomach or gizzard for the
trituration of the food intervenes between the oesophagus and the
true digestive stomach.) The walls of the stomach, h, have con-
siderable thickness; and they are beset with minute follicles, which
seem to have the character of a rudimentary liver. This, however,
is more obvious in some other members of the group. The stomach
is lined, especially at its upper part, with vibratile cilia, as seen at
c, g ; and by the action of these the food is kept in a state of
constant agitation during the digestive process. From the upper
part of the stomach, which is (as it were) doubled upon itself, the
intestine i opens, by a pyloric orifice, /, which is furnished with a
regular valve ; within the intestine are seen at k particles of ex-
crementitious matter, which are discharged by the anal orifice at
I. ]STo special circulating apparatus here exists ; but the liquid
which fills the cavity that surrounds the viscera, contains the
nutritive matter which has been prepared by the digestive opera-
tion, and which has transuded through the walls of the alimentary
canal ; a few corpuscles of irregular size are seen to float in it.
The visceral sacs of the different polypides put forth from the
same stem, appear to communicate with each other. ~No other
respiratory organs exist than the tentacula ; into whose cavity the
nutritive fluid is probably sent from the perivisceral cavity, for
aeration by the current of water that is continually flowing over them.
509. The production of gemmae, or buds may take place either
from the bodies of the polypides themselves, which is what always
happens when the cells are in mutual apposition ; or from the con-
necting stem or ■ stolon' where the cells are distinct one from the
other, as in Laguncula. In the latter case there is first seen a
bud-like protuberance of the horny external integument, into
which the soft membranous lining prolongs itself ; the cavity thus
formed, however, is not to become (as in Hydra and its allies) the
stomach of the new zooid ; but it constitutes the chamber sur-
rounding the digestive viscera, which organs have their origin in a
thickening of the lining membrane, that projects from one side of
the cavity into its interior, and gradually shapes itself into the
alimentary canal with its tentacular appendages. Of the pro-
duction of gemmae from the polypides themselves, the best ex-
amples are furnished by the Flustrae and their allies. From a
single cell of the Flustra, five such buds may be sent-off, which
PLATE XXII.
ewrir.
\ To face p. 818.
POLYZOA: — LAGUNCULA. 619
develope themselves into new p0l3rpid.es around it ; and these, in
their turn, produce buds from their unattached margins, so as
rapidly to augment the number of cells to a very large amount.
To this extension there seems no definite limit ; and it often
happens that the cells in the central portion of the leaf-like
expansion of a Flustra are devoid of contents and have lost their
vitality, whilst the edges are in a state of active growth. — In-
dependently of their propagation by gemmation, the Polyzoa have
a true sexual generation ; the sexes, however, being nsually, if not
invariably, nnited in the same polypides. The sperm-cells are
developed in a glandular body, the testis m, which lies beneath the
base of the stomach ; when mature, they rupture, and set free the
spermatozoa q q, which swim freely in the liquid of the visceral
cavity. The ova, on the other hand, are formed in an ovarium n,
which is lodged in the membrane lining the tegumentary sheath
near its outlet ; the ova, having escaped from this into the visceral
cavity, as at 0, are fertilized by the spermatozoa which they there
meet with ; and are finally discharged by an outlet at p, beneath
the tentacular circle.
510. These creatures possess a considerable number of muscles,
by which their bodies may be projected from their sheaths, or drawn
within them ; of these muscles, r, s, t, u, v, w, v, the direction
and points of attachment sufficiently indicate the uses ; they are
for the most part retractors, serving to draw-in and double-up the
body, to fold-together the circle of tentacnla, and to close the aper-
ture of the sheath, when the animal has been completely withdrawn
into its interior. The projection and expansion of the animal, on
the contrary, appear to be chiefly accomplished by a general
pressure upon the sheath, which will tend to force-out all that can
be expelled from it. The tentacles themselves are furnished with
distinct muscular fibres, by which their separate movements seem
to be produced. At the base of the tentacular circle, just above
the anal orifice, is a small body (seen at a, a), which is a nervous
ganglion ; as yet no branches have been distinctly seen to be con-
nected with it in this species ; but its character is less doubtful
in some other Polyzoa. — Besides the independent movements of the
individual polypides, other movements may be observed, which are
performed by so many of them simultaneously as to indicate the
existence of some connecting agency ; and such connecting agency
has lately been detected by Dr. Fritz Muller,* who has discovered
what he terms a ' colonial-nervous system' in a Serialaria having
a branching polyzoary that spreads itself on sea-weeds over a space
of three or four inches. A nervous ganglion may be distinguished
at the origin of each branch, and another ganglion at the origin of
each polypide-bud ; and all these ganglia are connected together,
not merely by principal trunks, but also by plexuses of nerve-fibres,
* See his Memoir in "Wiegmann's Arcniv.." 1860, p. 311; translated in
•' Quart. Journ. of Microsc. Science," New Ser., Vol. i. (1861), p. 300.
620 STRUCTURE OE EOLYZOA.
which may be distinctly made-out with the aid of Chromic acid in
the cylindrical joints of the polyzoary.
511. Of all the Polyzoa of our own coasts, the Flustrce or ' sea-
mats' are the most common ; these present flat expanded surfaces,
resembling in form those of many sea-weeds (for which they are
often mistaken), but exhibiting when viewed even with a low
magnifying power, a most beautiful network, which at once indi-
cates their real character. The cells are arranged on both sides ;
and it was calculated by Dr. Grant, that as a single square inch of
an ordinary Mustra contains 1800 such cells, and as an average
specimen presents about 10 square inches of surface, it will consist
of no fewer than 18,000 polypides. The want of transparence in the
cell-wall, however, and the infrequency with which the animal
projects its body far beyond the mouth of the cell, render the Polyzoa
of this genus less favourable subjects for microscopic examination
than are those of the Boiverbanhia, a Polyzoon with a trailing stem
and separated cells like those of Laguncula, which is very commonly
found clustering around the base of masses of Mustra?. It was in
this that many of the details of the organization of the interesting
group we are considering were first studied by Dr. A. Farre, who
discovered it in le.37, and subjected it to a far more minnte ex-
amination than any Polyzoon had previously received ;* and it is
one of the best-adapted of all the marine forms yet known, for the
display of the beauties and wonders of this type of organization. —
The Halodactylus (formerly called Alcyonidium), however, is one
of the most remarkable of all the marine forms for the comparatively
large size of the tentacular crowns ; these, when expanded, being
very distinctly visible to the naked eye, and presenting a spectacle
of the greatest beauty when viewed under a sufficient magnifying
power. The polyzoary of this genus has a spongy aspect and texture,
very much resembling that of certain Alcyonian Zoophytes (§ 487),
for which it might readily be mistaken when its contained
animals are all withdrawn into their cells ; when these are expanded,
however, the aspect of the two is altogether different, as the
minute plumose tufts which then issue from the surface of the
Halodactylus, making it look as if it were covered with the most
delicate downy film, are in striking contrast with the larger, solid-
looking polypes of the Alcyonium. The opacity of the poly-
zoary of the Halodactylus renders it quite unsuitable for the
examination of anything more than the tentacular crown and the
oesophagus which it surmounts ; the stomach and the remainder of
the visceral apparatus being always retained within the cell. It
furnishes, however, a most beautiful object for the Binocular
Microscope, when mounted with all its polypides expanded, in the
manner described in § 478. — Several of the fresh-water Polyzoa
are peculiarly interesting subjects for Microscopic examination ;
* See his Memoir ' On the Minute Structure of some of the higher forms of
Polypi,' in the "Philosophical Transactions" for 18137.
INFUNDIBULATE OE MAEINE POLYZOA. 621
alike on account of the remarkable distinctness with which the
various parts of their organization may be seen, and the very
beautiful manner in which their ciliated tentacula are arranged
upon a deeply-crescentic or horseshoe-shaped lophophore. By this
peculiarity the fresh- water Polyzoa are separated as a distinct sub-
class from the marine ; the former being designated as Hippo-
crepia (horseshoe-like), while the latter are termed Infundibulata
(funnel-like) .
512. The Infundibulata or Marine Polyzoa, constituting by far
the most numerous division of the class, are divided into four
Orders, as follows : — 1. Gheilostomata, in which the mouth of the
cell is sub-terminal, or not quite at its extremity (Fig. 326), is some-
what crescentic in form, and is furnished with a moveable (gene-
rally membranous) Up, which closes it when the animal retreats.
This includes a large part of the species that most abound on our
own coasts, notwithstanding their wide differences in form and
habit. Thus the polyzoaries of some (as Flustra) are horny and
flexible, whilst those of . others (as Eschara and Betepora) are so
penetrated with calcareous matter as to be quite rigid ; some grow
as independent plant-like structures (as Bugula and Gemellaria),
whilst others, having a like arborescent form, creep over the sur-
faces of rocks or stones (as Hippothoa) ; and others, again, have
their cells in close apposition, and form crusts which possess no
definite figure (as is the case with Lepralia and Membranipora).
— ii. The second order, Cyclostomata, consists of those Polyzoa
which have the mouth at the termination of tubular calcareous cells,
without any moveable appendage or lip (Fig. 327). This includes
a comparatively small number of genera, of which Crisia and Tubu-
Upora contain the largest proportion of the species that occur on
our own coasts. — in. The distinguishing character of the third
order, Gtenosomata, is derived from the presence of a comb-like
circular fringe of bristles, connected by a delicate membrane,
around the mouth of the cell, when the animal is projected from
it ; this fringe being drawn -in when the animal is retracted. The
Polyzoaries of this group are very various in character, the cells
being sometimes horny and separate (as in Laguncula and Bower-
bankia), sometimes fleshy and coalescent (as in Halodactylus). —
iv. In the fourth order, Pedicellinece, which includes only a single
genus, Pedicellina, the lophophore is produced upwards on the
back of the tentacles, uniting them at their base in a sort of
muscular calyx, and giving to the animal when expanded somewhat
the form of an inverted bell, like that of Vorticella (Fig. 257). —
The cells of the Hippocrepia or fresh-water Polyzoa are for the
most part lodged in a sort of gelatinous substratum, which spreads
over the leaves of aquatic plants, sometimes forming masses of
considerable size ; but in the very curious and beautiful Cristatella,
the polyzoary is unattached, so as to be capable of moving freely
through the waters. — As the Polyzoa altogether resemble Hydroid
ZoojDhytes in their habits, and are found in the same localities, it is
622 AVICULARIA AND VIBBACULA OF POLYZOA.
not requisite to add anything to what has already been said (§§ 478,
479), respecting the collection, examination, and mounting, of this
very interesting class of objects.*
513. A large proportion of the Polyzoa of the first Order are
furnished with very peculiar motile appendages, which are of two
kinds, avicularia and vihracula. The avicnlaria or ' bird's-head
processes,' so named from the striking resemblance they present
to the head and jaws of a bird (Fig. 327, b), are generally
' sessile' upon the angles or margins of the cells, that is, are
attached at once to them, without the intervention of a stalk, as in
Fig. 327, a, being either 'projecting' or 'immersed;' but in the
genera Bugula and Bicellaria, where they are present at all, they
are ' pedunculate,' or mounted on footstalks (b). Under one form
or the other, they are wanting in but few of the genera belonging
to this order ; and their presence or absence furnishes valuable
characters for the discrimination of species. Each avicularium has
two ' mandibles,' of which one is fixed, like the upper jaw of a
bird, the other moveable, like its lower jaw ; the latter is opened
and closed by two sets of muscles which are seen in the interior of
the ' head ;' and between them is a peculiar body, furnished with
a pencil of bristles, which is probably a tactile organ, being brought
forwards when the mouth is open, so that the bristles project
beyond it, and being drawn-back when the mandible closes. The
avicularia keep-up a continual snapping action during the life of
the polyzoary ; and they may often be observed to lay hold of
minute Worms or other bodies, sometimes even closing upon the
beaks of adjacent organs of the same kind, as shown in Fig. 327, b.
In the pedunculate forms, besides the snapping action, there is
a continual rhythmical nodding of the head upon the stalk ; and
few spectacles are more curious than a jDortion of the polyzoary of
Bugula avicularia (a very common British species) in a state
of active vitality, when viewed under a power sufficiently low to
allow a number of these bodies to be in sight at once. It is still
very doubtful what is their precise function in the economy of the
animal ; whether it is to retain within the reach of the ciliary
current bodies that may serve as food ; or whether it is, like the
Pedicellariss of Echini (§ 493), to remove extraneous particles that
may be in contact with the surface of the polyzoary. The latter
would seem to be the function of the vibr acuta, which are long
bristle-shaped organs (Fig. 326, a), each one springing at its base
out of a sort of cup that contains muscles by which it is kept in
* For a more detailed account of the Structure and Classification of this
group, see Prof. Van Beneden's lBecherches sur les Bryozoaires de la Cote
d'Ostende,' in "Me"m. de l'Acad. Boy. de Bruxelles," torn. xvii. ; Mr. G. Busk's
" Catalogue of the Marine Polyzoa in the Collection of the British Museum ;"
Mr. Huxley's ; Note on the Beproductive Organs of the Cheilostome Polyzoa,'
in " Quart. Journ. of Microsc. Sci.," Vol. iv. p. 191 ; Dr. G. Johnson's " History
of British Zoophytes;" and Prof. Mman's beautiful "Monograph of the
British Fresh-water Polyzoa," published by the Bay Society, 1857.
POLTZO A. — TUNIC ATA.
623
almost constant motion, sweeping slowly and carefully over the
surface of the polyzoary, and removing what might be injurious
to the delicate inhabitants of the cells when their tentacles are
Fig. 327.
A, Portion of CelhiJarla ciliata, enlarged ; B, one of the
'bird's-head ' processes of Bttgula avicularia, more highly
magnified, and seen in the act of grasping another.
protruded. Out of 191 species of Cheilostomatous Polyzoa de-
scribed by Mr. Busk, no fewer than 126 are furnished either with
Avicularia, or with Yibracula, or with both these organs.*
514. Tunic at a. — The Tunicated Mollusca are so named from the
enclosure of their bodies in a ' tunic,' which is sometimes leathery
or even cartilaginous in its texture, and which very commonly
includes calcareous spicules, whose forms are often very beautiful.
They present a strong resemblance to the Polyzoa, not merely in
their general plan of conformation, but also in their tendency to
produce composite structures by gemmation ; they are differ-
entiated from them, however, by the absence of the ciliated ten-
tacles which form so conspicuous a feature in the external aspect of
* See Mr. G. Busk's 'Eemarks on the Structure and Function of the Avieu-
larian and Vibracular Organs of Polyzoa,' in " Transact, of Microsc. Soc,"
Ser. 2, Vol. ii. (1854), p. 26.
624 STEUCTUEE OF TUNICATA.
the Polyzoa, by the presence of a distinct circulating apparatus,
and "by their peculiar respiratory apparatus, which may be re-
garded as a dilatation of their pharynx. In their habits, too, they
are for the most part very inactive, exhibiting scarcely anything
comparable to those rapid movements of expansion and retraction
which it is so interesting to watch among the Polyzoa ; whilst,
with the exception of the Salpi ce and other floating species which
are chiefly found in seas warmer than those that surround our
coast, and the curious Appendicular ia to be presently noticed
(§ 519), they are rooted to one spot during all but the earliest
period of their lives. The larger forms of the Ascidian group,
which constitutes the bulk of the class, are always solitary ; either
not propagating by gemmation at all, or, if this process does
take place, the gemrnas being detached before they have advanced
far in their development. — Although of special importance to the
Comparative Anatomist and the Zoologist, this group does not
afford much to interest the ordinary Microscopist, except in the
peculiar actions of its respiratory and circulatory apparatus. In
common with the composite forms of the group, the solitary
Ascidians have a large branchial sac, with fissured walls, resem-
bling that shown in Figs. 328 and 330 ; into this sac water is
admitted by the oral orifice, and a large proportion of it is caused
to pass through the fissures, by the agency of the cilia with which
they are fringed, into a surrounding chamber, whence it is expelled
through the anal orifice. This action may be distinctly watched
through the external walls in the smaller and more transparent
species ; and not even the ciliary action of the tentacles of the
Polyzoa affords a more beautiful spectacle. It is peculiarly
remarkable in one species that occurs on our own coasts, the
Ascidia parattelogramma* in which the wall of the branchial sac
is divided into a number of areolse, each of them shaped into
a shallow funnel ; and round one of these funnels each branchial
fissure makes two or three turns of a spiral. When the cilia of all
these spiral fissures are in active movement at once, the effect
is most singular. — Another most remarkable phenomenon pre-
sented throughout the group, and well seen in the solitary Ascidian
just referred-to, is the alternation in the direction of the Circula-
tion. The heart, which lies at the bottom of the branchial sac, is
composed of two chambers imperfectly divided from each other ;
one of these is connected with the principal trunk leading to the
body, and the other with that leading to the branchial sac. At
one time it will be seen that the blood flows from the respiratory
apparatus to the cavity of the heart in which its trunk terminates,
which then contracts so as to drive it into the other cavity, which
in its turn contracts and propels it through the systemic trunk
to the body at large ; but after this course has been maintained for
* See Alder in "Ann. of Nat. Hist.," 3rd Ser., Vol. xi. (1868), p. 157; and
Hancock in " Journ. of Linn. Soc," Vol. ix. p. 333.
ALTERNATING: CIRCULATION:— COMPOUND ASCLDIANS. 625
a time, the heart ceases to pulsate for a moment or two, and the
course is reversed, the blood flowing into the heart from the body
generally, and being propelled to the branchial sac. After this
reversed course has continued for some time, another pause occurs,
and the first course is resumed. The length of time intervening
between the changes does not seem by any means constant. It is
usually stated at from half-a-minute to two minutes in the com-
posite forms ; but in ' the solitary Ascidia parallelo gramma (a
species very common in Lamlash Bay, Arran), the Author has
repeatedly observed an interval of from five to fifteen minutes,
and in some instances he has seen the circulation go-on for half-
an-hour or even longer without change.
510. The Compound Ascidians are very commonly found adherent
to Sea-weeds, Zoophytes, and stones between the tide-marks ; and
they present objects of great interest to the Microscopist, since
the small size and transparence of their bodies, when they are
detached from the mass in which they are imbedded, not only
enables their structure to be clearly discerned without dissection,
but allows many of their living actions to be watched. Of these
we have a characteristic example in Amaroucium proliferum ; of
which the form of the composite mass and the anatomy of a single
individual are displayed in Fig. 328. Its clusters appear almost
completely inanimate, exhibiting no very obvious movements when
irritated ; but if they be placed when fresh in sea-water, a slight
pouting of the orifices will soon be perceptible, and a constant and
energetic series of currents will be found to enter by one set and to
be ejected by the other, indicating that all the machinery of active
life is going-on within these apathetic bodies. In the tribe of
Polyclinians to which this genus belongs, the body is elongated,
and may be divided into three regions, the thorax (a) which
is chiefly occupied by the respiratory sac, the abdomen (b) which
contains the digestive apparatus, and the post-abdomen (c) in
which the heart and generative organs are lodged. At the summit
of the thorax is seen the oral orifice c, which leads to the branchial
sac e ; this is perforated by an immense number of slits, which
allow part of the water to pass into the space between the
branchial sac and the muscular mantle, where it is especially col-
lected in the thoracic sinus /. At k is seen the oesophagus, which
is continuous with the lower part of the pharyngeal cavity ; this
leads to the stomach I, which is surrounded by biliary follicles ;
and from this passes-off the intestine m, which terminates at n in
the cloaca, or common vent. A current of water is continually
drawn-in through the mouth by the action of the cilia of the
branchial sac and of the alimentary canal ; a part of this current
passes through the fissures of the branchial sac into the thoracic
sinus, and thence into the cloaca ; whilst another portion, entering
the stomach by an aperture at the bottom of the pharyngeal sac,
passes through the alimentary canal, giving up any nutritive
materials it may contain, and carrying away with it any excremen-
s s
626
COMPOUND ASCIDIAXS.
titious matters to be discharged ; and this having met the respira-
tory current in the cloaca, the two mingled currents pass forth to-
gether by the anal orifice i. The long post-abdomen is principally
occupied by the large ovarium, p, which contains ova in various
Aj
nfc^
fe^
Fig. 328.
Compound mass of Amaroucium proliferam,
with the anatomy of a single zooid : — A, thorax;
B, abdomen ; c, post-abdomen : — c, oral orifice ;
e, branchial sac ; f, thoracic sinus; i, anal orifice ;
*', projection overhanging it ; j, nervous gan-
glion ; Jc, oesophagus ; I, stomach surrounded by
biliary tubuli; m, intestine; n, termination of
intestine in cloaca ; o, heart ; o', pericardium ; p7
ovarium ; p', egg ready to escape ; <?, testis ; r,
spermatic canal ; r', termination of this canal in
the cloaca.
stages of development. These, when
matured and set-free, find their way into
the cloaca ; where two large ova are seen
(one marked^/, and the other immediately
below it) waiting for expulsion. In this
position they receive the fertilizing in-
fluence from the testis, q, which discharges
its products hj the long spermatic canal,
r, that opens into the cloaca, r\ At the
very bottom of the post-abdomen we find
the heart o, enclosed in its pericardium,
o'. — In the group we are now considering,
a number of such animals are imbedded
together in a sort of gelatinous mass,
and covered with an integument common
to them all ; the composition of this
gelatinous substance is remarkable as
including cellulose, which generally ranks
as a Yegetable product. The mode in
which new individuals are developed in
this mass, is by the extension of stolons
AMAEOUCIUM :— BOTEYLLUS.
627
or creeping stems from the bases of those previously exist-
ing ; and from each of these stolons several bnds may be put-
forth, every one of which may evolve itself into the likeness of the
stock from which it proceeded, and may in its tnrn increase and
multiply after the same fashion. A communication between the
circulating systems of the different individuals is kept-up, through
their connecting stems, during the whole of life; and thus their
relationship to each other is somewhat like that of the several
polypes on the polypidom of a Campanularia (§ 476).
516. In the family of JJidemnians the post-abdomen is absent,
the heart and generative apparatus being placed by the side of the
intestine in the abdominal portion of the body. The zooids are
frequently arranged in star-shaped clusters, their anal orifices being
all directed towards a common vent which occupies the centre. —
This shortening is still more remarkable, however, in the family of
Fig. 32U.
Botryllus violaceus : — A, cluster on the surface of a Fucus : — B, portion
of the same enlarged.
BotrylMans, whose beautiful stellate gelatinous incrustations are
extremely common upon Sea-weeds and submerged rocks (Fig. 329).
The anatomy of these animals is very similar to that of the
Amaroucmm already described ; with this exception, that the
body exhibits no distinction of cavities, all the organs being brought
together in one, which must be considered as thoracic. In this
respect there is an evident approximation towards the solitary
species.
517. This approximation is still closer, however, in the ' social'
Ascidians, or Clavellimdce ; in which the general plan of structure
is nearly the same, but the zooids are simply connected by their
stolons (Fig. 330) instead of being included in a common invest-
ment ; so that their relation to each other is very nearly the same
ss 2
62S
SOCIAL ASCIDIANS.
as that of the polypides of Lagunculd (§ 508), the chief difference
being that a regular circulation takes-place through the stolon in
the one case, such as has no existence in the other A better
opportunity of studying the living actions of the Ascidians can
scarcely be found, than that which is afforded by the genus Pero-
phora, first discovered by Mr. Lister ; which occurs not unfre-
quently on the south coast of England and in the Irish Sea, living
attached to Sea-weeds, and looking like an assemblage of minute
globules of jelly, dotted with orange and brown, and linked by a
silvery winding thread. The isolation of the body of each zooid
from that of its fellows, and the extreme transparence of its tunics,
not only enable the movements of fluid within the body to be dis-
tinctly discerned, but also allow the action of the cilia that border
Fig. 330.
"t
A, Group of Perophora (enlarged), growing from a common stalk :—
B, single Perophora; a, test; 6, inner sac; c, branchial sac, attached to
the inner sac along the line c' c' ; e e, finger-like processes projecting
inwards ; /, cavity between test and internal coat ; /', anal orifice or
funnel ; g\ oral orifice ; g', oral tentacnla ; ft, downward stream of food ;
ft', oesophagus ; i, stomach ; Jc, vent ; I, ovaiy (?) ; n, vessels connect-
ing the circulation in the body with that in the stalk.
the slits of the Eespiratory sac to be clearly made-out. This sac
is perforated with four rows of narrow oval^ openings, through
which a portion of the water that enters its oral orifice {g)
escapes into the space between the sac and the mantle, and is
PEEOPHORA. — DEVELOPMENT OF ASCIDIANS. 629
thus discharged immediately by the anal funnel (/). Whatever
little particles, animate or inanimate, the current of water brings,
flow into the sac, unless stopped at its entrance by the tentacles
(g1), which do not appear fastidious. The particles which are
admitted usually lodge somewhere on the sides of the sac, and then
travel horizontally until they arrive at that part of it down which
the current proceeds to the entrance of the stomach (i), which is
situated at the bottom of the sac. Minute animals are often
swallowed alive, and have been observed darting about in the
cavity for some days, without any apparent injury either to them-
selves or to the creature which encloses them. In general, how-
ever, particles which are unsuited for reception into the stomach
are ejected by the sudden contraction of the mantle (or muscular
tunic), the vent being at the same time closed, so that they are
forced-out by a powerful current through the oral orifice. The
curious alternation of the circulation that is characteristic of the
Class generally (§ 504), may be particularly well studied in Pero-
phora. The creeping- stalk (Fig. 330) that connects the individuals
of any group, contains two distinct canals, which send-off branches
into each peduncle. One of these branches terminates in the
heart, which is nothing more than a contractile dilatation of the
principal trunk ; this trunk subdivides into vessels (or rather
sinuses, which are mere channels not having proper walls of their
own), of which some ramify over the respiratory sac, branching
off at each of the passages between the oval slits, whilst others are
first distributed to the stomach and intestine, and to the soft
surface of the mantle. All these reunite so as to form a trunk,
which passes to the peduncle and constitutes the returning branch.
Although the circulation in the different bodies is brought into
connection by the common stem, yet that of each is indepen-
dent of the rest, continuing when the current through its own
footstalk is interrupted by a ligature ; and the stream which
returns from the branchial sac and the viscera is then poured
into the posterior part of the heart, instead of entering the
peduncle.
518. The development of the Ascidians, the early stages of which
are observable whilst the ova are still within the cloaca of the
parent, presents some phenomena of much interest to the Micro-
scopist, After the ordinary repeated segmentation of the yolk,
whereby a ' mulberry mass' is produced (§ 540), a sort of ring is
seen encircling its central portion ; but this soon shows itself as a
tapering tail-like prolongation from one side of the yolk, which
gradually becomes more and more detached from it, save at the
part from which it springs. Either whilst the egg is still within
the cloaca, or soon after it has escaped from the vent, its envelope
bursts, and the larva escapes ; and in this condition it presents
very much the appearance of a tadpole, the tail being straightened
out, and propelling the body freely through the water by its
lateral strokes. The centre of the body is occupied by a mass of
630 DEVELOPMENT OF ASCIDIANS.
liquid yolk ; and this is continued into the interior of three pro-
longations which extend themselves from the opposite extremity,
each terminating in a sort of sucker. After swimming-about for
some hours with an active wriggling movement, the larva attaches
itself to some solid body by means of one of these suckers ; if dis-
turbed from its position, it at first swims about as before ; but it
soon completely loses its activity, and becomes permanently
attached ; and important changes manifest themselves in its
interior. The prolongations of the central yolk-substance into the
anterior processes and tail are gradually drawn back, so that the
whole of it is concentrated into one mass ; and the tail, now con-
sisting only of the gelatinous envelope, is either detached entire
from the body by the contraction of the connecting portion, or
withers, and is thrown-off gradually in shreds. The shaping of
the internal organs out of the yolk-mass takes-place very rapidly,
so that by the end of the second day of the sedentary state the
outlines of the branchial sac and of the stomach and intestine may
be traced ; no external orifices, however, being as yet visible. The
pulsation of the heart is first seen on the third day, and the forma-
tion of the branchial and anal orifices takes-place on the fourth ;
after which the ciliary currents are immediately established through
the branchial sac and alimentary canal. — The embryonic develop-
ment of other Ascidians, solitary as well as composite, takes-place
on a plan essentially the same as the foregoing, a, free tadpole-like
larva being always produced in the first instance.*
519. This larval condition is represented in a very curious adult
free-swimming form, termed Appendicidaria, which is frequently
to be taken with the Tow-net on our own coasts. This animal has
an oval or flask-like body, which in large specimens attains the
length of one-fifth of an inch, but which is often not more than
one-fourth or one-fifth of that size. It is furnished with a tail-
like appendage three or four times its own length, broad, flattened,
and rounded at its extremity ; and by the powerful vibrations of
this appendage it is propelled rapidly through the water. The
structure of the body differs greatly from that of the Ascidians, its
plan being much simpler ; in particular, the pharyngeal sac is
entirely destitute of ciliated branchial fissures opening into a sur-
rounding cavity ; but two canals, one on either side of the entrance
to the stomach, are prolonged from it to the external surface ; and
by the action of the long cilia with which these are furnished, in
conjunction with the cilia of the branchial sac, a current of water
is maintained through its cavity. From the observations of Prof.
Huxley, however, it appears that the direction of this current is
by no means constant ; since, although it usually enters by the
* For more special information respecting the Compound Ascidians, see
especially the admirable Monograph of Prof. Milne-Edwards on that group ;
Mr. Lister's Memoir l On the Structure and Functions of Tubular and Cellular
Polypi, and of Ascidiee,' in the " Philos. Transact.," 1834 ; and the Art. Tuni-
cata, in the " Cyclopaedia of Anatomy and Physiology."
APPENDICULAR^. 631
mouth and passes-out by the ciliated canals, it sometimes enters by
the latter and passes-out by the former. The caudal appendage
has a central axis, above and below which is a riband-like layer
of muscular fibres ; a nervous cord, studded at intervals with
minute ganglia, may be traced along its whole length. — By Mertens,
one of the early observers of this animal, it was said to be fur-
nished with a peculiar gelatinous envelope or Haus (house), very
easily detached from the body, and capable of being re-formed after
haviug been lost. Notwithstanding the great numbers of speci-
mens which have been studied by Miiller, Huxley, Leuekart, and
Gegenbaur, neither of these excellent observers ha s met with this
appendage ; but it has been recently seen by Prof. Allman, who
describes it as an egg-shaped gelatinous mass, in which the body is
imbedded, the tail alone being free ; whilst from either side of the
central plane there radiates a kind of double fan, which seems to
be formed by a semicircular membranous lamina folded upon
itself. It is surmised by Prof. Allman, with much probability,
that this curious appendage is ' nidamental,' having reference to
the development and protection of the young ; but on this point
further observations are much needed ; and any Microscopist, who
may meet with Appendicularia furnished with its 'house,' should
do all he can to determine its structure and its relations to the
body of the animal.*
* For details in respect to the structure of Appendicular ia, see Huxley, in
"Philos. Transact." for 1851, and in " Quart. Journ. of Microsc. Science," Vol. iv.
(1856), p. 181 ; also Allman in the same journal, Vol. vii. (1859), p. 86 ; Gegen-
baur in Siebold and Kolliker's "Zeitschrift," Bd. vi. (1855), p. 406; and Leuck-
art's " Zoologische Untersuchungen." Heft ii., 1854 — For the Tunicata gene-
rally, see Prof. T. Paipert Joues, in VoL iv. of the " Cyclop, of Anatomy and
Physiology;" Mr. Alder's ' Observations on the British Tunicata,' in "Ann. of
Nat. Hist./' Ser. 4, Vol. xi. (1863), p. 153 ; and Mr. Hancock's Memoir ' On the
Anatomy and Physiology of the Tunicata.' in the " Journal of the Linnasan
Society," Vol. ix. p. 309.
CHAPTEE XIV.
MOLLUSCOUS ANIMALS GENERALLY.
520. The various forms of ' Shell-fish/ with their ' naked' or
shelless allies, furnish a great abundance of objects of interest to
the Microscopist ; of which, however, the greater part may be
grouped under three heads : — namely, (1) the structure of the
shell, which is most interesting in the Conchifera and Brachiopoda,
in both of which classes the shells are ' bivalve,' while the animals
differ from each other essentially in general plan of structure ;
(2) the structure of the tongue or palate of the Gasteropoda, most
of which have ' univalve' shells, others, however, being ' naked ;'
(3) the developmental history of the embryo, for the study of which
certain of the Gasteropods present the greatest facilities. — These
three subjects, therefore, will be first treated of systematically ;
and a few miscellaneous facts of interest will be subjoined.
521. Shells of Mollusca. — These investments were formerly
regarded as mere inorganic exudations, composed of calcareous
particles, cemented together by animal glue ; Microscopic examina-
tion, however, has shown that they possess a definite structure,
and that this structure presents certain very remarkable variations
in some of the groups of which the Molluscous series is composed.
— We shall first describe that which may be regarded as the
characteristic structure of the ordinary Bivalves ; taking as a type
the group of Margaritacew, which includes the Avicula or ' pearl-
oyster' and its allies, the common Pinna ranking amongst the
latter. In all these shells we readily distinguish the existence of
two distinct layers ; an external, of a brownish-yellow colour ; and
an internal, which has a pearly or ' nacreous' aspect, and is com-
monly of a lighter hue.
522. The structure of the outer layer may be conveniently
studied in the shell of Pinna, in which it commonly projects beyond
the inner, and there often forms laminse sufficiently thin and
transparent to exhibit its general characters without any artificial
reduction. If a small portion of such a lamina be examined with
a low magnifying power by transmitted light, each of its surfaces
will present very much the appearance of a honeycomb ; whilst
its broken edge exhibits an aspect which is evidently fibrous to the
PKISMATIC SHELL-SUBSTANCE.
633
eye, but which, when examined under the Microscope with reflected
light, resembles that of an assemblage of segments of basaltic
columns (Fig. 433, p). This outer layer is thus seen to be com-
posed of a vast number of prisms, having a tolerably-uniform size,
and usually presenting an approach to the hexagonal shape.
These are arranged perpendicularly (or nearly so) to the surface of
the lamina of the shell;
Fig. 331.
^^fehCCife
so that its thickness is
formed by their length, -.. -
and its two surfaces . ]M
by their extremities. A
more satisfactory view ^
of these prisms is ob- ^|&
tained by grinding-down
a lamina until it pos-
sesses a high degree of
transparence ; and the
prisms are then seen (Fig.
331) to be themselves
composed of a very
homogeneous substance,
but to be separated by
definite and strongly
marked lines of division.
When such a lamina is submitted to the action of dilute acid, so
as to dissolve-away the carbonate of lime, a tolerably firm and con-
sistent membrane is left,
Fig. 332.
f
4B
^y%y--fy^m^K
Section of Shell of Pinna, taken transversely to
the direction of its prisms.
W\
which exhibits the pris-
matic structure just as
perfectly as did the ori-
ginal shell (Fig. 332);
its hexagonal divisions
bearing a stroug resem-
blance to the walls of
the cells of the pith or
bark of a Plant. By
making a section of the
shell perpendicularly to
its surface, we obtain a
view of the prisms cut
in the direction of their
length (Fig. 333) ; and
they are frequently seen
to be marked by delicate transverse striae (Fig. 334), closely resem-
bling those observable on the prisms of the enamel of teeth, to
which this kind of shell-structure may be considered as bearing
a very close resemblance, except as regards the mineralizing ingre-
dient. If a similar section be decalcified by dilute acid, the mem-
branous residuum will exhibit the same resemblance to the walls of
Membranous basis of the same.
634
SHELLS OF MOLLUSKS.
Fig. 333.
prismatic cells viewed longitudinally, and will be seen to be more
or less regularly marked by the transverse strise just alluded to.
It sometimes happens in recent, bnt still more commonly in fossil
shells, that the decay of the animal membrane leaves the contained
prisms without any connect-
ing medium : as they are then
quite isolated, they can be
readily detached one from
another ; and each one may
be observed to be marked by
the like striations, which,
when a sufficiently high mag-
nifying power is used, are
seen to be minute grooves,
apparently resulting from a
^fc&-^fp.:'i™fEj^^f-^^iji]^! thickening of the interme-
1 \ H-I^Pip^'l ^ \ I Hit ! - *3 : ' IIS ; I diate wa^ in those situations.
These appearances seem best
accounted-for by supposing
that each is lengthened by
successive additions at its
base, the lines of junction of which correspond with the transverse
striation ; and this view corresponds well with the fact that the
shell-membrane not unfrequently shows a tendency to split into
Section of the Shell of Pinna, in the
direction of its prisms.
Fig. 334.
7
f\
Oblique Section of Prismatic Shell-substance.
thin laminse along the lines of striation ; whilst we occasionally
meet with an excessively thin natural lamina lying between the
thicker prismatic layers, with one of which it would have probably
coalesced, but for some accidental cause which preserved its
distinctness. That the prisms are not formed in their entire
length at once, but that they are progressively lengthened
and consolidated at their lower extremities, would appear
PRISMATIC SHELL-SUBSTANCE; — NACEE. 635
also from the fact that where the shell presents a deep colour
(as in Pinna nigrina) this colour is usually disposed in distinct
strata, the outer portion of each layer being the part most deeply
tinged, whilst the inner extremities of the prisms are almost
colourless.
523. This 'prismatic' arrangement of the carbonate of lime in the
shells of Pinna and its allies, has been long familiar to Con-
chologists, and regarded by them as the result of crystallization.
When it was first more minutely investigated by Mr. Bowerbank*
and the Author ,f and was shown to be connected with a similar
arrangement in the membranous residuum left after the decalcifica-
tion of the shell-substance by acid, Microscopists generally J agreed
to regard it as a ' calcified' epidermis : the long prismatic cells
being supposed to be formed by the coalescence of the epidermic
cells in piles, and giving their shape to the deposit of carbonate of
lime formed within them. The progress of inquiry, however, has
led to an important modification of this interpretation ; the Author
being now disposed to agree with Prof. Huxley§ in the belief that
the entire thickness of the shell is formed as an excretion from
the surface of the epidermis, and that the horny layer which in
ordinary shells forms their external envelope or ' periostracum,'||
being here thrown out at the same time with the calcifying mate-
rial, is converted into the likeness of a cellular membrane by the
pressure of the prisms that are formed by crystallization at regular
distances in the midst of it. The peculiar conditions under which
calcareous concretions form themselves in an organic matrix, have
been carefully studied by Mr. Rainey ; whose researches (of which
some account will be given hereafter, § 669) are worthy of more
attention than they have received.^"
524. The internal layer of the shells of the ^Iarrjaritacem and
some other families has a 'nacreous' or iridescent lustre, which
depends (as Sir D. Brewster has shown**) upon the striation of
its surface with a series of grooved lines, which usually run
nearly parallel to each other (Fig. 335). As these lines are not
* ' On the Structure of the Shells of Molluscous and Conchiferous Animals,' in
" Transact, of Microsc. Society," 1st Ser. (1844), Vol. i. p. 123.
t 'On the Microscopic Structure of Shells,' in "Keports of British Associa-
tion " for 1844 and 1847.
% See Mr. Quekett's " Histological Catalogue of the College of Surgeons'
Museum," and his " Lectures on Histology," Vol. ii.
§ See his article ' Tegumentary Organs,' in "Cyclopaedia of Anatomy and
Physiology," Supplementary Volume, pp. 489-492.
|| The per lostracum is the yellowish-brown membrane covering the surface of
many shells, which is often (but erroneously) termed their epidermis.
^f See his Treatise " On the Mode of Formation of the Shells of Animals, of
Bone, and of several other structures, by a Process of Molecular Coalescence,
demonstrable in certain artificially-formed Products," 1858.
** "Philosophical Transactions," 1814. — The late Mr. Barton (of the Mint)
succeeded in producing an artificial Iridescence on metallic buttons, by draw-
ing closely-approximating lines with a diamond-point upon the surface of the
steel die by which they were struck.
636
SHELLS OF MOLLUSKS.
obliterated by any amount of polishing, it is obvious that their
presence depends upon something peculiar in the texture of this
substance, and not upon any mere superficial arrangement. "W men
a piece of the nacre (commonly known as 'mother-of-pearl')
of the Avicula or ' pearl-oyster' is carefully examined, it becomes
evident that the lines are produced by the cropping-out of laminae
of shell situated more or less obliquely to the plane of the surface.
The greater the dip of these laminae, the closer will their edges be ;
Fig. 335.
a r?'Y"-' "
I, ;>:,j.: ..,'-';'"'•■' V
HOI
.
/il ■" I '
:J- . ,-
•
," -..'
■
H ."..', i.
Jjfi
#*S
-"-.*y
, ^..,-----"1
■/ ■<--~'-^r7\;r-
■ "^'"''v^'^v^'j-'-'y.
'/-."-■v.. '*' /"V *"""~i/>
>' \K,S^
Section of nacreous lining of Shell of Avicula margarilacea (Pearl-oyster).
whilst the less the angle which they make with the surface, the
wider will be the interval between the lines. When the section
passes for any distance in the plane of a lamina, no lines will
present themselves on that space. And thus the appearance of a
section of nacre is such as to have been aptly compared by Sir J.
Herschel to the surface of a smoothed deal board, in which the
woody layers are cut perpendicularly to their surface in one part,
and nearly in their plane in another. Sir D. Brewster (loc. cit.)
appears to have supposed that nacre consists of a multitude of
layers of carbonate of lime alternating with animal membrane ;
and that the presence of the grooved lines on the most highly-
polished surface is due to the wearing away of the edges of the
animal laminae, whilst those of the hard calcareous lamina? stand
out. If each line upon the nacreous surface, however, indicates a
distinct layer of shell-substance, a very thin section of ' mother-of-
pearl' ought to contain many hundred laminae, in accordance with
the number of lines upon its surface ; these being frequently no
more than l-7500th of an inch apart.* But when the nacre is
MAKGAKITACE.E, UNIONID^, OSTKACE.E, ETC. 637
treated with dilute acid so as to dissolve its calcareous portion, no
such repetition of membranous layers is to be found ; on the contrary,
if the piece of nacre be the product of one act of shell-formation,
there is but a single layer of membrane. This layer, however, is
found to present a more or less folded or plaited arrangement ; and
the lineation of the nacreous surface may perhaps be thus
accounted for. — A similar arrangement is found in pearls ; which are
rounded concretions projecting from the inner surface of the shell of
Avicula, and possessing a nacreous structure corresponding to that
of ' mother-of-pearl.' Such concretions are found in many other
shells, especially the fresh-water mussels, Unio and Anodon ; but
these are usually less remarkable for their pearly lustre, and when
formed at the edge of the valves, they may be partly or even
entirely made up of the prismatic substance of the external layer,
and may be consequently altogether destitute, of the pearly
character.
525. In all the genera of the Margaritacece, we find the external
layer of the shell prismatic, and of considerable thickness ; the
internal layer being nacreous. But it is only in the shells of a few
families of Bivalves, that the combination of organic with mineral
components is seen in the same distinct form ; and these families
are for the most part nearly allied to Pinna. In the Unioriidce (or
' fresh-water mussels'), nearly the whole thickness of the shell is
made-up of the internal or ' nacreous' layer ; but a uniform
stratum of prismatic substance is always found between the nacre
and the periostracum, really constituting the inner layer of the
latter, the outer being simply horny. — In the Ostracece (or oyster
tribe) also, the greater part of the thickness of the shell is com-
posed of a ' sub-nacreous' substance (§ 527) representing the inner
layer of the shells of Margaritacese, its successively -formed laminaB,
however, having very little adhesion to each other ; and every one
of these laminaB is bordered at its free edge by a layer of the
prismatic substance, distinguished by its brownish-yellow coloui.
In these and some other cases, a distinct membranous residuum is
left after the decalcification of the prismatic layer by dilute acid ;
and this is most tenacious and substantial, where (as in the
Margaritacece) there is no proper periostracum. Generally
speaking, a thin prismatic layer may be detected upon the external
surface of Bivalve shells, where this has been protected by a
periostracum, or has been prevented in any other manner from
undergoing abrasion; thus it is found pretty generally in
Chama, Trigonia, and Solen, and occasionally in Anomia and
Pecten.
526. In many other instances, however, nothing like a cellular
structure can be distinctly seen in the delicate membrane left after
decalcification ; and in such cases the animal basis bears but a very
small proportion to the calcareous substance, and the shell is usually
extremely hard. This hardness appears to depend upon the mineral
arrangement of the carbonate of lime ; for whilst in the prismatic
63S
SHELLS OF MOLLUSKS.
and ordinary nacreous
dition of calcite, it can 1
Fig. 336.
Section of hinge-tooth of Mya arenaria.
layer this has the crystalline con-
3 shown in the hard shell of Pholas to
have the arrangement of arra-
gonite ; the difference between
the two being made evident
by Polarized light. A very
curions appearance is pre-
sented by a section of the
large hinge-tooth of Mya are-
naria (Fig. 336), in which the
carbonate of lime seems to be
deposited in nodnles that pos-
sess a crystalline structure
resembling that of the mine-
ral termed Wavellite. Ap-
proaches to this curions ar-
rangement are seen in many
other shells.
527. There are several Bivalve
shells which almost entirely
consist of what may be termed
a sub-nacreous substance ;
their polished surfaces being marked by lines, but these lines
being destitute of that regularity of arrangement which is necessary
to produce the iridescent lustre. This is the case, for example, with
most of the Pectinidce (or scallop tribe), also with some of the
Mytilacece (or mussel tribe), and with the common Oyster. In the
internal layer of by far the greater number of Bivalve shells, how-
ever, there is not the least approach to the nacreous aspect ; nor is
there anything that can be described as definite structure ;* and
the residuum left after its decalcification is usually a structureless
' basement-membrane.'
528. The ordinary account of the mode of growth of the shells
of Bivalve Mollusca, — that they are progressively enlarged by the
deposition of new laminas, each of which is in contact with the
internal surface of the preceding, and extends beyond it, — does not
express the whole truth ; for it takes no account of the fact that
most shells are composed of two layers of very different texture,
and does not specify whether both these layers are thus formed by
the entire surface of the ' mantle' whenever the shell has to be
extended, or whether only one is produced. An examination of
Fig. 337 will clearly show the mode in which the operation is
effected. This figure represents a section of one of the valves of
Unio occidens, taken perpendicularly to its surface, and passing
from the margin or lip (at the left hand of the figure) towards the
hinge (which would be at some distance beyond the right). This
section brings into view the two substances of which the shell is
* For an explanation of the real nature of what was formerly described by
the Author as ' tubular' Shell-substance, see § 297.
GROWTH OF BIVALVE-SHELLS:— BEACHIOPODS. 639
composed ; traversing the outer or prismatic layer in the direction
of the length of its prisms, and passing through, the nacreous
lining in such a manner as to bring into view its numerous laminae,
separated by the lines a a', b V, c d, &c. These lines evidently
indicate the successive formations of this layer: and it maybe
easily shown by tracing them towards the hinge on the one side
and towards the margin on the other, that at every enlargement
Vertical section of the lip of one of the valves of the shell
of Unio : — a, &, c, successive formations of the outer prismatic
layer ; a', b\ c', the same of the inner nacreous layer.
of the shell its whole interior is lined by a new nacreous lamina in
immediate contact with that which preceded it. The number of
such lamina?, therefore, in the oldest part of the shell, indicates the
number of enlargements which it has undergone. The outer or
prismatic layer of the growing shell, on the other hand, is only
formed where the new structure projects beyond the margin of the
old ; and thus we do not find one layer of it overlapping another,
except at the lines of junction of two distinct formations. When
the shell has attained its full dimensions, however, new laminae of
both layers still continue to be added ; and thus the lip becomes
thickened by successive formations of prismatic structure, each
being applied to the inner surface of the preceding, instead of to
its free margin. — A like arrangement may be well seen in the
Oyster ; with this difference, that the successive layers have but a
comparatively slight adhesion to each other.
5*29. The shells of Terebraiulce, however, and of most other
Brachiopods, are distinguished by peculiarities of structure which
differentiate them from all others. When thin sections of them
are microscopically examined, they exhibit the appearance of long
flattened prisms (Fig. 338, a, b), which are arranged with such
obliquity that their rounded extremities crop-out upon the inner
surface of the shell in an imbricated (tile-like) manner (a). All
true Terebratulidce, both recent and fossil, exhibit another very
remarkable peculiarity ; namely, the perforation of the shell by a
large number of canals, which generally pass nearly perpen-
dicularly from one surface to the other (as is shown in vertical
sections, Fig. 339), and terminate internally by open orifices
640
SHELLS OF BRACHTOPODS.
(Fig. 338, a), whilst externally they are covered by the periostra-
cum (b). Their diameter is greatest towards the external surface,
where they sometimes expand suddenly, so as to become trumpet-
shaped ; and it is usually narrowed rather suddenly, when, as
sometimes happens, a new internal layer is formed as a lining to
Fig. 338.
533lt3?<s§
A, Internal surface («), and oblique section (b), of Shell of
Terebrtaula (Waldlieimia) australis; B, external surface of the
same,
the preceding (Fig. 339, a, d d). Hence the diameter of these canals,
as shown in different transverse sections of one and the same shell,
will vary according to the part of its thickness which the section
happens to traverse. —
Fig. 339.
S^gSgg?
Vertical Sections of Shell of Terebratula
(Waldheimia) australis: — showing at A the
canals opening by large trumpet-shaped ori-
fices on the outer surface, and contracting at
d d into narrow tubes ; and showing at B a
bifurcation of the canals.
The shells of different
species of perforated Bra-
chiopods, however, present
very striking diversities
in the size and closeness
of their canals, as shown
by sections taken in cor-
responding parts ; three
examples of this kind are
given for the sake of com-
parison in Figs. 340-342.
These canals are occupied
in the living state by tu-
bular prolongations of the
mantle, whose interior is
filled with a fluid contain-
ing minute cells and gra-
nules, which, from its cor-
responding in appearance
with the fluid contained
in the great sinuses of the
TEREBRATULID.E ; RHTNCHONELLID^E.
641
mantle, may perhaps be considered to be the animal's blood. Of their
special function in the economy of the animal, it is difficult to form
any probable idea; but is interesting to remark (in connection
Fig. 340,
Fig. 341.
Fig. 342.
■<Wl
Fig. 340. Horizontal section of Shell of Terebratula bullata (fossil, Oolite).
Fig. 341. Ditto .... of MegerUa lima (fossil, Chalk).
Fig. 342. Ditto . • . . . of Spiriferina rostrata (Triassic).
with the hypothesis of a relationship between Brachiopods and
Polyzoa) that they seem to have their parallel in extensions of the
perivisceral cavity of many species of Flustra, Eschara, Lepralia,
&c, into passages excavated in the walls of the cells of the poly-
zoary.
530. In the Family Rhynclionellidce, which is represented by
only two recent species (the Rh. psittacea and Eh. nigricans, both
formerly ranking as Terebratulas), but which contains a very large
proportion of fossil Brachiopods, these canals are almost entirely
absent ; so that the uniformity of their presence in the Terebratu-
lidae, and their general absence in the Bhynchonellidas, supplies a
character of great value in the discrimination of the fossil shells
belonging to these two groups respectively. Great caution is
necessary, however, in applying this test; mere surface-markings
cannot be reliecl-on ; and no statement on this point is worthy of
reliance, which is not based on a Microscopic examination of thin
sections of the shell. — In the Families Spiriferidce and Stropho-
meniclo3, on the other hand, some species possess the perforations,
whilst others are destitute of them ; so that their presence or
absence there serves only to mark-out subordinate groups. This,
however, is what holds-good in regard to characters of almost
every description, in other departments of Natural History;
a character which is of fundamental importance from its
close relation to the general plan of organization in one group,
being, from its want of constancy, of far less account in
another.*
* For a particular account of the Author's researches on this group, see his
Memoir on the subject, forming part of the introduction of Mr. Davidson's
" Monograph of the British Fossil Brachiopoda," published by the Palseonto-
T T
642 SHELLS OF GASTEEOPODS.
531. There is not by any means the same amount of diversity
in the structure of the Shell in the class of Gasteropods ; a certain
typical plan of construction being common to by far the greater
number of them. The small proportion of animal matter con-
tained in most of these shells, is a very marked feature in their
character ; and it serves to render other features indistinct, since
the residuum left after the removal of the calcareous matter is
usually so imperfect, as to give no clue whatever to the explanation
of the appearances shown by sections. Nevertheless, the structure
of these shells is by no means homogeneous, but always exhibits
indications, more or less clear, of a definite arrangement. The
* porcellanous' shells are composed of three layers, all presenting
the same kind of structure, but each differing from the others in
the mode in which this is disposed. For each layer is made-up of
an assemblage of thin laminae placed side-by-side, which separate
one from another, apparently in the planes of rhomboidal cleavage,
when the shell is fractured ; and, as was first pointed out by Mr.
Bowerbank, each of these laminae consists of a series of elongated
spicules (considered by him as prismatic cells filled with carbonate
of lime) lying side-by-side in close apposition ; and these series are
disposed alternately in contrary directions, so as to intersect each
other nearly at right angles, though still lying in parallel planes.
The direction of the planes is different, however, in the three layers
of the shell, bearing the same relation to each other as have those
three sides of a cube which meet each other at the same angle ; and
by this arrangement, which is better seen in the fractured edge of
the Cyprcea or any similar shell, than in thin sections, the strength
of the shell is greatly augmented. — A similar arrangement obviously
answering the same purpose, has been shown by Mr. Tomes to exist
in the enamel of the teeth of Eodentia.
532. The principal departures from this plan of structure are
seen in Patella, Chiton, Haliotis, Turbo and its allies, and in the
' naked' Gasteropods, many of which last, both terrestrial and
marine, have some rudiment of a shell. Thus in the common
Slug, Limax rufus, a thin oval plate of calcareous texture is found
imbedded in the shield-like fold of the mantle covering the fore-
part of its back ; and if this be examined in an early stage of its
growth, it is found to consist of an aggregation of minute calcareous
nodules, generally somewhat hexagonal in form, and sometimes
quite transparent, whilst in other instances presenting an appear-
ance closely resembling that delineated in Fig. 336. — In the epi-
dermis of the mantle of some species of Boris, on the other hand,
we find long calcareous spicules, generally lying in parallel
graphical Society. — A very remarkable example of the importance of the
presence or absence of the perforations, in distinguishing shells whose internal
structure shows them to be generically different, whilst from their external
conformation they would be supposed to be not ouly generically but specifically
identical, will be found in the " Annals of Natural History," Ser. 8, Vol. xx.
(1867), p. 68.
SHELLS OF GASTEEOPODS AND CEPHALOPODS. 643
directions, but not in contact with each other, giving firmness to
the whole of its dorsal portion ; and these are sometimes covered
with small tubercles, like the spicules of Gorgonia (Fig. 309). They
may be separated from the soft tissue in which they are imbedded,
by means of caustic potash ; and when treated with dilute acid,
whereby the calcareous matter is dissolved-away, an organic basis
is left, retaining in some degree the form of the original spicule.
This basis cannot be said to be a true cell ; but it seems to be
rather a cell in the earliest stage of its formation, being an isolated
particle of sarcode without wall or cavity ; and the close corre-
spondence between the appearance presented by thin sections of
various Univalve shells, and the forms of the spicules of Doris,
seems to justify the conclusion that even the most compact shells
of this group are constructed out of the like elements, in a state
of closer aggregation and more definite arrangement, with the
occasional occurrence of a layer of more spheroidal bodies of
the same kind, like those forming the rudimentary shell of Limax.
533. The animals composing the class of Cephalopoda (cuttle-
fish and nautilus tribe) are for the most part unpossessed of
shells ; and the structure of the few that we meet- with in the
genera Nautilus, Argonauta (' paper-nautilus '), and Spirvla, does
not present any peculiarities that need here detain us. The rudi-
mentary shell or sepiostcuire of the common Cattle-fish, however,
which is frequently spoken-of as the ' cuttle-fish bone,' exhibits a
very beautiful and remarkable structure, such as causes sections of
it to be very interesting Microscopic objects. The outer shelly
portion of this body consists of horny layers, alternating with cal-
cified layers, in which last may be seen a hexagonal arrangement
somewhat corresponding with that in Fig. 336. The soft friable
substance that occupies the hollow of this boat- shaped shell, is
formed of a number of delicate calcareous plates, running across
it from one side to the other in parallel directions, but separated
by intervals several times wider than the thickness of the plates ;
and these intervals are in great part filled-up by what appear to be
fibres or slender pillars, passing from one plate or floor to another.
A more careful examination shows, however, that instead of a
large number of detached pillars, there exists a comparatively
small number of very thin sinuous laminae, which pass from one
surface to the other, winding and doubling upon themselves, so
that each lamina occupies a considerable space. Their precise
arrangement is best seen by examining the parallel plates, after
the sinuous laminae have been detached from them ; the lines of
junction being distinctly indicated upon these. By this arrange-
ment each layer is most effectually supported by those with which
it is connected above and below ; and the sinuosity of the
thin intervening laminae, answering exactly the same purpose as
the ' corrugation ' given to iron plates for the sake of diminishing
their flexibility, adds greatly to the strength of this curious
texture ; which is at the same time lightened by the large amount
T t 2
PALATES OF GASTEEOPODS.
of open space between the parallel plates, that intervenes among
the sinuosities of the lamina?. The best method of examining this
structure, is to make sections of it with a sharp knife in various
directions, taking care that the sections are no thicker than is re-
quisite for holding-together; and these may be mounted on a
Black Ground as opaque objects, or in Canada balsam as transpa-
rent objects, under which last aspect they furnish very beautiful
objects for the Polariscope.
534. The structure of Shells generally is best examined by
making sections in different planes, as nearly parallel as may be
possible to the surfaces of the shell ; and other sections at right
angles to these : the former may be designated as horizontal, the
latter as vertical. Nothing need here be added to the full direc-
tions for making such sections which have already been given
(§§ 154-156). Much valuable information may also be derived,
however, from the examination of the surfaces presented by
fracture. The membranous residua left after the decalcification of
the shell by dilute acid, may be mounted in weak spirit or in
Goadby's solution.
535. Palate of Gasteropocl Mollushs. — The organ which is some-
times referred to under this designation, and sometimes as the
' tongue,' is one of a very singular nature ; and cannot be likened
to either the tongue or the palate of higher animals. For it is a
tube that passes backwards and downwards beneath the mouth,
closed at its hinder end, whilst in front it opens obliquely upon the
floor of the mouth, being:
Fig. 343.
(as it were) slit-up and
spread-out so as to form
a nearly flat surface. On
the interior of the tube,
as well as on the flat ex-
pansion of it, we find nu-
merous transverse rows of
minute teeth, which are
set upon flattened plates ;
each principal tooth some-
times having a basal plate
of its own, whilst in other
instances one plate carries
several teeth. — Of the
former arrangement we
Portion of the left half of the Palate of have an example in the
Helix hortensis; the rows of teeth near the palate of many terrestrial
edge separated from each other to show their Gasteropods, such as the
form- Snail {Helix) and Slug
(Limax), in which the
number of plates in each row is very considerable (Figs. 343, 344),
amounting to 180 in the large garden Slug (Limax maximus)
whilst the latter prevails in many marine Gasteropods, such as the
PALATES OF GASTEROPODS.
645
Palate of Zonites cellarius.
common Whelk (Buccinum unclatum), the palate of which has
only three plates in each row, one bearing the small central teeth,
and the two others the large lateral teeth (Fig. 347). The length
of the palatal tube, and the
number of rows of teeth, vary Fig. 344.
greatly in different species.
Generally speaking, the tube
of the terrestrial Gasteropods
is short, and is contained en-
tirely within the nearly glo-
bular head ; but the rows of
teeth being closely set together
are usually very numerous,
there being frequently more
than 100, and in some species
as many as 160 or 170; so
that the total amount of
teeth may mount-up, as in
Helix pomatia, to 21,000,
and in Limax maximus, to 26,800. The transverse rows are
usually more or less curved, as shown in Fig. 344, whilst the longi-
tudinal rows are quite straight ; and the curvature takes its
departure on each side from a central longitudinal row, the teeth
of which are symmetrical, whilst those of the lateral portions of
each transverse row present
a modification of that sym-
metry, the prominences on
the inner side of each tooth
being suppressed, whilst
those on the outer side are
increased ; this modification
being observed to augment
in degree, as we pass from
the central line towards the
edges.
536. The palatal tube of
the marine Gasteropods is
generally longer, and its
teeth larger ; and in many
instances it extends far be-
yond the head, which may,
indeed, contain but a small
part of it. Thus in the com-
mon Limpet {Patella), we
find the principal part of
the tube to he f olded-up, but
perfectly free, in the abdo-
minal cavity, between the intestine and the muscular foot ; and in
some species its length is twice or even three times as great as that
Fig. 345.
n\
-W
'%,
m%r
in
Palate of Trochus zizyphinus.
wfr
646
PALATES OF GASTEKOPODS.
of the entire animal. In a large proportion of cases, these palates
exhibit a very marked separation between the central and the
lateral portions (Figs. 345, 347) ; the teeth of the central band
being frequently small and smooth at their edges, whilst those of
the lateral are large and serrated. The palate of Trochus zizy-
pliinus, represented in Fig. 345, is one of the most beantifnl
examples of this form ; not only the large teeth of the lateral
bands, but the delicate leaf -like teeth of the central portion, having
their edges minutely serrated.
Fig. 346. Ayetmore complex type, how-
ever, is found in the palate of
Haliotis ; in which there is a
central band of teeth having
nearly straight edges instead
of points : then, on each side,
a lateral band consisting of
large teeth shaped like those
of the Shark ; and beyond
this, again, another lateral
band on either side, composed
of several rows of smaller
teeth. — Very curious diffe-
rences also present them-
selves among the different
species of the same genus.
Thus in Boris pilosa, the cen-
tral band is almost entirely
wanting, and each lateral band is formed of a single row of very
large hooked teeth, set obliquely like those of the lateral band in
Fig. 345 ; whilst in Doris tuberculata, the central band is the part
most developed, and contains a number of rows of conical teeth,
standing almost perpendicularly, like those of a harrow (Fig. 346).
537. Many other varieties might be described, did space permit ;
but we must be content with adding, that the form and arrange-
ment of the teeth of these ' palates ' afford characters of great
value m classification, as was first pointed-out by Prof. Loven
(of Stockholm) in 1847, and has been since very strongly urged by
Dr. J. E. Gray, who considers that the structure of these organs
is one of the best guides to the natural affinities of the species,
genera, and families of this group, since any important alteration
in the form or position of the teeth must be accompanied by some
corresponding peculiarity in the habits and food of the animal.*
Hence a systematic examination and delineation of the structure
and arrangement of these organs, by the aid of the Microscope and
Camera Lucida, would be of the greatest service to this depart-
ment of Natural History. The short thick tube of the Limax and
Palate of Doris tuberculata.
Annals of Natural History," Ser. 2, Vol. x. (1852), p. 413.
PALATES OF GASTEROPODS.
647
other terrestrial Gasteropoda, appears adapted for the trituration
of the food previously to its passing into the oesophagus ; for in
these animals we find the roof of the mouth furnished with a large
strong horny plate, against which the flat end of the tongue can
work. On the other hand, the flattened portion of the palate of
JBnccinum (whelk) and its allies is used by these animals as a file,
with which they bore holes through the shells of the Mollusks that
serve as their prey ; this they are enabled to effect by everting that
part of the proboscis-shaped mouth whose floor is formed by the
flattened part of the tube, which is thus brought to the exterior,
and by giving a kind of sawing-motion to the organ by means of
the alternate action of two pairs of muscles, — a protractor, and a
retractor,— which put-forth and draw-back a pair of cartilages
whereon the tongue is supported, and also elevate and depress its
teeth. Of the use of the long blind tubular part of the palate in
these Gasteropods, however, scarcely any probable guess can be
made ; unless it be a sort of ' cavity of reserve,' from which a new
toothed surface may be continually supplied as the old one is worn-
away, somewhat as the front teeth of the Eodents are constantly
being regenerated from the surface of the pulps which occupy their
hollow conical bases, as fast as they are rubbed-down at their
edges.
538. The preparation- of these Palates for the Microscope can,
of course, be only accomplished by carefully dissecting them from
their attachments within the head ; and it will be also necessary to
remove the membrane that forms the sheath of the tube, when this
is thick enough to interfere with its
transparency. The tube itself should
be slit-up with a pair of fine scissors
through its entire length ; and should
be so opened out, that its expanded
surface may be a continuation of that
which forms the floor of the mouth.
The mode of mounting it will depend
upon the manner in which it is to be
viewed. For the ordinary purposes of
Microscopic examination, no method is
so good as mounting in fluid ; either
weak Spirit or Goadby's solution an-
swering very well. But many of these
palates, especially those of the marine
Gasteropods, become most beautiful
objects for the Polariscope when they
are mounted in Canada balsam; the
form and arrangement of the teeth
being very strongly brought-out by it
(Fig. 347), and a gorgeous play of
colours being exhibited when a selenite
Fig. 347.
Palate of Buccinum undatum a
seen under Polarized Light.
648
DEVELOPMENT OF MOLLUSKS.
plate is placed behind the object, and the analyzing prism is made
to rotate *
539. Development of Mollusks. — Although no application of the
Microscope is more important to the scientific Physiologist than
that which enables him to watch the successive steps of the process
of the Development of organized structures, yet the ordinary Micro -
scopist cannot be expected to feel the same interest in its history,
Fig. 348.
Parasitic Larva (GlocMdmrn) of Anodon: — A, glochidia
attached to the tail of a Stickleback; b, side view of glochi-
dium still enclosed in the egg-membrane, showing the hooks
of its valves and the byssus-filament a ,• c, glochidium with
its valves widely opened, showing the addiictor-nruscle a; D.
side view of glochidium, with the valves opened to show the
origin of the byssus-filament and the three pairs of tenta-
cular (?) organs, the barbed hooks b, and the muscular or
membranous folds c, c, connected with them.
and will expect only to have his attention directed to such of its
phenomena as are of most general interest. The study of the early
stages of the Embryonic Development of Bivalve Mollusks is
attended with considerable difficulty, and has been, with few excep-
tions, but very incompletely prosecuted. Of the very unsatisfactory
* For additional details on the organization of the Palate and Teeth of
the Gasteropod Mollusks, see Mr. W. Thomson, in "Cyclop, of Anat. and
Physiol.," Vol. iv. pp. 1142, 1143 ; and in " Ann. of Nat. His.," Ser. 2, Vol. vii.
p. 86.
GLOCHIDIUM-LAKVA OF ANODON. 649
nature of our present knowledge of its history, we have a marked
example in the fact that what are undoubtedly the embryoes of a
fresh-water Mussel, the Anodon cijgneus, when found adhering to
the gills of their parent, have been described as parasites, under the
name of Glochidium, and were long maintained to be such by some
persons who assumed to be authorities on the subject. It has been
shown,* however, that these embryoes, after being excluded from
between the valves of their parent, attach themselves in a peculiar
manner to the fins and gills of small Fishes (Fig. 348, a). In this
stage of the existence of the young Anodon, its valves are provided
with curious barbed or serrated hooks (d b), and are continually
snapping together (so as to remind the observer of the avicularia
of Polyzoa, § 513), until they have inserted their hooks into the
skin of the Fish, which seems so to retain the barbs as to prevent
the re-opening of the valves. In this stage of its existence no
internal organ is definitely formed, except the strong ' adductor'
muscle (c, a) which draws the valves together, and the long, slender,
byssus-filament (b, a, d) which makes its appearance while the
embryo is still within the egg-membrane, lying coiled-up between
the lateral lobes. The hollow of each valve is filled with a soft
granular-looking mass, in which are to be distinguished what are
perhaps the rudiments of the branchia3 and of oral tentacles ; but
their nature can only be certainly determined by further observation,
which is rendered difficult by the opacity of the valves. By keeping
an adequate supply of Fish, however, with these embryoes attached,
any dexterous Microscopist may overcome this difficulty, and may
work out the entire history of the development of the fresh-water
Mussel as successfully as M. Lacaze Duthier's has worked out
an important part of that of the common Mytilus edulis or true
Mussel.f
540. The history of embryonic Development may be studied with
peculiar facility in certain members of the Class of Gasteropoda,
and presents numerous phenomena of great interest. The eggs
(save among the terrestrial species) are usually deposited in aggre-
gate masses, each enclosed in a common protective envelope or
nidctmentum. The nature of this envelope, however, varies
greatly : thus in the common Limnceus stogncdis or ' water-snail'
of our ponds and ditches, it is nothing else than a mass of soft
jelly about the size of a sixpence, in which from 50 to 60 eggs are
imbedded, and which is attached to the leaves or stems of aquatic
plants ; in the Buccinum undatum, or common Whelk, it is a
membranous case, connected with a considerable number of similar
cases by short stalks, so as to form large globular masses which
* See the Kev. W. Houghton ' On the Parasitic Nature of the Fry of
the Anodonta cijgnea, in " Quart. Journ. of Microsc. Sci.," N.S., Vol. ii.
(1861), p. 162.
t See his admirable ' Mdmoire sur le DeVeloppement des Branchies des Mol-
lusques Aedphales Lamellibranches,' in "Ann. des Sciences Nat.," Sex. 4,
Tom. v. (1856), p. 5.
650 EMBRYONIC DEVELOPMENT OF MOLLUSKS.
Fig. 349.
Embryonic Development of Doris bilamellata: — A, Ovum, consisting of
enveloping membrane a and yolk b ; B, c, D, E, F, successive stages of
segmentation of yolk; G, first marking-out of the shape of the embryo ;
H, embryo on the 8th day ; I, the same on the 9th day ; K, the same on
the 12th day, seen on the left side at L ; M, still more advanced embryo,
seen at N as retracted within its shell : — a, superficial layer of yolk-
segments coalescing to give origin to the shell ; c, c, ciliated lobes ; d,
foot ; g, hard plate or operculum attached to it ; h, stomach ; i, intes-
tine, m, n, masses (glandular?) at the sides of the oesophagus ; o,
heart (?); s, retractor muscle (?); t, situation of funnel; t', membrane
enveloping the body; x, auditory vesicles; y, mouth.
DEVELOPMENT OE NUDIBEANCHS. 651
may often be picked-up on onr shores, especially between April and
June ; in the Purpura lamillus, or 'rock-whelk,' it is a little flask-
shaped capsule, having a firm horny wall, which is attached by a
short stem to the surface of rocks between the tide-marks, great
numbers being often found standing erect side by side ; whilst in
the Nudibrancliiate order generally (consisting of the Doris, Eolis,
and other ' sea-slugs') it forms a long tube with a membranous wall,
in which immense numbers of eggs (even half a million or more)
are packed closely together in the midst of a jelly-like substance,
this tube being disposed in coils of various forms, which are usually
attached to Sea-weeds or Zoophytes. — The course of development,
in the first and last of these instances, may be readily observed
from the very earliest period down to that of the emersion of the
embryo ; owing to the extreme transparence of the nidamentum
and of the egg-membranes themselves. The first change which
will be noticed by the ordinary observer, is the ' segmentation' of
the yolk-mass, which divides itself (after the manner of a cell
undergoing binary subdivision) into two parts, each of these two
into two others, and so on until a mulberry-like mass of minute
yolk-segments is produced (Fig. 349, a — r), which next evolves itself
into a gastrula (§ 468), whose form is shown at c. The
' gastrula' soon begins to exhibit a very curious alternating rotation
within the egg, two or three turns being made in one direction, and
the same number in a reverse direction : this movement is due to
the cilia fringing a sort of fold of the ectoderm termed the velum,
which afterwards usually gives origin to a pair of large ciliated
lobes (h — l, c) resembling those of Eotifers. The velum is so little
developed in Limnasus, however, that its existence has been com-
monly overlooked until recognized by Mr. Ray Lankester,* who
also has been able to distinguish its fringe of minute cilia. This,
however, has only a transitory existence ; and the later rotation of
the embryo, which presents a very curious spectacle when a number
of ova are viewed at once under a low magnifying power, is due to
the action of the cilia fringing the head and foot.
541. A separation is usually seen at an early period, between the
anterior or ' cephalic' portion, and the posterior or ' visceral' por-
tion, of the embryonic mass ; and the development of the former
advances with the greater activity. One of the first changes
which is seen in it consists of its extension into a sort of fin-like
membrane on either side, the edges of which are fringed with
long cilia (Fig. 349, h — l, c), whose movements may be clearly
distinguished whilst the embryo is still shut-up within the egg ;
at a very early period may also be discerned the ' auditory vesi-
cles' (k, x) or rudimentary organs of hearing (§ 546), which
scarcely attain any higher development in these creatures during
* See his valuable ' Observations on the Development of Limnceus stagnalis,
and on the early stages of other Mollusca,' in " Quart. Journ. Microsc. Science,"
Oct. 1874. See also Lereboullet, 'Eecherches sur le Developpenient du Lini-
ne'e,' in "Ann. des Sci. Nat. Zool.," 4ieme Ser., Tom. xviii. p. 47.
652 EMBRYONIC DEVELOPMENT OF GASTEROPODS.
the whole of life ; and from the immediate neighbourhood of these
is pnt-forth a projection, which is afterwards to be evolved into
the ' foot' or muscular disk of the animal. While these organs
are making their appearance, the shell is being formed on the
surface of the posterior portion, appearing first as a thin covering
over its hinder part, and gradually extending itself until it be-
comes large enough to enclose the embryo completely, when this
contracts itself. The ciliated lobes are best seen in the embryoes
of Nudibranchs ; and the fact of the universal presence of a shell in
the embryoes of that group is of peculiar interest, as it is destined
to be cast-off very soon after they enter upon active life. These
embryoes may be seen to move-about as freely as the narrowness
of their prison permits, for some time previous to their emersion ;
and when set free by the rupture of the egg-cases, they swim
forth with great activity by the action of their ciliated lobes, —
these, like the ' wheels' of Rotifera, serving also to bring food to the
mouth, which is at that time unprovided with the reducing appa-
ratus subsequently found in it. The same is true of the embryo
of Lymnceus, save that its swimming movements are less active, in
consequence of the non-development of the ciliated lobes ; and the
currents produced by the cilia that fringe the head and the orifice
of the respiratory sac, seem to have reference chiefly to the pro-
vision of supplies of food, and of aerated water for respiration.
The disappearance of the cilia has been observed by Mr. Hogg to
be coincident with the development of the teeth to a degree sufficient
to enable the young water- snail to crop its vegetable food ; and he
has further ascertained that if the growing animal be kept in fresh
water alone for some time, without vegetable matter of any kind,
the gastric teeth are very imperfectly developed, and the cilia are
still retained*
542. A very curious modification of the ordinary plan of develop-
ment is presented in the Purpura lapillus ; and it is probable
that something of the same kind exists also in Buccinum, as well as
in other Gasteropods of the same extensive Order (Pectinibran-
chiata). — Each of the capsules already described (§ 540) contains
from 500 to 600 egg-like bodies (Fig. 350, a), imbedded in a viscid
gelatinous substance ; but only from 12 to 30 embryoes usually
attain complete development ; and it is obvious from the large com-
parative size which these attain (Fig. 351, b), that each of them
must include an amount of substance equal to that of a great
number of the bodies originally found within the capsule. The
explanation of this fact (long since noticed by Dr. J. E. Gray, in
regard to Buccinum) seems to be as follows : — Of those 500 or 600
egg-like bodies, only a small part are true ova, the remainder being
merely yolk-spherules, which are destined to serve for the nutrition
of the embryoes. The distinction between them manifests itself at
a very early period, even in the first segmentation ; for while the
* See " Transact, of Microsc. Soc," 2nd Ser., Vol. ii. (1854), p. 93.
EMBRYONIC DEVELOPMENT OF PUBPUKA,
653
Fig. 350.
Early stages of Embryonic Develop-
ment of Purpura lapillus: — A, egg-like
spherule ; B, c, E, F, G, successive stages
of segmentation of yolk-spherules ; D, H,
I, J, K, successive stages of development
of early embryoes.
yolk- spherules divide into two equal hemispheres (Fig. 350, b), the
real ova divide into a larger and a smaller segment (d) ; in the cleft
between these are seen the
minute ' directive vesicles,'
which appear to be always
double or even triple, although,
from being seen ' end on,' only
one may be visible ; and near
these is generally to be seen
a clear space in each segment.
The difference is still more
strongly marked in the subse-
quent divisions ; for whilst
the cleavage of the yolji-sphe-
rules goes-on irregularly, so
as to divide each into from 14
to 20 segments, having no de-
finiteness of arrangement (c,
e, f, g), that of the ova takes
place in such a manner as to
mark-out the distinction al-
ready alluded-to between the
' cephalic' and the ' visceral'
portions of the mass (h) ; and
the evolution of the former
into distinct organs very speedily commences. In the first instance,
a narrow transparent border is seen around the whole embryonic
mass, which is broader at the cephalic portion (i) ; next, this
border is fringed with short cilia, and the cephalic extension into
two lobes begins to show itself; and then between the lobes a
large mouth is formed, opening through a short, wide oesophagus,
the interior of which is ciliated, into the visceral cavity, occupied
as yet only by the yolk-particles originally belonging to the
ovum (k).
543. Whilst these developmental changes are taking place in
the embryo, the whole aggregate of segments formed by the sub-
division of the yolk-spherules coalesces into one mass, as shown
at a, Fig. 351 ; and the embryoes are often, in the first instance,
so completely buried within this, as only to be discoverable by
tearing its portions asunder ; but some of them may commonly be
found upon its exterior ; and those contained in one capsule very
commonly exhibit the different stages of development represented
in Fig. 350, h — k. After a short time, however, it becomes ap-
parent that the most advanced embryoes are beginning to swalloiv
the yolk-segments of the conglomerate mass ; and capsules will
not unfrequently be met-with, in which embryoes of various sizes,
as a, b, c, d, e (Fig. 351, a), are projecting from its surface, their
difference of size not being accompanied by advance in develop-
ment, but merely depending upon the amount of this ' supple-
654
EMBKYONIC DEVELOPMENT OF PURPURA.
mental ' yolk which the embryoes have respectively gulped-down.
For during the time in which they are engaged in appropriating
this additional supply of nutriment, although they increase in
size, yet they scarcely exhibit any other change ; so that the large
embryo, Fig. 351, e, is not apparently more advanced as regards
Fig. 351.
Later stages of embryonic Development of Purpuru Inpil-
lus : — A, conglomerate mass of vitelline segments, to which
were attached the embryoes, a, &, c, rf, e: — B, full-size embryo,
in more advanced stage of development.
the formation of its organs, than the small embryo, Fig. 350, k.
So soon as this operation has been completed, however, and the
embryo has attained its full bulk, the evolution of its organs
takes-place very rapidly ; the ciliated lobes are much more highly
developed, being extended in a long sinuous margin, so as almost
to remind the observer of the ' wheels' of Rotifera (§ 405), and
being furnished with very long cilia (Fig. 351, b) ; the auditory
vesicles, the tentacula, the eyes, and the foot, successively make
their appearance ; a curious rhythmically-contractile vesicle is
seen, just beneath the edge of the shell in the region of the neck,
which may, perhaps, serve as a temporary heart; a little later,
the real heart may be seen pulsating beneath the dorsal part of
the shell ; and the mass of yolk-segments of which the body is
made-up, gradually shapes itself into the various organs of di-
gestion, respiration, &c, during the evolution of which (and while
they are as yet far from complete) the capsule thins-away at its
summit, and the embryoes make their escape from it.*
* The Author thinks it worth while to mention the method which he has
EMBRYONIC DEVELOPMENT OF PURPURA. 655
544. It happens not unfrequently that one of the embryoes
which a capsule contains does not acquire its ' supplemental ' yoliv
in the manner now described, and can only proceed in its develop-
ment as far as its original yolk will afford it material ; and thus,
at the time when the other embryoes have attained their full size
and maturity, a strange-looking creature, consisting of two large
ciliated lobes with scarcely the rudiment of a body, may be seen
in active motion among them. This may happen, indeed, not only
to one but to several embryoes within the same capsule, especially
if their number should be considerable ; for it sometimes appears
as if there were not food enough for all, so that whilst some attain
their full dimensions and complete development, others remain of
unusually small size, without being deficient in any of their organs,
and others again are more or less completely abortive, — the supply
of supplemental yolk which they have obtained having been too
small for the development of their viscera, although it may have
afforded what was needed for that of the ciliated lobes, eyes, ten-
tacles, auditory vesicles, and even the foot, — or, on the other hand,
no additional supply whatever having been acquired by them, so
that their development has been arrested at a still earlier stage. —
These phenomena are of so remarkable a character, that
they furnish an abundant source of interest to any Microscopist
who may happen to be spending the months of August and Sep-
tember in a locality in which the Purpura abounds; since, by
opening a sufficient number of capsules, no difficulty need be expe-
rienced in arriving at all the facts which have been noticed in this
brief summary.* It is much to be desired that such Microscopists
as possess the requisite opportunity, would apply themselves to
the study of the corresponding history in other Pectinibranchiate
Gasteropods, with a view of determining how far the plan now
described prevails through the Order. And now that these
found most convenient for examining the contents of the egg-capsules of Pur-
pura; as he believes that it maybe advantageously adopted in man y other
cases. This consists in cutting off the two ends of the capsule (taking care
not to cut far into its cavity), and in then forcing a jet of water through it, by
inserting the end of a fine-pointed syringe (§ 115) into one of the orifices thus
made, so as to drive the contents of the capsule before it through the other.
These should be received into a shallow cell, and first examined under the
Simple Microscope.
* Fuller details on this subject will be found in the Author's account of his
researches, in "Transactions of the Microscopical Society," 2nd Ser.. Vol. iii.
(1855), p. 17. His account of the process was called in question by MM.Koren
and Danielssen, who had previously given an entirely different version of it,
but was fully confirmed by the observations of Dr. Dyster; see "Ann. of Nat.
Hist.," 2nd Ser., Vol. xx. (1857), p. 16. The independent observations of M.
Claparede on the development of Xeritina fluviatilis ("Midler's Archiv," 1857,
p. 109, and abstract in "Ann. of Nat. Hist.," 2nd Ser., Vol. xx., 1857, p. 196)
showed the mode of development in that species to be the same in all essential
particulars as that of Purpura. The subject has again been recently studied
with great minuteness by Selenka, " Niederlandisches Archiv fur Zoologie,"
Bd. i., Julv, 1862.
656 MOLLUSCOUS ANIMALS GENERALLY.
Mollusks have "been brought not only to live, bnt to breed, in
artificial aquaria, it may be anticipated that a great addition to
our knowledge of this part of their life-history will ere long be
made.
545. Ciliary Motion on Gills. — There is no object that is better
suited to exhibit the general phenomena of Ciliary motion (§ 402),
than a portion of the gill of some bivalve Mollusk. The Oyster
will answer the purpose sufficiently well ; but the cilia are much
larger on the gills of the Mussel* as they are also on those of the
Anodon or common ' fresh -water mussel ' of our ponds and streams.
Nothing more is necessary than to detach a small portion of one
of the riband-like bands, which will be seen running parallel with
the edge of each of the valves when the shell is opened ; and to
place this, with a little of the liquor contained within the shell,
upon a slip of glass, — taking care to spread it out sufficiently with
needles to separate the oars of which it is composed, since it is on
the edges of these, and round their knobbed extremities, that the
ciliary movement presents itself,— and then covering it with a
thin-glass disk. Or it will be convenient to place the object in the
Live-box (§ 108), which will enable the observer to subject it to any
degree of pressure that he may find convenient. A magnifying
power of about 120 diameters is amply sufficient to afford a
general view of this spectacle ; but a much greater amplification
is needed to bring into view the peculiar mode in which the stroke
of each cilium is made. Few spectacles are more striking to the
unprepared mind, than the exhibition of such wonderful activity
as will then become apparent, in a body which to all ordinary ob-
servation is so inert. . This activity serves a double purpose ; for
it not only drives a continual current of water over the surface of
the gills themselves, so as to affect the aeration of the blood, but
also directs a portion of this current (as in the Tunicata, § 514)
to the mouth, so as to supply the digestive apparatus with the
aliment afforded by the Biatomacece, Infusoria, &c, which it
carries-in with it.
546. Organs of Sense of Mollusks. — Some of the minuter and
more rudimentary forms of the special organs of sight, hearing, and
touch, which the Molluscous series presents, are very interesting
objects of Microscopic examination. Thus just within the margin
of each valve of Pecten, we see (when we observe the animal in
its living state, under water) a row of minute circular points of
great brilliancy, each surrounded by a dark ring; these are the
eyes, with which this creature is provided, and by which its
peculiarly-active movements are directed. Each of them, when
their structure is carefully examined, is found to be protected
by a sclerotic coat with a transparent cornea in front, and to
possess a coloured iris (having a pupil) that is continuous
* This Shell-fish may be obtained, not merely at the sea-side, but likewise
at the shops of the fishmongers who supply the humbler classes, even in mid-
land towns.
OEGAXS OF SEXSE IN MOLLUSKS. 657
with a layer of pigment lining the sclerotic, a crystalline lens and
vitreous body, and a retinal expansion proceeding from an optic
nerve which passes to each eye from the trunk that runs along the
margin of the mantle. — Eyes of still higher organization are borne
upon the head of most Gasteropod Mollusks, generally at the base
of one of the pairs of tentacles, but sometimes, as in the Snail and
Slug, at the points of these organs. In the latter case, the ten-
tacles are furnished with a very peculiar provision for the protec-
tion of the eyes ; for when the extremity of either of them is
touched, it is drawn-back into the basal purt of the organ, much
as the finger of a glove may be pushed-back into the palm. The
retraction of the tentacle is accomplished by a strong muscular slip,
which arises within the head, and proceeds to the extremity of the
tentacles ; whilst its protrusion is effected by the agency of the cir-
cular bands with which the tubular wall of the tentacle is itself
furnished, the inverted portion being (as it were) squeezed-out by
the contraction of the lower part into which it has been drawn back .
The structure of the eyes, and the curious provision just described,
may easily be examined by snipping-off one of the e)re-bearing
tentacles with a pair of scissors. — Xone but the Cephalopod Mollusks
have distinct organs of hearing ; but rudiments of such organs may
be found in most Gasteropods (Fig. 349, k, x), attached to some
part of the nervous collar that surrounds the oesophagus ; and even
in many Bivalves, in connection with the nervous ganglion imbedded
in the base of the foot. These ' auditory vesicles,' as they are
termed, are minute sacculi, each of which contains a fluid, wherein
are suspended a number of minute calcareous particles (named
otoliths or ear-stones), which are kept in a state of continual
movement by the action of cilia lining the vesicles. This " won-
derful spectacle," as it was truly designated by its discoverer
Siebold, may be brought into view without any dissection, by sub-
mitting the head of any small and not very thick-skinned Gas-
teropod, or the young of the larger forms, to gentle compression
under the Microscope, and transmitting a strong light through it,
The very early appearance of the auditory vesicles in the embryo
Gasteropod has been already alluded-to (§ 541). — Those who have
the opportunity of examining young specimens of the common
Peden, will find it extremely interesting to watch the action of the
very delicate tentacles which they have the power of putting-forth
from the margin of their mantle, the animal being confined in a
shallow cell, or in the zoophyte-trough ; and if the observer should
be fortunate enough to obtain a specimen so young that the valves
are quite transparent, he will find the spectacle presented by the
ciliary movement of the gills, as well as the active play of the foot
(of which the adult animal can make no such use), to be worthy of
more than a cursory glance.
547. Chromaiojjhores of Cejplialopods. — Almost any species of
Cuttle-fish (Sepia) or Squid (Loligo) will afford the opportunity of
examining the very curious provision which their skin contains for
658 CHEOMATOPHOEES OF CEPHALOPODS.
changing its hue. This consists in the presence of numerous large
' pigment- cells,' containing colouring-matter of various tints ; the
prevailing colour, however, being that of the fluid of the ink-bag.
These pigment-cells may present very different forms, being some-
times nearly globular, whilst at other times they are flattened and
extended into radiating prolongations ; and, by the peculiar con-
tractility with which they are endowed, they can pass from one to
the other of these conditions, so as to spread their coloured con-
tents over a comparatively-large surface, or to limit them within
a comparatively- small area. Very commonly there are different
layers of these pigment-cells, their contents having different hues
in each layer ; and thus a great variety of coloration may be
given, by the alteration in the form of the cells of which one
or another layer is made-up. It is curious that the changes in the
hue of the skin appear to be influenced, as in the case of the
Chameleon, by the colour of the surface with which it may be in
proximity. The alternate contractions and extensions of these
pigment-cells or chromatophores may be easily observed in a
piece of skin detached from the living animal and viewed as a
transparent object ; since they will continue for some time, if the
skin be placed in sea- water. And they may also be well seen in
the embryo cuttle-fish, which will sometimes be found in a state
of sufficient advancement in the grape-like eggs of these animals
attached to Sea-weeds, Zoophytes, &c. — The eggs of the small
cuttle-fish termed the Sepiola, which is very common on our
southern coasts, are imbedded, like those of the Doris, in gelatinous
masses, which are attached to Sea-weeds, Zoophytes, &c. ; and their
embryoes, when near maturity, are extremely beautiful and in-
teresting objects, being sufficiently transparent to allow the action
of the heart to be distinguished, as well as to show most advan-
tageously the changes incessantly occurring in the form and hue
of the ' chromatophores.'
CHAPTEE XV.
AXXUIOSA, OB. WORMS.
548. Under the general designation of ' Annnlose' animals, or
Worms, may be grouped-together all that lower portion of the
great Articulated Sub-kingdom, in which the division of the body
into longitudinally-arranged segments is not distinctly marked-out,
and in which there is an absence of those ' articulated' or jointed
limbs that constitute so distinct a feature of Insects and their
allies. This group includes the classes of Entozoa or Intestinal
Worms, Botifera or Wheel-animalcules, Turbellaria, and Annelida;
each of which furnishes many objects for ^licroscopic examination,
that are of the highest scientific interest. As our business,
however, is less with the professed Physiologist, than with the
general inquirer into the minute wonders and beauties of Nature,
we shall pass over these classes (the Kotifera having been already
treated-of in detail, Chap. IX.) with only a notice of such points as
are likely to be specially deserving the attention of observers of
the latter order.
549. Extozoa.. — This class consists almost entirely of animals
of a very peculiar plan of organization, which are parasitic within
the bodies of other animals, and which obtain their nutriment by
the absorption of the juices of these, — thus bearing a striking
analogy to the parasitic Fungi (§§293-297). The most remarkable
feature in their structure consists in the entire absence or the
extremely low development of their nutritive system, and the
extraordinary development of their reproductive apparatus. Thus,
in the common Taenia ('tape- worm'), which may be taken as the
type of the Cestoid group, there is neither mouth nor stomach,
the so-called ' head' being merely an organ for attachment, whilst
the segments of the ' body' contain repetitions of a complex gene-
rative apparatus, the male and female sexual organs being so
united in each as to enable it to fertilize and bring to maturity its
own very numerous eggs ; and the chief connection between these
segments is established by two pairs of longitudinal canals, which,
though regarded by some as representing a digestive apparatus,
and by others as a circulating system, appear really to represent
the ' water-vascular system,' whose simplest condition has been
u u 2
660 EELATION OF CYSTIC TO CESTOID ENTOZOA.
noticed in the Wheel-animalcule (§ 410). — Few among the recent
results of Microscopic inquiry have been more curious, than the
elucidation of the real nature of the bodies formerly denominated
Cystic Entozoa, which had been previously ranked as a distinct
group. These are not found, like the preceding, in the cavity
of the alimentary canal of the animals they infest ; but always
occur in the substance of solid organs, such as the glands, muscles,
&c. They present themselves to the eye as bags or vesicles of
various sizes, sometimes occurring singly, sometimes in groups ;
but upon careful examination each vesicle is found to bear upon
some part a ' head' furnished with hooklets and suckers ; and this
may be either single, as in Gysticercus (the entozoon whose
presence gives to pork what is known as the ' measly' disorder), or
multiple, as in Ccenurus, which is developed in the brain, chiefly of
sheep, giving rise to the disorder known as ' the staggers.' Now
in none of these Cystic forms has any generative apparatus ever
been discovered, and hence they are obviously to be considered as
imperfect animals. The close resemblance between the ' heads' of
certain Cysticerci and that of certain Tcenice first suggested that
the two might be different states of the same animal ; and ex-
periments made by those who have devoted themselves to the
working-out of this curious subject have led to the assured con-
clusion, that the Cystic Entozoa are nothing else than Cestoid
Worms, whose development has been modified by the peculiarity
of their position, — the large bag being formed by a sort of dropsical
accumulation of fluid when the young are evolved in the midst of
solid tissues, whilst the very same bodies, conveyed into the
alimentary canal of some carnivorous animal which has fed upon
the flesh infested with them, begin to bud-forth the generative
segments, the long succession of which, united end-to-end, gives to
the entire series a Worm-like aspect.
550. The higher forms of Entozoa, belonging to the Nematoicl or
thread-like Order,— of which the common Ascaris may be taken as
a type, one species of it (the A. lumbricoides, or 'roundworm')
being a common parasite in the small intestine of man, while
another (the A. vermicularis, or ' thread worm') is found rather in
the lower bowel, — approach more closely to the ordinary type of
conformation of Worms ; having a distinct alimentary canal,
which commences with the mouth at the anterior extremity of the
body, and which terminates by an anal orifice near the other ex-
tremity ; and also possessing a regular arrangement of circular
and longitudinal muscular fibres, by which the body can be
shortened, elongated, or bent in any direction. The smaller species
of Ascaris, by some or other of which almost every Yertebrated
animal is infested, are so transjDarent that every part of their
internal organization may be made-out, especially with the assis-
tance of the Compressorium (§ 111), without any dissection ; and
the study of the structure and actions of the generative apparatus
has yielded many very interesting results, especially in regard to
NEMATOID EXTOZOA; — AXGUILLUL^E. 661
tlie first forniation of tlie ova, the mode of their fertilization, and
the history of their subsequent development. — Some of the Worms
belonging to this group are not parasitic in the bodies of other
animals, but live in the midst of dead or decomposing Vegetable
matter. The Gordius or ' hair-worm,' which is peculiar in not
having any perceptible anal orifice, seems to be properly a parasite
in the intestines of water-insects ; but it is frequently found in
large knot-like masses (whence its name) in the water or mud of
the pools inhabited by such insects, and may apparently be deve-
loped in these situations. The Anguittulce are little eel-like worms,
of which one species, A. jkwiatilis, is very often found in fresh-
water amongst Desmidiece, Gonfervce, &c, also in wet moss and
moist earth, and sometimes also in the alimentary canal of snails,
frogs, fishes, insects, and larger worms ; whilst another species,
A. tritici, is met- with in the ears of Wheat affected with the blight
termed the 'cockle;' another, the A. glutinis, is found in sour
paste ; and another, the A. aceti, was often found in stale vinegar,
until the more complete removal of mucilage and the addition of
sulphuric acid, in the course of the manufacture, rendered this
liquid a less favourable ' habitat' for these little creatures. A
writhing mass of any of these species of ' eels,' is one of the most
curious spectacles which the Microscopist can exhibit to the un-
scientific observer ; and- the capability which they all possess (in
common with Rotifers and Tardigrades (§ 413), of revival after
desiccation, at however remote an interval, enables him to com-
mand the spectacle at any time. A grain of wheat within which
these worms (often erroneously called Vibriones) are being deve-
loped, gradually assumes the appearance of a black pepper-corn ;
and if it be divided in two, the interior will be found almost
completely filled with a dense white cottony mass, occupying the
place of the flour, and leaving merely a small place for a little
glutinous matter. The cottony substance seems to the eye to
consist of bundles of fine fibres closely packed-together ; but on
taking-out a small portion, and putting it under the Microscope
with a little water under a thin glass-cover, it will be found after a
short time (if not immediately) to be a wriggling mass of life, the
apparent fibres being really Anguittulce, or the ' eels' of the Mi-
croscopist. If the seeds be soaked in water for a couple of hours
before they are laid open, the eels will be found in a state of
activity from the first ; their movements, however, are by no means
so energetic as those of the A. glutinis or ' paste-eel.' This last
frequently makes its appearance spontaneously in the midst of
paste that is turning sour ; but the best means of securing a supply
for any occasion, consists in allowing any portion of a mass of
paste in which they may present themselves to dry up, and then,
laying this by so long as it may not be wanted, to introduce it into
a mass of fresh paste, which, if it be kept warm and moist,
will be found after a few days to swarm with these curious little
creatures.
662 TREMATODE ENTOZOA.— TUKBELLARIA.
551. Besides the foregoing Orders of Entozoa, the Trematode
group must be named ; of which the Distoma hepaticum, or ' fluke,'
found in the livers of Sheep affected with the 'rot,' is a typical
example. Into the details of the structure of this animal, which
has the general form of a sole, there is no occasion for us here to
enter : it is remarkable, however, for the branching form of its
digestive cavity, which extends throughout almost the entire body,
very much as in the Planarige (Fig. 352) ; and also for the curious
phenomena of its development, several distinct forms being passed
through between one sexual generation and another. These have
been especially studied in the Distoma which infests the Lymnceus ;
the ova of which are not developed into the likeness of their
parents, but into minute worm-like bodies, which seem to be little
else than masses of cells enclosed in a contractile integument, no
formed organs being found in them ; these cells, in their turn, are
developed into independent zooids, which escape from their con-
taining cyst in the condition of free ciliated Animalcules ; in this
condition they remain for some time, and then imbed themselves
in the mucus that covers the tail of the Mollusk, in which they
undergo a gradual development into true Distomata ; and having
thus acquired their perfect form, they penetrate the soft integu-
ment, and take-up their habitation in the interior of the body.
Thus a considerable number of Distomata may be produced from
a single ovum, by a process of cell-multiplication in an early stage
of its development. In some instances the free ciliated larva
possesses distinct eyes ; although tbey are wanting in the fully
developed Distoma, the peculiar ' habitat' of which would render
them useless.
552. Tukbellaria. — This group of animals, which is distin-
guished by the presence of cilia over the entire surface of the body,
seems intermediate in some respects between the ' trematode' En-
tozoa and the Leech-tribe among Annelida. It deserves special
notice here, chiefly on account of the frequency with which the
worms of the Planarian tribe present themselves among collections
both of marine and of fresh-water animals (particular species in-
habiting either locality), and on account of the curious organization
which many of these possess. Most of the members of this tribe
have elongated flattened bodies, and move by a sort of gliding or
crawling action over the surfaces of aquatic Plants and Animals.
Some of the smaller kinds are sufficiently transparent to allow of
their internal structure being seen by transmitted light, es]3ecially
when they are slightly compressed ; and the accompanying figure
(Pig. 352) displays the general conformation of their principal
organs, as thus shown. The body has the flattened sole-like shape
of the Trematode Entozoa ; its mouth, which is situated at a con-
siderable distance from the anterior extremity of the body, is
surrounded by a circular sucker that is applied to the living
surface from which the animal draws its nutriment ; and the
buccal cavity (&) opens into a short oesophagus (c), which leads at
TUEBELLAEIA :— PLANAEIAN WOEMS.
663
once to the cavity of the stomach. In the trne Planarice the
month is furnished with a sort of long fnnnel- shaped proboscis ;
and this, even when detached from the body, continues to swallow
anything presented to it. The cavity of the stomach does not give
origin to any intestinal tube, nor is it provided with any second
orifice ; but a large number
of ramifying canals are Fig. "52.
prolonged from it, which
carry its contents into
every part of the body.
This seems to render un-
necessary any system of
vessels for the circulation
of nutritive fluids ; and the
two principal trunks, with
connecting and ramifying
branches, which may be
observed in them, are pro-
bably to be regarded in the
light of a water- vascular
system, the function of
which is essentially re-
spiratory. Both sets" of
sexual organs are combined
in the same individuals ;
though the congress of two,
each impregnating the ova
of the other, seems to be
generally necessary. The
ovaria, as in the Entozoa,
extend through a large part
of the body, their ramifi-
cations proceeding from
the two oviducts (h, h),
which have a dilatation (I)
at their point of junction.
— There is much obscurity
about the history of the Structure of .g m pw
embryonic Development of rian WOrm):-a, Mouth, surrounded by its
these animals ; and the circular sucker ; b, buccal cavity ; c, oeso-
facts observed by Siebold pbageal orifice ; d, stomach ; e, ramifications
seem to be best explained of gastric canals;/, cephalic ganglia and their
upon the hypothesis, that
nervous filaments; g,g, testes; k, vesicular
seminalis : ?*, male genital canal ; k. k, ovi-
wnat has been usually dufits. ^ dilatation at their point of junc-
considered as an egg is tion ; m, female genital orifice,
really an egg-capsule con-
taining several embryoes with a store of supplemental yolk, as
in Purpura (§ 543), which yolk is swallowed by the embryoes
at a very early period of their development within the cap-
664 PLANAEIAN WORMS.— ANNELIDS.
sule.* After their emersion from the capsule, the embryoes bear
so strong a resemblance to certain Infusoria, as to have led
Prof. Agassiz to the conclusion that the genera Paramecium and
Kolpocla are nothing else than Planarian larvae, — an idea decisively-
negatived by the discovery of their sexual generation (§ 398). The
Planariaa, however, do not multiply by eggs alone ; for they
occasionally undergo spontaneous fission in a transverse direction,
each segment becoming a perfect animal ; and an artificial division
into two or even more parts may be practised with a like result.
In fact, the power of the Planarias to reproduce portions which
have been removed, seems but little inferior to that of the Hydra
(§ 472) ; a circumstance which is peculiarly remarkable, when the
much higher character of their organization is borne in mind.
They possess a distinct pair of nervous ganglia (/, /), from which
branches proceed to various parts of the body ; and in the neigh-
bourhood of these are usually to be observed a number (vary-
ing from 2 to 40) of ocelli or rudimentary eyes, each having its
refracting body or crystalline lens, its pigment-layer, its nerve-
bulb, and its cornea-like bulging of the skin. The integument
of many of these animals is furnished with ' thread-cells' or
'filiferous capsules,' very much resembling those of Zoophytes
(§ 486).
553. Annelids. — This Class includes all the higher kinds of
Worm-like animals, the greater part of which are marine, though
there are several species which inhabit fresh water, and some
which live on land. The body in this class is usually very long,
and nearly always presents a well-marked segmental division, the
segments being for the most part similar and equal to each other,
except at the two extremities; bnt in the lower forms, such as
the Leech and its allies, the segmental division is very indistinctly
seen, on account of the general softness of the integument. A
large proportion of the marine Annelids have special respiratory
appendages, into which the fluids of the body are sent for aeration ;
and these are situated upon the head (Fig. 353), in those species
which (like the Serjmla, Terebella, Sabellaria, &c.) have their
bodies enclosed by tubes, either formed of a shelly substance pro-
duced from their own surface, or built-up by the agglutination of
grains of sand, fragments of shell, &c. ; whilst they are distributed
along the two sides of the body in such as swim freely through the
water, or crawl over the surfaces of rocks, as is the case with the
Nereidce, or simply bury themselves in the sand, as the Arenicola
or ' lob-worm.' In these respiratory appendages the circulation of
the fluids may be distinctly seen by Microscopic examination ; and
these fluids are of two kinds, — first, a colourless fluid, containing
numerous cell-like corpuscles, which can be seen in the smaller
and more transparent species to occupy the space that intervenes
* See § 129 of Siebold and Stannius's " Vergleichende Anatoinie ;" also
" Miiller's Archiv.," 1850, p 485.
ANNELIDS : — TEEEBELLA.
665
Fig. 353.
between the outer surface of the alimentary canal and the inner
wall of the body, and to pass from this into canals which often
ramify extensively in the respi-
ratory organs, but are never fur-
nished with a returning series of
passages, — and second, a fluid
which is usually red, contains
few floating particles, and is en-
closed in a system of proper vessels
that communicates with a central
propelling organ, and not only
carries the fluid away from this,
but also brings it back again.
In Terebella we find a distinct
provision for the aeration of both
fluids ; for the first is transmitted
to the tendril-like tentacles which
surround the mouth (Fig. 353,
b, b), whilst the second circulates
through the beautiful arborescent
gill-tufts {h,-h), situated just be-
hind the head. The former are
covered with cilia, the . action of
which continually renews the
stratum of water in contact with
them, whilst the latter are desti-
tute of these organs ; and this
seems to be the general fact as to
the several appendages to which
these two fluids are respectively
sent for aeration, the nature of
their distribution varying greatly
in the different members of the
class. The red fluid is commonly
considered as blood, and the tubes
through which it circulates as
blood-vessels ; but the Author
has elsewhere given his reasons*
for coinciding in the opinion of
Prof. Huxley, that the colourless
corpusculated fluid which moves
in the peri- visceral cavity of the
body and in its extensions, is that
which really represents the blood
of other Articulated animals ; and
that the system of vessels carrying
the red fluid is to be likened on
the one hand to the ' water- vas-
m
i
Circulating Apparatus of Terebella
conchilega : — a, labial ring ; b, b, ten-
tacles ; c, first segment of the trunk;
(?, skin of the back ; e, pharynx ; /,
intestine; g, longitudinal muscles of
the inferior surface of the body ; h,
glandular organ (liver ?) ; i, organs
of generation ; j,feet ; k, le, branchiae ;
?, dorsal vessel acting as a respira-
tory heart ; m, dorso-intestinal ves-
sel; n, venous sinus surrounding
oesophagus ; n', inferior intestinal
vessel ; o, o, ventral trunk ; p, lateral
vascular branches.
* See his "Principles of Comparative Physiology," 4th Edit., §§ 218, 219, 292.
m DEVELOPMENT OF ANNELIDS.
cular system' of the inferior Worms, and on the other to the
tracheal apparatus of Insects (§ 594). — In the observation of
the beautiful spectacle presented by the respiratory circulation
of the various kinds of Annelids which swarm on most of our
shores, and in the examination of what is going-on in the
interior of their bodies (where this is rendered possible by their
transparence), the Microscopist will find a most fertile source of
interesting occupation ; and he may easily, with care and patience,
make many valuable additions to our present stock of know-
ledge on these points. There are many of these marine Annelids,
in which the appendages of various kinds put-forth from the sides
of their bodies furnish very beautiful microscopic objects ; as do
also the different forms of teeth, jaws, &c, with which the mouth
is commonly armed in the free or non-tubicolar species, these being
eminently carnivorous.
554. The early history of the Development of Annelids, too, is
extremely curious ; for they come forth from the egg in a condition
very little more advanced than the ciliated gemmules of Polypes,
consisting of a globular mass of untransformed cells, certain parts
of whose surface are covered with cilia ; in a few hours, however,
this embryonic mass elongates, and indications of a segmental
division become apparent, the head being (as it were) marked-off
in front, whilst behind this is a large segment thickly covered with
cilia, then a narrower and non-ciliated segment, and lastly the
caudal or tail-segment, which is furnished with cilia. A little
later, a new segment is seen to be interposed in front of the
caudal ; and the dark internal granular mass shapes itself into the
outline of an alimentary canal.* The number of segments pro-
gressively increases by the interposition of new ones between the
caudal and its preceding segments ; the various internal organs
become more and more distinct, eye-spots make their appearance,
little bristly appendages are put-forth from the segments, and
the animal gradually assumes the likeness of its parent ; a few
days being passed by the tubicolar kinds, however, in the actively-
moving condition, before they settle down to the formation of
a tube.f
* A most curious transformation once occurred within the Author's experi-
ence in the larva of an Annelid, which was furnished with a broad collar or
disk fringed with very long cilia, and showed merely an appearance of seg-
mentation in its hinder part; for in the course of a few minutes, during which
it was not under observation, this larva assumed the ordinary form of a marine
Worm three or four times its previous length, and the ciliated disk entirely
disappeared. An accident unfortunately prevented the more minute examina-
tion of this Worm, which the Author would have otherwise made ; but he may
state that he is certain that there was no fallacy as to the fact above stated ;
this larva having been placed by itself in a cell, on purpose that it might be
carefully studied, and having been only laid aside for a short time whilst other
selections were being made from the same gathering of the Tow-net.
t See especially the admirable Memoir of Prof. Milne-Edwards, 'Sur le
DeVeloppement des Annelides,' in the " Ann. des Sci. Nat.," Se'r. 3, Zool.,
DEVELOPMENT OF ANNELIDS : — ACTINOTEOCHA.
Fig. 354.
555. To carry out any systematic observations on the embryonic
development of Annelids, the eggs should be searched-for in the
situations which these animals haunt ; but in places where Anne-
lids abound, free-swimming larvae are often to be obtained at the
same time and in the same manner
as small Medusae (§ 480) ; and there
is probably no part of our coasts off
which some very curious forms may
not be met with. The following may
be specially mentioned as departing
widely from the ordinary type, and
as in themselves extremely beautiful
objects. — The Adinotrocha (Fig. 354)
bears a strong resemblance in many
particulars to the ' bipinnarian' larva
of a Star-fish (§ 502), having an elon-
gated body, with a series of ciliated
tentacles (d) symmetrically arranged;
these tentacles, however, proceed
from a sort of disk which somewhat
resembles the ' lophophore' of certain
Polyzoa (§ 508). The mouth (e) is
concealed by a broad but pointed hood
or ' epistome' (a), which sometimes
closes-down upon the tentacular disk,
but is sometimes raised and extended
forwards. The nearly cylindrical
body terminates abruptly at the other
extremity, where the anal orifice of
the intestine (b) is surrounded by
a circlet of very large cilia. This
animal swims with great activity,
sometimes by the tentacular cilia,
sometimes by the anal circlet, some-
times by both combined ; and besides stomach; d, ciliated tentacles; e,
its movement of progression, it mouth,
frequently doubles itself together,
so as to bring the anal extremity and the epistome almost into
contact. It is so transparent that the whole of its alimentary
canal may be as distinctly seen as that of Bowerbankia (§ 511) ;
and, as in that Polyzoon, the alimentary masses often to be seen
within the stomach (c) are kept in a continual whirling movement
by the agency of cilia with which its walls are clothed. This
very interesting creature was for a long time a puzzle to Zoologists ;
since, although there could be little doubt of its being a larval
form, there was no clue to the nature of the adult produced from it,
Actinotrocha branchiata : — er,
Epistome or hood; b, anus; c,
Tom. iii. ; and the recent Systematic Treatise of M. de Quatrefages, entitled,
'Histoire Naturelle des Annelides,' in the "Suites a Buffon."
668 LARVAL ZOOLDS OF WORMS.
until this was discovered by Krohn to be a Sipunculide worm * The
process of transformation has been subsequently more fully
described by Dr. A. Schneider, and seems to consist in a sort of
turning-inside-out of the Actinotrocha. A long convoluted tube
which was previously to be seen within the cavity of its body,
closed at one end and opening at the other upon the ventral sur-
face, is the body-wall of the future Worm ; this everts itself, and
issues from the body of the larva, at the same time completely
taking-in its intestine, which is doubled together (as in a hernial
protrusion), so that the mouth and anus are brought into close
apposition with each other at the anterior end of the body. The
entire body- wall of the larva, with the hood and the anal circlet of
cilia, disappears ; the tentacles remain for a time at the anterior
extremity of the tube, contracted into a close circlet ; this circlet
is subsequently cast-off, however, by a kind of moult, at which
period the whole surface of the body has become clothed with cilia.
The development of the circulating apparatus commences before
the transformation, and this apparatus comes soon afterwards into
active operation.f
556. An even more extraordinary departure from the ordinary
type is presented by the larva which has received the name
PUidium (Fig. 355) ; its shape being that of a helmet, the plume
of which is replaced by a single long bristle-like appendage that
is in continual motion, its point moving round and round in a
circle. This curious organism, first noticed by Muller, has been
since ascertained to be the larva of the well-known Nemertes, a
Turbellarian worm of enormous length, which is commonly found
entwining itself among the roots of AlgaB.J
557. Among the animals captured by the Tow-net, the marine
Zoologist will be not unlikely to meet with an Annelid which,
although by no means Microscopic in its dimensions, is an admi-
rable subject for Microscopic observation, owing to the extreme
transparence of its entire body, which is such as to render it difficult
to be distinguished when swimming in a glass jar, except by a very
favourable light. This is the Tomopteris, so named from the divi-
sion of the lateral portions of its body into a succession of wing-like
segments (Plate XXIII., b), each of them carrying at its ex-
tremity a pair of pinnules, by the movements of which the animal
is rapidly propelled through the water. The full-grown animal,
which measures nearly an inch in length, has first a curious pair
* 'Ueber PUidium und Actinotrocha,'1 in "Muller's Archiv.," 1858, p. 293 ; see
also Wagener, ' Ueber den Ban der Actinotrocha branchiata' op.cit., 1857, p. 202.
•j- ' On the Development of Actinotrocha branchiata ' in the " Monatsberichte "
of the Berlin Academy for Oct. 1861, p. 934, and in "Ann. of Nat. Hist.,"
Se'r. 3, Vol. ix. (1862), p. 486. — The Author has met with Actinotrocha, some-
times in large numbers together, in Lamlash Bay, Arran ; and Dr. Cobbold has
taken it in the Frith of Forth.
\ See especially Leuckart and Pagenstecher's ' Untersnchungen uber niedere
Seethiere,' in " Muller's Archiv.," 1853, p. 569. The Author has frequently
met with PUidium in Lamlash Bay.
ANNELIDS ; — TOMOPTEEIS.
669
of 'frontal horns' projecting laterally from the head, so as to give
the animal the appearance of a ' hammer-headed' Shark ; behind
these there is a pair of very long antennas, in each of which we
distinguish a rigid bristle-Eke stem or seta, enclosed in a soft sheath,
Fig. 355.
Pilidium gyrans : — A, young, shewing at a the alimentary
canal, and at b the rudiment of the Xeniertid ; — B, more ad-
vanced stage of the same ; — c, newly-freed Nemertid.
and moved at its base by a set of mnscles contained within the
lateral protuberances at the head. Behind these are about sixteen
pairs of the ordinary pinnnlated segments, of which the hinder
ones are much smaller than those in front, gradually lessening in
size until they become almost rudimentary ; and where these cease,
the body is continued onwards into a tail-like prolongation, the
length of which varies greatly according as it is contracted or ex-
tended. This prolongation, however, bears four or five pairs of
very minute appendages, and the intestine is continued to its very
extremity ; so that it is really to be regarded as a continuation of
670 ANNELIDS :— TOMOPTEEIS.
the body. In the head we find, between the origins of the antenna?,
a ganglionic mass, the component cells of which may be clearly
distinguished under a sufficient magnifying power, as shown at p ;
seated upon this are two pigment-spots (b, b), each bearing a double
pellucid lens-like body, which are obviously rudimentary eyes :
whilst imbedded in its anterior portion are two peculiar nucleated
vesicles, a, a, which are probably the rudiments of some other
sensory organs. On the under side of the head is situated the
mouth, which, like that of many other Annelids, is furnished with a
sort of proboscis that can be either projected or drawn-in ; a short
oesophagus leads to an elongated stomach, which, when distended
with fluid, occupies the whole cavity of the central portion of the
body, as shown in fig. b, but which is sometimes so empty and con-
tracted as to be like a mere cord, as shown in fig. c. In the caudal
appendage, however, it is always narrowed into an intestinal canal ;
this, when the appendage is in extended state as at c, is nearly
straight ; but when the appendage is contracted, as seen at b, it is
thrown into convolutions. The perivisceral cavity is occupied by
fluid in which some minute corpuscles may be distinguished ; and
these are kept in motion by cilia which clothe some parts of the outer
surface of the alimentary canal and line some parts of the wall of
the body. ISTo other more special apparatus either for the circulation
or for the aeration of the nutrient fluid, exists in this curious
Worm ; unless we are to regard as subservient to the respiratory
function the ciliated canal which may be observed in each of the
lateral appendages except the five anterior pairs. This canal com-
mences by two orifices at the base of the segment, as shown at fig.
e, b, and on a larger scale at fig. D ; each of these orifices (d, a, b)
is surrounded by a sort of rosette ; and the rosette of the larger one
(a) is furnished with radiating ciliated ridges. The two branches
incline towards each other, and unite into a single canal, that runs
along for some distance in the wall of the body, and then terminates
in the perivisceral cavity ; and the direction of the motion of the
cilia which line it is from without inwards.
558. The Reproduction and Developmental history of this
Annelid present many points of great interest. The sexes appear
to be distinct, ova being found in some individuals, and spermatozoa
in others. The development of the ova commences in certain
' germ-cells' situated within the extremities of the pinnulated seg-
ments, where they project inwards from the wall of the body ; these,
when set free, float in the fluid of the perivisceral cavity, and
multiply themselves by self -division ; and it is only after their
number has thus been considerably augmented, that they begin to
increase in size and to assume the characteristic appearance of ova.
In this stage they usually fill the perivisceral cavity not only of the
body but of its caudal extension, as shown at c ; and they escape
from it through transverse fissures which form in the outer wall of
the body, at the third and fourth segments. The male reproductive
organs, on the other hand, are limited to the caudal prolongation,
PLATE XXIII.
TOMOPTEEIS OSTSCIJOBMIS.
[To face p. 670.
EEPBODUCTION AND DEVELOPMENT OF TOMOPTEKIS. 671
where the sperm-cells are developed within the pinrmlated appen-
dages, as the germ-cells of the female are within the appendages of
the body. Instead of being set free, however, into the perivisceral
cavity, they are retained within a saccular envelope forming a testis
(a, a, a) which fills up the whole cavity of each appendage ; and
within this the spermatozoa may be observed, when mature, in
active movement. They make their escape externally by a passage
that seems to communicate with the smaller of the two just-men-
tioned rosettes ; but they also appear to escape into the perivisceral
cavity by an aperture that forms itself when the spermatozoa are
mature. "Whether the ova are fertilized while yet within the body
of the female, by the entrance of spermatozoa through the ciliated
canals, or after they have made their escape from it, has not yet
been ascertained. — Of the earliest stages of embryouic development
nothing whatever is yet known ; but it has been ascertained that
the animal passes through a larval form, which differs from the
adult not merely in the number of the segments of the body (which
successively augment by additions at the posterior extremity), but
also in that of the antennae. At g is represented the earliest larva
hitherto met with, enlarged as much as ten times in proportion to
the adult at b ; and here we see that the head is destitute of the
frontal horns, but carries a pair of setigerous antennas, a, a, behind
which there are five pairs of bifid appendages, b, c, d, e, f. in the
first of which, b, one of the pinnules is furnished with a seta. In
more advanced larvas having eight or ten segments, this is de-
veloped into a second pair of antenna? resembling the first ; and the
animal in this stage has been described as a distinct species, T.
quadricornis. At a more advanced age, however, the second pair
attains the enormous development shown at b ; and the first or
larval antennas disappear, the setigerous portions separating at a
sort of joint (g, a, a) whilst the basal projections are absorbed into
the general wall of the body. — This beautiful creature has been
met-with on so many parts of our coast, that it cannot be con-
sidered at all uncommon ; and the Microscopist can scarcely have
a more pleasing object for study.* Its elegant form, its crystal
clearness, and its sprightly, graceful movements render it
attractive even to the unscientific observer ; whilst it is of special
interest to the Physiologist, as one of the simplest examples yet
known of the Annelid type.
559. To one phenomenon of the greatest interest, presented by
various small Marine Annelids, the attention of the Microscopist
should be specially directed ; this is their luminosity, which is not
a steady glow like that of the Glow-worm or Fire -fly, but a
series of vivid scintillations (strongly resembling those produced
by an electric discharge through a tube spotted with tin-foil),
that pass along a considerable number of segments, lasting for
an instant only, but capable of being repeatedly excited by any
* See the Memoirs of the Author and M. Claparede in Vol. xxii. of the
" Linnsean Transactions," and the authorities there referred to.
672 FEESH-WATEE ANNELIDS : — NAIS.
irritation applied to the body of the animal. These scintillations
may be discerned under the Microscope, even in separate seg-
ments, when they are subjected to the irritation of a needle-
point or to a gentle pressure ; and it has been ascertained by
the careful observations of M. de Quatrefages, that they are given
out by the muscular fibres in the act of contraction.*
560. Among the fresh- water Annelids, those most interesting to
the Microscopist are the worms of the Nais tribe, which are
common in our rivers and ponds, living chiefly amidst the mud at
the bottom, and especially among the roots of aquatic plants.
Being blood-red in colour, they give to the surface of the mud,
when they protrude themselves from it in large numbers and keep
the protruded portion of their bodies in constant undulation, a very
peculiar appearance ; but if disturbed, they withdraw themselves
suddenly and completely. These Worms, from the extreme trans-
parence of their bodies, present peculiar facilities for Microscopic
examination, and especially for the study of the internal circulation
of the red liquid commonly considered as blood. There are here
no external respiratory organs ; and the thinness of the general
integument appears to supply all needful facility for the aeration
of the fluids. One large vascular trunk (dorsal) may be seen lying
above the intestinal canal, and another (ventral) beneath it ; and
each of these enters a contractile dilatation, or heart-like organ,
situated just behind the head. The fluid moves forwards in the
dorsal trunk as far as the heart, which it enters and dilates ; and
when this contracts, it propels the fluid partly to the head, and
partly to the ventral heart, which is distended by it. The ventral
heart, contracting in its turn, sends the blood backwards along the
ventral trunk to the tail, whence it passes towards the head as
before. In this circulation, it branches-ofi2 from each of the
principal trunks into numerous vessels proceeding to different
parts of the body, which then return into the other trunk ; and
there is a peculiar set of vascular coils, hanging down in the peri-
visceral cavity that contains the corpusculated liquid representing
the true blood, which seem specially destined to convey to it the
aerating influence received by the red fluid in its circuit, thus
acting (so to speak) like internal gills. — The Naiad-worms have
been observed to undergo spontaneous division during the summer
months ; a new head and its organs being formed for the posterior
segment behind the line of constriction, before its separation from
the anterior. It has been generally believed that each segment
continues to live as a complete worm ; but it is asserted by Dr. T.
Williams that from the time when the division occurs, neither half
takes in any more food, and that the two segments only retain
vitality enough to enable them to be (as it were) the ' nurses' of
the eggs which both include. — In the Leech tribe, the dental ap-
t See his Memoirs on the Annelida of La Manche, in " Ann. des Sci. Nat.,"
Ser, 2, Zool., Tom. xix., and Ser. 3, Zool., Tom. xiv.
MOUTH OF LEECH. 673
paratus with, which the mouth is furnished is one of the most curious
among their points of minute structure ; and the common ' medi-
cinal' Leech affords one of the most interesting examples of it.
What is commonly termed the ' bite' of the leech, is really a saw-
cut, or rather a combination of three saw-cuts, radiating from a
common centre. If the mouth of the leech be examined wita a
hand-magnifier, or even with the naked eye, it will be seen to be a
triangular aperture in the midst of a sucking disk ; and on turning
back the lips of that aperture, three little white ridges are brought
into view. Each of these is the convex edge of a horny semi-
circle, which is bordered by a row of eighty or ninety minute hard
and sharp teeth ; whilst the straight border of the semicircle is
imbedded in. the muscular substance of the disk, by the action of
which it is made to move backwards and forwards in a saw-like
manner, so that the teeth are enabled to cut into the skin to which
the suctorial disk has affixed itself.*
* Among the more recent sources of information as to the Anatomy and
Physiology of the Annelids, the following may be specially mentioned: — The
" Histoire Naturelle des Anneles Marin et d'Eau douce" of M. de Quatrefages,
forming part of the " Suites a Buffon ;" the successive admirable Monographs of
the late M. Ed. Claparede, " Becherches Anatomiques sur les Annelides, Tur-
bellarie's, Opalines, et Gregarines, observes dans les Hebrides " (Geneva, 1861) ;
"Becherches Anatomiques sur les Oligochetes" (Geneva, 1862); "Beobaeh-
tungen iiber Anatomie und- Entwickelungsgeschichte Wirbellosen Thiere
an der Kiiste von Normandie " (Leipzig, 1863); and "Les Annelides Che"to-
podes du Golfe de Naples" (Geneva, 1868-70); the Monograph of Dr.
Ehlers, "Die Borstenwiirmer (Annelida Chsetopoda)," 1864-8; and lastly, Dr.
Macintosh's "Monograph of the British Annelids," now in course of publication
by the Bay Society.
X X
CHAPTER XYI.
CRUSTACEA.
561. Passing from the lower division of the Articulated series
to that of Arthropods, in which the body is furnished with distinctly
articulated or jointed limbs, we come first to the Class of Crus-
tacea, which includes (when used in its most comprehensive sense)
all those animals belonging to this group, which are fitted for
aquatic respiration. It thus comprehends a very extensive range
of forms ; for although we are accustomed to think of the Crab,
Lobster, Cray-fish, and other well-known species of the order
Decapoda (ten-footed), as its typical examples, yet all these belong
to the highest of its many orders ; and among the lower are many
of a far simpler structure, and not a few which would not be
recognized as belonging to the class at all, were it not for the
information derived from the study of their development as to
their real nature, which is far more apparent in their early than it
is in their adult condition. Many of the inferior kinds of Crus-
tacea are so minute and transparent, that their whole structure
may be made-out by the aid of the Microscope without any pre-
naration ; this is the case, indeed, with nearly the whole group of
Entomostraca (§ 563), and with the larval forms even of the Crab
and its allies (§ 574) ; and we shall give our first attention to these,
afterwards noticing such points in the structure of the larger kinds
as are likely to be of general interest.
562. A curious example of the reduction of an elevated type to
a very simple form is jDresented by the group of Pycnogonida,
some of the members of which may be found by attentive search
in almost every locality where Sea- weeds abound ; it being their
habit to crawl (or rather to sprawl) over the surfaces of these, and
probably to imbibe as food the gelatinous substance with which
they are invested.* The general form of their bodies (Fig. 356)
usually reminds us of that of some of the long-legged Crabs ; the
abdomen being almost or altogether deficient, whilst the head is
very small, and fused (as it were) into the thorax ; so that the last-
named region, with the members attached to it, constitutes nearly
* It is remarkable that very large forms of this group, sometimes extending
to nearly twelve inches across, have been brought up from great depths of
the sea, where (as there are no sea-weeds) they would seem to feed upon
Bathybius (§ 366).
CRUSTACEA
■PYCNOGONIDA.
or;
the whole bulk of the animal. The head is extended in front
into a proboscis-like projection, at the extremity of which is the
narrow orifice of the mouth ; which seems to be furnished with
vibratile cilia, that serve to draw into it the semi-fluid aliment.
Instead of being furnished (as in the higher Crustaceans) with
two pairs of antennae and numerous pairs of ' feet-jaws,' it has
but a single pair of either ; it also bears four minute ocelli, or
rudimentary eyes, set at a little distance from each other on a sort
of tubercle. From the thorax proceed four pairs of legs, each
Fig. 356.
Ammothea pycnorjonoides : — a, narrow oesophagus; b, stomach;
c, intestine ; d, digestive caeca of the feet-jaws ; e e, digestive
ceeca of the legs.
composed of several joints, and terminated by a hooked claw ;
and by these members the animal drags itself slowly along, in-
stead of walking actively upon them like a crab. The mouth
leads to a very narrow oesophagus (a), which passes back to the
central stomach (b) situated in the midst of the thorax, from the
hinder end of which a narrow intestine (c) passes-ofx, to terminate
at the posterior extremity of the body. From the central stomach
five pairs of caecal prolongations radiate ; one pair (d) entering the
feet-jaws, the other four (e, e) penetrating the legs, and passing
along them as far as the last joint but one ; and those extensions
xx2
676 OEUSTACEA: — PYCNOGONIDA.
are covered with a layer of brownish-yellow granules, which are
probably to be regarded as a diffused and rudimentary condi-
tion of the liver. The stomach and its cascal prolongations are
continually executing peristaltic movements of a very curious
kind ; for they contract and dilate with an irregular alternation,
so that a flux and reflux of their contents is constantly taking
place between the central portion and its radiating extensions, and
between one of these extensions and another. The perivisceral
space between the widely-extended stomach and the walls of the
body and limbs is occupied by a transparent liquid, in which are
seen floating a number of minute transparent corpuscles of irre-
gular size ; and this fluid, which represents the blood, is kept in
continual motion, not only by the general movements of the
animal, but also by the actions of the digestive apparatus ; since,
whenever the csecuni of any one of the legs undergoes dilatation, a
part of the circumambient liquid will be pressed-out from the
cavity of that limb, either into the thorax, or into some other limb
whose stomach is contracting. The fluid must obtain its aeration
through the general surface of the body, as there are no special
organs of respiration. The nervous system consists of a single
ganglion in the head (formed by the coalescence of a pair), and of
another in the thorax (formed by the coalescence of four pairs),
with which the cephalic ganglion is connected in the usual mode,
namely, by two nervous cords which diverge from each other to
embrace the oesophagus. Of the reproduction of these animals,
very little is yet known.* — In the study of the very curious phe-
nomena exhibited by the digestive apparatus, as well as of the
various points of internal conformation which have been described,
the Achromatic Condenser will be found useful, even with the 1 inch,
2-3rds inch, or -| inch Objectives ; for the imperfect transparence of
the bodies of these animals renders it of importance to drive a
large quantity of light through them, and to give to this light such
a quality as shall define the internal organs as sharply as possible.
563. Entomostraca. — This group of Crustaceans, nearly all the
existing members of which are of such minute size as to be only
just visible to the naked eye, is distinguished by the enclosure of
the entire body within a horny or shelly casing ; which sometimes
closely resembles a bivalve shell in form and in the mode of junc-
tion of its parts, whilst in other instances it is formed of only a
single piece, like the hard envelope of certain Rotifera (§ 414, in.).
The segments into which the body is divided, are frequently very
numerous, and are for the most part similar to each other ; but
there is a marked difference in regard to the appendages which they
bear, and to the mode in which these minister to the locomo-
tion of the animals. For in the Lophyropoda, or ' bristly-footed '
tribe, the number of legs is small, not exceeding five pairs, and
* A curious account is given by Mr. Hodge in "Ann. of Nat. Hist.," Ser. 8,
Vol. ix., p. 33, of the development of a species of Pycnogon, which in its larval
state is parasitic on the polypary of Coryne.
ENTOMOSTRACA : — OSTEACODA ; CYPRIS, CYTHERE. 677
their function is limited to locomotion, the respiratory organs being
attached to the parts in the neighbourhood of the month ; whilst in
the Bramchiopoda, or ' gill-footed' tribe, the same members (known
as ' fin-feet ') serve both for locomotion and for respiration, and the
number of these is commonly large, being in Apus not less than
sixty pairs. The character of their movements differs accordingly ;
for whilst all the members of the first-named tribe dart through
the water in a succession of jerks, so as to have acquired the com-
mon name of 'water-fleas,' those among the latter which possess a
great number of ' fin-feet,' swim with an easy gliding movement,
sometimes on their back alone (as in the case with Branchijpus),
and sometimes with equal facility on the back, belly, or sides (as is
done by Artemia salina, the ; brine shrimp'). — Some of the most
common forms of both tribes will now be briefly noticed.
564. The tribe of Lophyropoda is divided into two Orders ; of
which the first, Ostracoda, is distinguished by the complete enclo-
sure of the body in a bivalve shell, by the small number of legs,
and by the absence of an external ovary. One of the best known
examples is the little GypHs, which is a common inhabitant of
pools and streams : this may be recognized by its j)ossession of
two pairs of antenna?, the first having numerous joints with a
pencil-like tuft of filaments, and projecting forwards from the
front of the head, whilst the second has more the shape of legs,
and is directed downwards ; and by the limitation of its legs to
two pairs, of which the posterior does not make its appearance
outside the shell, being bent upwards to give support to the
ovaries/" The valves are generally opened sufficiently widely to
allow the greater part of both pairs of antennas and of the front
pair of legs to pass-out between them ; but when the animals are
alarmed, they draw these members within the shell, and close the
valves firmly. They are very lively creatures, being almost con-
stantly seen in motion, either swimming by the united action of
their foot-like antennas and legs, or walking upon plants and other
solid bodies floating in the water. — Nearly allied to the preceding
is the Cythere, whose body is furnished with three pairs of legs, all
projecting out of the shell, and whose superior antennae are desti-
tute of the filamentous brush ; this genus is almost entirely
marine, and some species of it may almost invariably be met-with
in little pools among the rocks between the tide-marks, creeping
about (but not swimming) amongst Confervas and Corallines. —
There is abundant evidence of the former existence of Crustacea of
this group, of larger size than any now existing, to an enormous
extent; for in certain fresh-water strata, both of the Secondar}-
and Tertiary series, we find layers, sometimes of great extent and
thickness, which are almost entirely composed of the fossilized
shells of Cyprides ; whilst in certain parts of the Chalk, which
was a marine deposit, the remains of bivalve shells resembling
those of Cythere present themselves in such abundance as to form
a considerable part of its composition.
678
ENTOMOSTEACOUS CEUSTACEA.
Af**
565. In the order Copepoda, there is a jointed shell forming a
kind of buckler or carapace that almost entirely encloses the head
and thorax, an opening being left beneath, through which the
members project ; and there are five pairs of legs, mostly adapted
for swimming, the fifth pair, however, being rudimentary in the
genus Cyclops, the commonest example of the group. This genus
receives its name from possessing only a single eye, or rather a
single cluster of ocelli ;
Fig. 357. which character, how-
ever, it has in common
with the two genera
already named, as well
as with Dcvphnia(§ 566),
and with many other
Entomostraca. It con-
tains numerous species,
some of which belong
to fresh water, whilst
others are marine. The
Fresh-water species
often abound in the
muddiest and most stag-
nant pools, as well as in
the clearest springs ;
the ordinary water with
which London is sup-
plied frequently con-
tains large numbers of
them. Of the marine
species, some are to be
found in the localities
in which the Cythere
is most abundant, whilst
others inhabit the open
ocean, and must be col-
lected by the Tow-
net. The body of the
Cyclops is soft and ge-
latinous, and it is com-
posed of two distinct
parts, a thorax (Fig. 357, a) and an abdomen (b), of which the latter,
being comparatively slender, is commonly considered as a tail, though
traversed by the intestine which terminates near its extremity.
The head, which coalesces with the thorax, bears one very large
pair of antennas (c), possessing numerous articulations, and fur-
nished with bristly appendages, and another small pair (d) ; it is
also furnished with a pair of mandibles or true jaws, and with two
pairs of ' feet-jaws,' of which the hinder pair is the longer and
more abundantly supplied with bristles. The legs (e) are all beset
A, Female of Cyclops quadricornis : — a, body ;
&, tail ; c, antenna ; d, antennule ; e, feet ; /, plu-
mose setae of tail: — b, tail, with external egg-
sacs: — c, d, e, f, g, successive stages of deve-
lopment of young.
BRANCHIPODA— PHYLLOPODA. 679
with plumose tufts, as is also the tail (/, /) which is borne at the
extremity of the abdomen. On either side of the abdomen of the
female, there is often to be seen an egg- capsule or external ovarium
(b) ; within which the ova, after being fertilized, undergo the
earlier stages of their development. — The Cyclops is a very active
creature, and strikes the water in swimming, not merely with its
legs and tail, but also with its antennae. The rapidly-repeated
movements of its feet- jaws serve to create a whirlpool in the sur-
rounding water, by which minute animals of various kinds, and
even its own young, are brought to its mouth to be devoured.
566. The tribe of Brancliiopocla also is divided into two Orders,
of which the Gladocera present the nearest approach to the pre-
ceding, having a bivalve carapace, no more than from four to six
pairs of legs, two pairs of antennae, of which one is large and
branched and adapted for swimming, and a single eye. The com-
monest form of this is the JDaplinia index, sometimes called the
' arborescent water-flea' from the branching form of its antennas.
It is very abundant in many ponds and ditches, coming to the sur-
face in the mornings and evenings and in cloudy weather, but seek-
ing the depths of the water during the heat of the day. It swims
by taking short springs ; and feeds on minute particles of vegetable
substances, not, however, rejecting animal matter when offered.
Some of the peculiar phenomena of its reproduction will be pre-
sently described (§ 569).
567. The other Order, PhijJlopoda, includes those Branchiopoda
whose body is divided into a great number of segments, nearly all
of which are furnished with leaf -like members, or ' fin-feet.' The
two Families which this order includes, however, differ considerably
in their conformation ; for in that of which the genera Apus and
Nebalia are representatives, the body is enclosed in a shell, either
shield-like or bivalve, and the feet are generally very numerous;
whilst in that which contains Branchi/pus and Artemia, the body
is entirely unprotected, and the nnmber of pairs of feet does not
exceed eleven. The Apus cancriformis, which is an animal of
comparatively large size, its entire length being about 2^ inches,
is an inhabitant of stagnant waters ; but although occasionally
very abundant in particular pools or ditches, it is not to be met-
with nearly so commonly as the Entomostraca already noticed. It
is recognized by its large oval carapace, which covers the head and
body like a shield ; by the nearly cylindrical form of its body,
which is composed of thirty articulations ; and by the multiplica-
tion of its legs, which amount to about sixty pairs. The number
of joints in these and in the other appendages is so great, that in a
single individual they may be safely estimated at not less than two
millions. These organs, however, are for the most part small ; and
the instruments chiefly used by the animal for locomotion are the
first pair of feet, which are very much elongated (bearing such a
resemblance to the principal antennas of other Entomostraca, as
to be commonly ranked in the same light), and are distinguished
680 ENTOMOSTKACOUS CKUSTACEA.
as rami or oars. "With these they can swim freely in any position ;
but when the rami are at rest and the animal floats idly on the
water, its fin-feet may be seen in incessant motion, causing a
sort of whirlpool in the water, and bringing to the mouth the
minute animals (chiefly the smaller Entomostraca inhabiting the
same localities) that serve them as food.- — The Branchi'pus stag-
nails has a slender, cylindriform, and very transparent body of
nearly an inch in length, furnished with eleven pairs of fin-feet,
but is destitute of any protecting envelope ; its head is furnished
with a pair of very curious prehensile organs (which are really
modified antennas), whence it has received the name of ClieirO'
cephalus ; but these are not used by it for the seizure of prey, the
food of this animal being vegetable, and their function is to clasp
the female in the act of coj)ulation. The Branchipus or Cheiro-
cephalus is certainly the most beautiful and elegant of all the
Entomostraca, being rendered extremely attractive to the view
by " the uninterrupted undulatory wavy motion of its graceful
branchial feet, slightly tinged as they are with a light reddish hue,
the brilliant mixture of transparent bluish-green and bright red of
its prehensile antenna?, and its bright red tail with the beautiful
plumose setae springing from it;" unfortunately, however, it is a
comparatively rare animal in this country .• — The Artemia salina
or ' brine shrimp' is an animal of very similar organization, and
almost equally beautiful in its appearance and movements, but
of smaller size, its body being about half an inch in length. Its
' habitat' is very peculiar ; for it is only found in the salt-pans or
brine-pits in which sea-water is undergoing concentration (as at
Lymington) ; and in these situations it is sometimes so abundant
as to communicate a red tinge to the liquid.
568. Some of the most interesting j>oints in the history of the
Entomostraca lie in the peculiar mode in which their generative
function is performed, and in their tenacity of life when desiccated,
in which last respect they correspond with many Kotifers (§ 413).
By this provision they escape being completely exterminated, as
they might otherwise soon be, by the drying-up of the pools, ditches,
and other small collections of water which constitute their usual
' habitats.' It does not appear, however, that the adult Animals
can bear a complete desiccation, although they will preserve their
vitality in mud that holds the smallest quantity of moisture ; but
their eggs are more tenacious of life, and there is ample evidence
that these will become fertile on being moistened, after having con-
tinued for a long time in the condition of fine dust. Most Ento-
mostraca, too, are killed by severe cold, and thus the whole race of
adults perishes every winter ; but their eggs seem unaffected by
the lowest temperature, and thus continue the species, which would
be otherwise exterminated.— Again, we frequently meet in this group
with that agamic reproduction, which we have seen to prevail so
extensively among the lower Radiata and Mollusca. In many
species there is a double mode of multiplication, the sexual and
EEPEODUCTIOX OF ENTOMOSTBACA: — DAPHNIA. 6S1
the non-sexual. The former takes-place at certain seasons only ;
the males (which are often so different in conformation from the
females, that they would not be supposed to belong to the same
species, if they were not seen in actual congress) disappearing
entirely at other times. The latter, on the other hand, continues
at all periods of the year, so long as warmth and food are supplied ;
and is repeated many times (as in the Hydra), so as to give origin
to as many successive ' broods.' Further, a single act of impreg-
nation serves to fertilize not merely the ova which are then mature
or nearly so, but all those subsequently produced by the same
female, which are deposited at considerable intervals. In these two
modes, the multijDlication of these little creatures is carried on with
great rapidity, the young animal speedily coming to maturity and
beginning to propagate ; so that according to the computation of
Jurine, founded upon data ascertained by actual observation, a
single fertilized female of the common Cyclops quadricornis maybe
the progenitor in one year of 4,442,189,120 young.
569. The eggs of some Entomostraca are deposited freely in the
water, or are carefully attached in clusters to aquatic Plants ; but
they are more frequently carried for some time by the parent in
special receptacles developed from the posterior part of the body ;
and in many cases they are retained there until the young are ready-
to come-forth, so that .these animals may be said to be ovo-vivi-
parous. In Daphnia, the eggs are received into a large cavity
between the back of the animal and its shell, and there the young
undergo almost their whole development, so as to come-forth in a
form nearly resembling that of their parent. Soon after their birth,
a moult or exuviation of the shell takes-place ; and the egg-cover-
ings are cast-off with it. In a very short time afterwards, another
brood of eggs is seen in the cavity, and the same process is repeated,
the shell being again exuviated after the young have been brought
to maturity. At certain times, however, the Daplinia may be seen
with a dark opaque substance within the back of the shell, which
has been called the ephippium from its resemblance to a saddle.
This, when carefully examined, is found to be of dense texture, and
to be composed of a mass of hexagonal cells ; and it contains two
oval bodies, each consisting of an ovum covered with a horny casing,
enveloped in a capsule which opens like a bivalve shell. From the
observations of Sir J. Lubbock,* it appears that the ephippium is
really only an altered portion of the carapace ; its outer valve
being a part- of the outer layer of the epidermis, and its inner
valve the corresponding part of the inner layer. The development
of the ephippial eggs takes-place at the posterior part of the ovaries,
and is accompanied by the formation of a greenish -brown mass of
granules ; and from this situation the eggs pass into the receptacle
formed by the new carapace, where they become included between
the two layers of the ephippium. This is cast-off, in process of time,
* 'An account of the two methods of Pieproduction in Daphnia, and of the
structure of the Ephippium,' in "Philosophical Transactions," 1857, p. 79.
682 ENTOMOSTRACOUS CRUSTACEA.
with tlie rest of the skin, from which, however, it soon becomes
detached : and it continnes to envelope the eggs, generally floating
on the surface of the water until they are hatched with the return-
ing warmth of spring. This curious provision obviously affords
protection to the eggs which are to endure the severity of winter
cold ; and some approach to it may be seen in the remarkable
firmness 'of the envelopes of the ' winter eg;gs' of some Eotif era
(§ 412). There seems a strong probability, from the observations
of Sir J. Lubbock, that the ' ephippial' eggs are true sexual pro-
ducts, since males are to be found at the time when the ephippia
are developed ; whilst it is certain that the ordinary eggs can be
produced non- sexually, and that the young which spring from
them can multiply the race in like manner. It has been ascertained
by Dr. Baird, that the young produced from the ephippial eggs have
the same power of continuing the race by non-sexual reproduction,
as the young developed under ordinary circumstances.
570. In most Entomostraca, the young at the time of their
emersion from the egg differ considerably from the parent, espe-
cially in having only the thoracic portion of the body as yet evolved,
and in possessing but a small number of locomotive appendages
(see Fig. 357, c-g) ; the visual organs, too, are frequently wanting
at first. The process of development, however, takes place with
great rapidity ; the animal at each successive moult (which process
is very commonly repeated at intervals of a day or two) presenting
some new parts, and becoming more and more like its parent, which
it very early resembles in its power of multiplication, the female
laying eggs before she has attained her own full size. Even when
the Entomostraca have attained their full growth, they continue
to exuviate their shell at short intervals during the whole of
life ; and this repeated moulting seems to prevent the animal
from being injured, or its movements obstructed, by the over-
growth of parasitic Animalcules and Confervas ; weak and sickly
individuals being frequently seen to be so covered with such para-
sites, that their motion and life are soon arrested, apparently
because they have not strength to cast-off and renew their enve-
lopes. The process of development appears to depend in some
degree upon the influence of light, being retarded when the animals
are secluded from it ; but its rate is still more influenced by heat ;
and this appears also to be the chief agent that regulates the time
which elapses between the moultings of the adult, these, in
Dcvphnia, taking-place at intervals of two days in warm summer
weather, whilst several days intervene between them when the
weather is colder. The cast shell carries with it the sheaths not
only of the limbs and plumes, but of the most delicate hairs and
setae which are attached to them. If the animal have previously
sustained the loss of a limb, it is generally renewed at the next
moult, as in higher Crustacea.*
* For a systematic and detailed account of this group, see Dr. Baird's " Natu-
ral History of the British Entomostraca," published by the Ray Society.
SUCTORIAL CRUSTACEA; — AEG-ULUS ; LEK1LEA. 6S3
571. Closely connected with the Entomostracons group is the
tribe of suctorial Crustacea ; which for the most part live as
parasites upon the exterior of other animals (especially Fish),
whose juices they imbibe by means of the peculiar proboscis-like
organ which takes in them the place of the jaws of other Crus-
taceans ; whilst other appendages, representing the feet- jaws, are
furnished with hooks, by which these parasites attach themselves
to the animals from whose juices they derive their nutriment.
Many of the suctorial Crustacea bear a strong resemblance even
in their adult condition, to certain Entomostraca ; but more com-
monly it is between the earlier forms of the two groups that the
resemblance is the closest, most of the Suctoria undergoing such
extraordinary changes in their progress towards the adult con-
dition, that, if their complete forms were alone attended-to, they
might be excluded from the class altogether, as has (in fact) been
done by many Zoologists. — Among those Suctorial Crustacea which
present the nearest approach to the ordinary Entomostracous type,
may be specially mentioned the Argulus foliaceus, which attaches
itself to the surface of the bodies of fresh-water Fish, and is com-
monly known under the name of the ' fish louse.' This animal has
its body covered with a large firm oval shield, which does not
extend, however, over the posterior part of the abdomen. The
mouth is armed with a pair of styliform mandibles ; and on each
side of the proboscis there is a large short cylindrical appendage,
terminated by a curious sort of sucking-disk, with another pair of
longer jointed members, terminated by prehensile hooks. These
two pairs of appendages, which are probably to be considered as
representing the feet-jaws, are followed by four pairs of legs,
which, like those of the Branchiopods, are chiefly adapted for
swimming ; and the tail, also, is a kind of swimmeret. This little
animal can leave the fish upon which it feeds, and then swims
freely in the water, usually in a straight line, but frequently and
suddenly changing its direction, and sometimes turning over and
over several times in succession. The stomach is remarkable for
the large caecal prolongations which it sends out on either side,
immediately beneath the shell ; for these subdivide and ramify in
such a manner, that they are distributed almost as minutely as the
caBcal prolongations of the stomach of the Planar la (Fig. 352).
The proper alimentary canal, however, is continued backwards
from the central cavity of the stomach, as an Intestinal tube,
which terminates in an anal orifice at the extremity of the ab-
domen.— A far more marked departure from the typical form of the
class is shown in the Lerncea, which is found attached to the gills
of Fishes. This creature has a long suctorial proboscis ; a short
thorax, to which is attached a single pair of legs, which meet at
their extremities, where they bear a sucker which helps to give
attachment to the parasite ; a large abdomen ; and a pair of
pendent egg-sacs. In its adult condition it buries its anterior
portion in the soft tissues of the animal it infests, and appears to
684 SUCTORIAL CEUSTACEA :— CIEEHIPEDA.
have little or no power of changing its place. But the young,
when they come forth from the egg, are as active as the young of
Cyclops (Fig. 357, c, d), which they much resemble, and only
attain the adult form after a series of metamorphoses, in which
they cast off their locomotive members and their eyes. It is curious
that the original form is retained with comparatively slight change
by the males, which increase but little in size, and are so unlike the
females that no one would suppose the two to belong to the same
family, much less to the same species, but for the Microscopic
study of their development *
572. From the parasitic Suctorial Crustacea, the transition is not
really so abrupt as it might at first sight appear to the group of
Cirrhipeda, consisting of the Barnacles and their allies : which
like many of the Suctoria, are fixed to one spot during the adult
portion of their lives, but come into the world in a condition that
bears a strong resemblance to the early state of many of the true
Crustacea. The departure from the ordinary Crustacean type in
the adults is, in fact, so great, that it is not surprising that Zoolo-
gists in general should have ranked them in a distinct Class ; their
superficial resemblance to the Mollusca, indeed, having caused most
systematists to place them in that series, until due weight was
given to those structural features which mark their ' articulated'
character. "We must limit ourselves, in our notice of this group,
to that very remarkable part of their history, the Microscopic
study of which has contributed most essentially to the elucidation
of their real nature. The observations of Mr. J. V . Thompson,f
with the extensions and rectifications which they have subsequently
received from others (especially Mr. Spence Bate^ and Mr.
Darwin§) show that there is no essential difference between the
early forms of the sessile (Balanidee or ' acorn-shells') and of the
pedunculated Cirrhipecls (Lepadidse or ' barnacles') ; for both are
active little animals (Fig. 358, a), possessing three pairs of legs
and a pair of compound eyes, and having the body covered with an
expanded carapace, like that of many Entomostracous Crusta-
ceans, so as in no essential particular to differ from the larva of
Cyclops (Fig. 357, c). After going through a series of metamor-
phoses, one stage of which is represented in Fig. 358, b, c, these
larvae come to present a form, d, which reminds us strongly of that
of Daplinia ; the body being enclosed in a shell composed of two
valves, which are united along the back, whilst they are free along
their lower margin, where they separate for the protrusion of a
* As the group of Suctorial Crustacea is rather interesting to the professed
Naturalist than to the amateur Microscopist, even an outline view of it would
be unsuitable to the present work ; and the Author would refer such of his
readers as may desire to study it, to the excellent Treatise by Dr. JBaird already
referred to.
t "Zoological Eesearches," No ill,, 1830.
% 'On the Development of the Cirripedia,' in "Ann. of Nat. Hist.," Ser. ii.,
Vol. viii. (1851), p. 324.
§ "Monograph of the Sub-Class Cirripedia" published by the Eay Society.
METAMORPHOSIS OF CIREHIPEDS.
685
large and strong anterior pair of prehensile limbs provided with
an adhesive sucker and hooks, and of six pairs of posterior legs
adapted for swimming. This bivalve shell, with the members of
both kinds, is subsequently thrown-off ; the animal then attaches
itself by its head, a portion of which, in the Barnacle, becomes
excessively elongated into the ' peduncle' of attachment, whilst in
Fig. 358.
Development of Balanus balanoides: — A, earliest form; B,
larva after second moult ; c, side view of the same ; D, stage
immediately preceding the loss of activity ; a, stomach (?) ;
b, nucleus of future attachment (?).
Balanus it expands into a broad disk of adhesion ; the first
thoracic segment sends backwards a prolongation which arches
over the rest of the body so as completely to enclose it, and of
which the exterior layer is consolidated into the ' multivalve' shell ;
whilst from the other thoracic segments are evolved the six pairs
of cirrhl, from whose peculiar character the name of the group is
derived. These are long, slender, many-jointed, tendril-like
appendages, fringed with delicate filaments covered with cilia,
whose action serves both to bring food to the mouth, and to main-
tain aerating currents in the water. The Balani are peculiarly
interesting objects in the Aquarium, on account of the pumping
action of their beautiful feathery appendages, which may be
watched through a Tank-Microscope ; and their cast skins, often
collected by the Tow-net, are well worth mounting.
686 SHELL OF DECAPOD CEUSTACEA.
573. Malacostbaca. — The chief points of interest to the Mi-
croscopist in the more highly-organized forms of Crustacea, are
furnished by the structure of the shell, and by the phenomena of
metamorphosis, both which may be best studied in the commonest
kinds. — The Shell of the Decapods in its most complete form
consists of three strata ; namely, 1, a horny structureless layer
covering the exterior ; 2, an areolated stratum ; and 3, a laminated
tubular substance. The innermost and even the middle layers,
however, may be altogether wanting ; thus in the Phyllosomce or
* glass-crabs,' the envelope is formed by the transparent horny
layer alone; and in many of the small crabs belonging to the
genus Portuna, the whole substance of the carapace beneath the
horny investment presents the areolated structure. It is in the
large thick-shelled Crabs, that we find the three layers most
differentiated. Thus in the common Cancer pagurus, we may
easily separate the structureless horny covering after a short
maceration in dilute acid; the areolated layer, in which the
pigmentary matter of the coloured parts of the shell is chiefly
contained, may be easily brought into view by grinding-away from
the inner side as flat a piece as can be selected, having first
cemented the outer surface to the glass slide, and by examining this
with a magnifying power of 250 diameters, driving a strong light
through it with the Achromatic Condenser ; whilst the tubular
structure of the thick inner layer may be readily demonstrated,
by means of sections parallel and perpendicular to its surface.
This structure, which resembles that of dentine (§ 615), save that
the tubuli do not branch, but remain of the same size through their
whole course, may be particularly well seen in the black extre-
mity of the claw, which (apparently from some peculiarity in the
molecular arrangement of its mineral particles) is much denser
than the rest of the shell ; the former having almost the semi-
transparence of ivory, whilst the latter has a chalky opacity. In
a transverse section of the claw, the tubuli may be seen to radiate
from the central cavity towards the surface, so as very strongly to
resemble their arrangement in a tooth ; and the resemblance is
still further increased by the presence, at tolerably regular
intervals, of minute sinuosities corresponding with the laminations
of the shell, which seem, like the ' secondary curvatures' of the
dentinal tubuli, to indicate successive stages in the calcification of
the animal basis. In thin sections of the areolated layer it may be
seen that the apparent walls of the areolse are merely translucent
spaces from which the tubuli are absent, their orifices being
abundant in the intervening spaces.* The tubular layer rises-up
* The Author is bow quite satisfied of the correctness of the interpretation
put by Prof. Huxley (see his Article, ' Tegumentary Organs,' in the " Cyclop,
of Anat. and Phys.," Vol. v. p. 487) and by Prof. W. C. Williamson ('On some
Histological Features in the Shells of Crustacea,' in " Quart. Journ. of Microsc.
Science," Vol. viii., 1860, p. 38), upon the appearances which he formerly de-
scribed ("Keports of British Association" for 1847, p. 128) as indicating a
cellular structure in this layer.
METAMORPHOSIS OF DECAPODS.
687
through the pigmentary layer of the Crab's shell in little papillary
elevations, which seem to be concretionary nodules ; and it is from
the deficiency of the pigmentary layer at these parts, that the
coloured portion of the shell derives its minutely-speckled ap-
pearance.— Many departures from this type are presented by the
different species of Decapods ; thus in the Prawns, there are large
stellate pigment- spots (resembling those of Frogs, Fig. 410, c), the
colours of which are often in remarkable conformity with those of
the bottom of the rock- pools frequented by these creatures ; whilst
in the Shrimps there is seldom any distinct trace of the areolated
layer, and the calcareous portion of the skeleton is disposed in the
form of concentric rings, which seem to be the result of the con-
cretionary aggregation of the calcifying deposit (§ 669).
574. It is a very curious circumstance, that a strongly-marked
difference exists between Crustaceans that are otherwise very
closely allied, in regard to the degree of change to which their
young are subject in their progress towards the adult condition.
For whilst the common Crab, Lobster, Spiny Lobster, Prawn, and
Shrimp undergo a regular metamorphosis, the young of the Land-
crab and the Cray-fish come-forth from the egg in a form which
corresponds in all essential particulars with that of their parents.
Generally speaking, a strong resemblance exists among the young
of all the species of Decapods which undergo a metamorphosis,
whether they are afterwards to belong to the macrourous (long-
tailed) or to the brachyourous (short-tailed) division of the group ;
and the forms of these larva? are so peculiar, and so entirely
Metamorphosis of Carciinis manas: — A, first or Zoea stage ;
B, second or Megalopa stage ; c, third stage, in which it begins
to assume the adult fomi ; D, perfect form.
different from any of those into which they are ultimately to be
developed, that they were considered as belonging to a distinct
genus, Zoea, until their real nature was first ascertained by Mr.
J. Y. Thompson. Thus, in the earliest state of Carcinus mcenas
(small edible Crab), we see the head and thorax, which form the
688 METAMORPHOSIS OF DECAPOD CRUSTACEA.
principal bulk of the body, included within a large carapace or
.shield (Fig. 359, a) furnished with a long projecting spine, beneath
which the fin-feet are put-forth : whilst the abdominal segments
narrowed and prolonged, carry at the end a flattened tail-fin, by
the strokes of which upon the water, the propulsion of the animal
is chiefly effected. Its condition is hence comparable, in almost
all essential particulars, to that of Cyclops (§ 565). In the case
of the Lobster, Prawn, and other ' macrourous ' species, the meta-
morphosis chiefly consists in the separation of the locomotive and
respiratory organs ; true legs being developed from the thoracic
segments for the former, and true gills (concealed within a special
chamber formed by an extension of the carapace beneath the body)
for the latter ; and the abdominal segments increase in size, and
become furnished with appendages (false feet) of their own. In
the Crabs, or ' brachyourous ' species, on the other hand, the altera-
tion is much greater ; for besides the change first noticed in the
thoracic members and respiratory organs, the thoracic region
becomes much more developed at the expense of the abdominal, as
seen at b, in which stage the larva is remarkable for the large size
of its eyes, and hence received the name of Megalojpa when it was
supposed to be a distinct type. In the next stage, c, we find the
abdominal portion reduced to an almost rudimentary condition,
and bent under the body ; the thoracic limbs are more completely
adapted for walking, save the first pair, which are developed into
chelce or pincers ; and the little creature entirely loses the active
swimming habits which it originally possessed, and takes-on the
mode of life peculiar to the adult.
575. In collecting minute Crustacea, the Eing-net should be
used for the fresh-water species, and the Tow-net for the marine.
In localities favourable for the latter, the same ' gathering' will
often contain multitudes of various species of Entomostraca, ac-
companied, perhaps, by the larvae of higher Crustacea, Echinoderm
larvae, Annelid-larvse, and the smaller Medusas. The water con-
taining these should be put into a large glass jar, freely exposed
to the light ; and after a little practice, the eye will become so far
habituated to the general appearance and modes of movement of
these different forms of animal life, as to be able to distinguish
them one from the other. In selecting any specimen for Micro-
scopic examination, the Dipping-tube (§ 114) will be found invaluable.
If the collector should happen to gather any floating leaves of
Zostera, he will do well to examine these for Megalojja-l&rvse, which
the Author has frequently found clinging to their surface, his
attention being directed to them by the brightness of their two
black eye-spots. — The study of the Metamorphosis will be best
prosecuted, however, by obtaining the fertilized eggs which are
carried-about by the females, and watching the history of their
products. — For preserving specimens, whether of Entomostraca, or
of larvas of the higher Crustacea, the Author would recommend
Glycerine -jelly as the best medium.
CHAPTEE XYII.
INSECTS AXD ABACHSIDA.
576. There is no Class in the whole Animal Kingdom, which
affords to the Microscopist snch a wonderful variety of interesting
objects, and snch facilities for obtaining an almost endless succession
of novelties, as that of Insects. For, in the first place, the number
of different kinds that may be brought-together (at the proper
time) with extremely little trouble, far surpasses that which any
other group of animals can supply to the most painstaking col-
lector ; then again, each specimen will afford, to him who knows
how to employ his materials, a considerable number of Microscopic
objects of very different kinds ; and, thirdly, although some of these
objects require much care and dexterity in their preparation, a
large proportion may be got-out, examined, and mounted, with
very little skill or trouble. Take, for example, the common House-
Fry : — its eyes may be easily mounted, one as a transparent, the
other as an opaque object (§ 586) ; its antennce, although not such
beautiful objects as those of many other Diptera, are still well worth
examination (§ 588) ; its tongue or ' proboscis' is a peculiarly in-
teresting object (§ 589), though requiring some care in its prepara-
tion ; its spiracles, which may be easily cut-out from the sides of its
body, have a very curious structure (§ 595) ; its alimentary canal
affords a very good example of the minute distribution of the
tracheae (§ 594) ; its wing, examined in a living specimen newly
come-forth from the pupa state, exhibits the circulation of the blood
in the ' nervures' (§ 593), and when dead shows a most beautiful
play of iridescent colours, and a remarkable areolation of surface,
when examined by light reflected from its surface at a particular
angle (§ 598) ; its foot has a very peculiar conformation, which is
doubtless connected with its singular power of walking over smooth
surfaces in direct opposition to the force of gravity, and on the
action of which additional light has lately been thrown (§ 600) ;
while the structure and physiology of its sexual apparatus, with
the history of its development and metamorphoses, would of
itself suffice to occupy the whole time of an observer who should
desire thoroughly to work it out, not only for months but for
690 INSECTS AND AKACHNIDA.
years.* Hence, in. treating of this department in such a work as
the present, the Author labours under the embarras des richesses ;
for to enter into such a description of the parts of the structure of
Insects most interesting to the Microscopist, as should be at all
comparable in fulness with the accounts which it has been thought
desirable to give of other Classes, would swell-out the volume to
an inconvenient bulk ; and no course seems open, but to limit the
treatment of the subject to a notice of the hinds of objects which
are likely to prove most generally interesting, with a few illustra-
tions that may serve to make the descriptions more clear, and with
an enumeration of some of the sources whence a variety of specimens
of each class may be most readily obtained. And this limitation
is the less to be regretted, since there already exist in our
language numerous elementary treatises on Entomology, wherein
the general structure of Insects is fully explained, and the conforma-
tion of their minute parts as seen with the Microscope is adequately
illustrated.
577. A considerable number of the smaller Insects — especially
those belonging to the Orders Coleoptera (Beetles), Neuroptera
(Dragon-fly, May-fly, &c), Hymenoptera (Bee, Wasp, &c), and
JDiptera (two-winged Flies), — may be mounted entire as opaque
objects for low magnifying powers ; care being taken to spread out
their legs, wings, &c, so as adequately to display them, which may
be accomplished, even after they have dried in other positions, by
softening them by steeping them in hot water, or, where this is
objectionable, by exposing them to steam. Full directions on this
point, applicable to small and large Insects alike, will be found in
all Text-books of Entomology. There are some, however, whose
translucence allows them to be viewed as transparent objects ; and
these are either to be mounted in Canada balsam, or in Deane's
medium, Glycerine- jelly, or Farrant's gum, according to the degree
in which the horny opacity of their integument requires the
assistance of the balsam to facilitate the transmission of light
through it, or the softness and delicacy of their textures render a
preservative medium more desirable. Thus an ordinary Flea or
Bug will best be mounted in balsam ; but the various parasites of
the Louse kind, with some or other of which almost every kind
of animal is affected, should be set-up in some of the ' media.'
Some of the aquatic larvae of the Diptera and Neuroptera, which
are so transparent that their whole internal organization can be
made-out without dissection, are very beautiful and interesting
objects when examined in the living state, especially because they
allow the Circulation of the blood and the action of the dorsal
vessel to be discerned (§ 592). Among these, there is none pre-
ferable to the larva of the Ephemera marginata (Day-fly), which
is distinguished by the possession of a number of beautiful appen-
* See Mr. Lowne's valuable Treatise on " The Anatomy and Physiology of
the Blow-flv " 1870.
STRUCTURE OF INTEGUMENT. 691
dages on its body and tail, and is, moreover, an extremely common
inhabitant of onr ponds and streams. This insect passes two
or even three years in its larva state, and during this time it
repeatedly throws-off its skin ; the cast skin, when perfect, is an
object of extreme beauty, since, as it formed a complete sheath to
the various appendages of the body and tail, it continues to exhibit
their outlines with the utmost delicacy; and by keeping these
larvas in an Aquarium, and by mounting the entire series of their
cast skins, a record is preserved of the successive changes they
undergo. Much care is necessary, however, to extend them upon
slides, in consequence of their extreme fragility ; and the best plan
is to place the slip of glass under the skin whilst it is floating on
water, and to lift the object out upon the slide. — Thin sections of
Insects, Caterpillars, &c, which bring the internal parts into view
in their normal relations, may be cut with the Section-instrument
(§ 152), by first soaking the body (as suggested by Dr. Halifax) in
thick gum-mucilage, which passes into its substance, and gives
support to its tissues, and then enclosing it in a casing of melted
paraffin, made to fit the cavity of the Section-instrument.
578. Structure of the Integument. — In treating of those separate
parts of the organization of Insects which furnish the most inte-
resting objects of Microscopic study, we may most appropriately
commence with their Integument and its appendages (scales,
hairs, &c). The body and members are closely invested by a
hardened skin, which acts as their skeleton, and affords points of
attachment to the muscles by which their several parts are moved ;
being soft and flexible, however, at the joints. This skin is usually
more or less horny in its texture, and is consolidated by the
animal substance termed Chitine, as well as, in some cases, by a
small quantity of mineral matter. It is in the Coleoptera that it
attains its greatest development ; the ' derm o- skeleton' of many
Beetles being so firm as not only to confer upon them an extra-
ordinary power of passive resistance, but also to enable them to
put forth enormous force by the action of the powerful muscles
which are attached to it. It may be stated as a general rule, that
the outer layer of this dermo-skeleton is always cellular, taking the
place of an epidermis ; and that the cells are straight- sided and
closely fitted-together, so as to be polygonal (usually hexagonal) in
form. Of this we have a very good example in the superficial
layers (Fig. 372, b) of the thin horny lamellae or blades which
constitute the terminal portion of the antenna of the Cockchafer
(Fig. 371) ; this layer being easily distinguished from the inter-
mediate portion of the lamina (a), by careful focussing. In many
Beetles, the hexagonal areolation of the surface is distin-
guishable when the light is reflected from it at a particular angle,
even when not discernible in transparent sections. The integument
of the common Bed Ant exhibits the hexagonal cellular arrange-
ment very distinctly throughout; and the broad flat expansion of
the leg of the Crabro (' sand-wasp'), affords another beautiful example
y t2
692 INSECTS AND ARACHNIDA.
of a distinctly-cellular structure in the outer layer of the integu-
ment. The inner layer, however, which constitutes the principal
part of the thickness of the horny casing of the Beetle-tribe, seldom
exhibits any distinct organization ; though it may be usually sepa-
rated into several lamellae, which are sometimes traversed by tubes
that pass into them from the inner surface, and extend towards the
outer without reaching it.
579. Tegumentary Appendages. — The surface of Insects is often
beset, and is sometimes completely covered, with appendages,
having either the form of broad flat Scales, or that of Hairs
more or less approaching the cylindrical shape, or some form
intermediate between the two. — The scaly investment is most
complete among the Lepidoptera (Butterfly and Moth tribe) ;
the distinguishing character of the insects of this order being
derived from the presence of a regular layer of scales upon each
side of their large membranous wings. It is to the peculiar
coloration of the scales that the various hues and figures are due,
by which these wings are so commonly distinguished ; all the scales
of one patch (for example) b^ing green, those of another red, and
so on: for the subjacent membrane remains perfectly transparent
and colourless, when the scales have been brushed-off from its
surface. Each scale seems to be composed of two or more mem-
branous lamellae, often with an intervening deposit of pigment, on
which, especially in Lepidoptera, their colour depends. Certain
scales, however, especially in the Beetle-tribe, have a metallic
lustre, and exhibit brilliant colours that vary with the mode in
which the light glances from them ; and this ' iridescence,' which
is specially noteworthy in the scales of the Gurculio imperialis
('diamond-beetle'), seems to be a purely optical effect, depending
either (like the prismatic hues of a soap-bubble) on the extreme
thinness of the membranous lamellse, or (like those of " mother-of-
pearl,' § 526) on a lineation of surface produced by their corru-
gation. Each scale is furnished at one end with a sort of handle
or ' pedicle' (Figs. 360, 361), by which it is fitted into a minute
socket attached to the surface of the insect ; and on the wings of
Lepidoptera these sockets are so arranged that the scales lie in
very regular rows, each row overlapping a portion of the next, so
as to give to their surface, when sufficiently magnified, very much
the appearance of being tiled like the roof of a house. Such an
arrangement is said to be ' imbricated.' The forms of these scales
are often very curious, and frequently differ a good deal on the
several parts of the wings and of the body of the same individual ;
being usually more expanded on the former, and narrower and
more hair-like on the latter. A peculiar type of scale, which has
been distinguished by the designation plumule, is met with among
the Pieridw, one of the principal families of the Diurnal Lepi-
doptera. The ' plumules' are not flat, but cylindrical or bellows
shaped, and are hollow ; they are attached to the wing by a bulb,
at the end of a thin elastic peduncle that differs in length in
STRUCTURE OF TEST-SCALES. 693
different species, and proceeds from the broader, not from the
narrower end of the scale ; whilst the free extremity usually tapers
off, and ends in a kind of brash, though sometimes it is broad and
has its edge fringed with minute filaments. These ' plumules,' which
are peculiar to the males, are found on the upper surface of the
wings, partly between and partly under the ordinary scales. They
seem to be represented among the Lyccenidce by the ' battledore'
scales to be presently described (§ 581).*
580. The peculiar markings which many of these Scales exhibit,
very early attracted the attention of those engaged in the improve-
ment of the Microscope by the correction of the , Spherical and
Chromatic Aberrations (§§ 9-20) ; since these markings are entirely
invisible, however great may be the magnif}dng power employed,
under Microscopes of the older construction, owing to the necessary
limitation of their angular aperture ; whilst, as they are brought
into view with a clearness and strength that are proportionate,
within certain limits, to the extension of the angular aperture, but
still more to the perfection with which the aberrations are corrected,
they serve as ' tests' for the goodness of an Achromatic combina-
tion. At first, the scale of the Podura (Fig. 365) was the most
difficult test known for the highest powers ; and a Microscope which
could only exhibit an alternation of dark and light bands or strias
upon its surface, was considered a good one. But even the com-
plete resolution of these stria? into component markings resembling
' notes of admiration' (Plate II., fig. 2) is now considered as but a
very ordinary test for the medium powers of the Microscope ; and
' tests' of much greater difficulty, and therefore more suitable for
the higher, are afforded (as we have seen, § 148) by the valves of
Diatoms, the true structure of which may now be considered
as satisfactorily determined. Of late, however, new questions have
been raised in regard to the ' test-scales' of Insects : first, as to the
meaning or import of those stronger markings, which all accept as
the ' optical expressions' of a structure, though there are differences
of opinion as to the nature of that structure : and second, as to the
cause of the appearance of a very minute ' beading,' first brought
into notice as existing in the Podura-scale by Dr. Royston-Pigott,
but since detected in other scales ; some regarding it as an optical
illusion, whilst by Dr. B-oyston-Pigott himself it is considered as
the indication of a true ultimate structure only discernible by the
most perfectly-corrected objectives.f It seems to the Author that
in considering both these questions, it is desirable to begin with a
clear conception of what a scale is ; and to satisfy ourselves in the
first instance as to the meaning of the appearances presented in
those larger and more strongly-marked forms which can be inter-
preted with tolerable certainty, before committing ourselves to any
* See Mr. "Watson's Memoirs ' On the Scales of Battledore Butterflies,' in
"Monthly Microscopical Journal," Vol. ii. pp. 73, 314.
f See his paper 'Oq High Power Definition,' in " Monthly Microscopical
Journal," Vol. ii. p. 295.
694
INSECTS AND AEACHNIDA.
theory as to the import of those which are more minute and less
clearly defined. — That the Scales are in reality cells, analogous to
the Epidermic cells of higher animals (§ 631), can scarcely "be doubted
by any Physiologist. Their ordinary flattening is simply the result
of their drying-up ; and the exception presented by the ' plumules'
and ' battledore' scales, which have the two surfaces separated by
a considerable cavity, helps to prove the rule. It is perfectly clear
in some of these, that the membranous wall of the cell is strengthened
by longitudinal ribs, which diverge from the peduncle ; as is parti-
cularly well seen in the plumules of two West African butterflies,
Pieris Agathina and Pleris Ghloris, in which the plumules are as
much as 1 -300th of an inch in length (large enough to be studied
under the Binocular Microscope), and are of cylindrical form, save
that they are drawn -in as if by a cord at about one-half or one-
third of their length, the ribs curving inwards to this constriction.*
In ordinary scales we find similar ribs, sometimes running parallel
to each other, or nearly so (Figs. 360, 361), and occasionally con-
nected by distinct cross-bars (Fig. 364), but sometimes diverging
from the 'quill;' and where, as in Lepisma (Fig. 363), the ribs are
23arallel on one surface and divergent on the other, a very curious set
of appearances is presented by their optical intersection, which throws
considerable light on the meaning of
the PocZ^ra-raarkings. That an ap-
pearance of minute beading is really
to be seen in many scales, alike in the
ribs and in the intervening spaces, the
Author has perfectly satisfied himself
by the aid of the black-ground illumi-
nation ; and he is disposed to regard
it as resulting either from the drying-
up of the membranou s lamellae, or from
a deposit between them. But he feels
equally certain that the ribbing of the
scales, and the markings which repre-
sent that ribbing, are alike indepen-
dent of it.f
581. Among the most beautiful of
all these scales, both for colour and
for regularity of marking, are those
of the butterfly termed Morplw
Menelaus (Fig. 360). These are of
a rich blue tint, and exhibit strong
longitudinal strise, which seem due
to ribbed elevations of one of the
superficial layers. There is also
Fig. 360.
f
Scale of MorpJw Menelaus.
* See Watson, loc. cit., p. 75.
t See Dr. Maddox's 'Eemarks on the General and Particular Construction
of the Scales of some of the Lephloptera? in "Monthly Microscopical Journal,"
Vol. v. p. 247.
STRUCTURE OF TEST-SCALES.
695
Fig. 361.
an appearance of transverse striation, which cannot be seen at all
with an inferior objective, "but becomes very decided with a good
objective of medium foe as ; and is found, when submitted to the
test of a high power and good illumination, to depend upon the
presence of transverse thickenings
or corrugations, probably on the
internal surface of one of the mem-
branes, as in Fig. 360. — The large
scales of the Pol/yommatus argus
(' azure-blue' butterfly) resemble
those of the Menelaus in form and
structure, but are more delicately
marked. Their ribs are more
nearly parallel than those of the
Menelaus scale, and do not show
the same transverse striation.
When one of these scales lies
partly over another, the effect of
the oj)tical intersection of the two Scales of Pohjommatus argus (Azure-
sets of ribs at an oblique angle blue) »— a> battledore-scale,
is to produce a set of interrupted
striations, very much resembling those of the Poc7?.rra-scale. The
same Butterfly furnishes smaller scales, which are commonly
termed the 'battledore' scales, from the resem-
blance which their form presents to that
object (Fig. 361, a). These scales, which occur
in the males of several genera of the family
LyccBiiidcB, and present a considerable variety
of shape,* are marked by narrow longitudinal
ribbings, which at intervals expand into
rounded or oval elevations that give to the
scales a dotted appearance (Fig. 362) ; at the
lower part of the scale, however, these dots are
wanting. The nature of the structure which
gives rise to these appearances has lately been
a matter of considerable discussion. Dr. An-
thony describes and figures the scales as pre-
senting a series of elevated bodies, somewhat
resembling dumb-bells or shirt-studs, ranged
along the ribs, and standing out from the
general surface.f Other good observers, how-
ever, whilst recognizing the stud-like bodies
described by Dr. Anthony, regard them as not
projecting from the external surface of the
scale, but as interposed between its two la-
Fig. 362.
Battledore Scale of
Pohjommatus argus
(Azure-bine).
* See Watson, loc. cit.
t ' The Markings on the Battledore Scales of some of the LepidopteraJ in
"Monthly Microsc. Journal/' Vol. vii. pp. I, 250.
696 INSECTS AND AEACHNIDA.
mellaa ;# and this view seems to the Author to be more conformable
than Dr. Anthony's to general probability. The question affords a
very good illustration of the uncertainty often attending the inter-
pretation of appearances presented under high magnifying power ;
it would be pretty certainly resolvable by the aid of the Stereo-
scopic Binocular, if this should ever be made capable of use with
objectives of very short focus.
582. The most valuable ' test- scales,' however, are furnished by
little wingless insects ranked together by Latreille in the order
Thysanura, but now separated by Sir John Lubbock into the two
groups Collembola and true Thysanura, on account of important
differences in internal structure.f Of the former of these, the
Lepismidce constitute the typical family ; and the scale of the com-
mon Lepisma saccharina, or ' sugar-louse,' very early attracted the
attention of Microscopists on account of its beautiful shell-like
sculpture. This scale has been recently examined with great atten-
tion, and with all the advantage of the most improved powers of
amplification and illumination, on account of the aid which the re-
sults of such examination is well fitted to afford in the determination
of the vexed question of the structure of the Podura-scale (§ 583).
The insect may be found in most old houses, frequenting damp
warm cupboards, and especially such as contain sweets ; it may be
readily caught in a small pill-box, which should have a few pin-
holes in the lid ; and if a drop of chloroform be put over the holes,
the inmate will soon become insensible, and maybe then turned out
upon a piece of clean paper, and some of its scales transferred to a
slip of glass by simply pressing this gently on its body. When
viewed under a low magnifying power, this scale presents a beau-
tiful ' watered silk' appearance, which, with higher amplification,
is found to depend (as Mr. R. Beck first pointed out)J upon the in-
tersection of two sets of striee, representing the different structural
arrangements of its two superficial membranes. One of its surfaces
(since ascertained by Mr. Joseph Beck§ to be the under or attached
surface of the scale) is raised, either by corrugation or thickening,
into a series of strongly -marked longitudinal ribs, which run nearly
parallel from one end of the scale to the other, and are particularly
distinct at its margins and at its free extremity ; whilst the other
surface (the free or outer, according to Mr. J. Beck) presents a set
of less definite corrugations, radiating from the pedicle, where they
are strongest, towards the sides and free extremity of the scale, and
therefore crossing the parallel ribs at angles more or less acute
(Fig. 363). It was further pointed out by Mr. R. Beck, that the
intersection of these two sets of corrugations at different angles
produces most curious effects upon the appearances which optically
* " Proceedings of the Microscopical Society," op. cit. 278.
f See his " Monograph of the Collembola and Thysanura" published by the
Bay Society.
\ " The Achromatic Microscope," p. 50.
§ See his Appendix to Sir John Lubbock's " Monograph."
STRUCTURE OF TEST-SCALES.
697
Fig. 363.
Hit?:
represent them. For where the diverging ribs cross the longi-
tudinal ribs very obliquely, as they do near the free extremity of
the scale, the longitudinal ribs seem broken up into a series of
'notes of admiration,' like those of the Podura; but where the
crossing is transverse or nearly so, as at the sides of the scale, an
appearance is presented as of successions of large bright beads.
The conclusion drawn by the
Messrs. Beck, that these inter-
rupted appearances are " produced
by two sets of uninterrupted lines
on different surfaces," has been
confirmed by the recent careful
investigations of Mr. Morehouse.*
— With regard to the more
minute structure of this scale as
seen under the highest powers,
there is at present considerable
difference of opinion. Dr. Eoyston-
Pigott (loc. cit.) represents not
only the longitudinal and the di-
verging ribs, but also the spaces
between them, as minutely beaded.
Mr. Morehouse (loc. cit.) regards
the whole of this ' beading' as
' spurious ;' attributing it in part
to ' transverse corrugations of the
membranes' on the same surface
with the longitudinal ribs, and in
part to " faint irregular veins
branching from the diverging
ridges, and generally taking a
transverse direction." Dr. An-
thony^ again, examining the
scales by reflected light, sees a
minute beading in the longitudinal ribs, which disappears when
they are viewed by transmitted light ; but he also sees by reflected
light a series of longitudinal parallel lines between the longitu-
dinal ribs (four in each interspace), which, by transmitted light,
present interruptions that make them resemble the finer Podura-
markings. These, he thinks, may represent longitudinal plica-
tions of the membrane between the principal ribs. Of other trans-
verse markings than the beading of the longitudinal ribs, he says
nothing. — The Author is himself disposed, for the reasons pre-
viously given (§ 580), to agree on this point rather with Dr. Eoy-
ston-Pigott than with either Mr. Morehouse or Dr. Anthony. — It
is a point of some importance, that, in the scale of a type nearly
allied to Lepisma, the Macliilis polypoda, the very distinct ribbing
* ''Monthly Microscopical Journal," Yol. xi. p. 13,
f Op. cit., p. 193,
Scale of Lepisma saccliarina.
INSECTS AND ABACHNIDA.
Fig. 364.
(Fig. 364) is produced by the corrugation of the under membranous
lamina alone ; the upper or exposed lamina being smooth, with the
exception of slight undulations near the
pedicle ; and the cross-markings being
due to structure between the superposed
membranes, probably a deposit on the
interior surface of one or both of them.*
583. We now come to that which
is pre-eminently the qucestio vexata
among Microscopists at the present
time, — the real structure of the scale
of the Lepidocyrtiis curvicollis, com-
monly known as the Podura or ' spring-
tail.' The question is really one of
greater importance than might at first
sight appear ; since not only is there a
general agreement among Opticians that
the Podura- scale is a pre-eminently good
'test' both for spherical and for chromatic
aberration, but its markings are regarded
by Physiologists as affording a more
satisfactory 'test' than those of Diatom-
valves, for those qualities of an Objec-
tive which fit it for the ordinary pur-
poses of scientific investigation. So long
as it cannot be certainly known what
ought to be seen, it is obvious that the
performance of any particular glass
cannot be rightly estimated. Thus we
are now assured by Dr. Royston-Pigott,
not only that what a lens most perfectly
corrected for spherical aberration (which he maintains to be in-
compatible with perfect correction for chromatic aberration, and to
be the more important of the two) ought to show, is a minute beaded
structure, alike in the ' exclamation-markings' and in the spaces be-
tween them; but that the markings whose perfect definition had been
previously considered the aim of all constructors of high-power
Objectives, are altogether illusory, these markings representing
nothing else than the manner in which the rouleaux of beads He
with reference to one another.f It is maintaiued, on the other
hand, by a large majority of observers, that the 'beading' does not
represent a true structure ; and that, as it is reasonable to interpret
the structure of the scale of Podura according to the analogy fur-
nished by that of the Lepisma-scole, the best Objective is that
which brings the ' exclamation-marks' into most distinct view ; these
marks being affirmed to be the optical expressions of a 'ribbed'
* See Mr. Joseph Beck, op cit., p. 255.
f See his paper 'On High Power Definition,' in "Monthly Microscopical
Journal," vol. ii. p. 295, and several subsequent papers.
Scale of MacMUs polypoda.
STRUCTURE OF TEST-SCALES.
699
Fig. 365.
or corrugated arrangement of one of the membranous lamellae of
the scale, with interruptions as to the meaning of which there is
some divergence of opinion. The conclusions at which the Author
has himself arrived will be presently stated. — Although the Podu-
ridce and L&pismidce now rank as distinct Families, jet they
approximate sufficiently in general organization, as well as in
habits, to justify the expectation that their scales would be
framed upon the same plan. The Poduridce are found amidst the
sawdust of wine-cellars, in garden tool-houses, or near decaying
wood ; and derive their popular name of ' spring-tails' from
the possession by many of them of a curious caudal appendage,
by which they can leaj) like fleas. This is particularly well developed
in the species now designated Lepidocyrtus curvicollis, which fur-
nishes what are ordinarily known as ' Podura' -scales. " When full-
grown and unrubbed," says Sir John Lubbock, "this species is
very beautiful, and reflects the most gorgeous metallic tints."
Its scales are of different sizes and of different degrees of strength
of marking (Fig. 365, a, b), and are therefore by no means of uni-
form value as tests. The general
appearance of their surface, under
a power not sufficient to resolve
their marking, is that of watered
silk, light and dark bands passing
across with wavy irregularity ;
but a well-corrected Objective of
very moderate angular aperture
now suffices to resolve every dark
band into a row of short lines,
each of them thick at one end
and coming to a point at the
other, which have been called the
' exclamation' marks, from their
resemblance to 'notes of admi-
ration' (! !). Under a well-cor-
rected l-8th inch Objective, the
appearance of the markings by
transmitted light is that which is
represented in Plate II., fig. 2 ;
if, however, they are illuminated by
oblique light from above (the scales
being placed under the objective
without any cover, so as to avoid
the loss of light by reflection from
its surface), the appearances pre- lis]
sented are those shown in fig. 4 B, small scale, more'faintly marked,
when the markings are at right .
angles to the direction of the light, and in fig. 5 when they lie m the
same direction as the light with their narrow ends pointing to it.
When this last direction is reversed, the light from the points is so
Test-scales of Lepidocyrtus curvicoh-
■A, large strongly-marked scale;
700
INSECTS AND ABACHNIDA.
i'
slight, that the scales appear to have lost their markings altogether.
If moisture should insinuate itself between the scale and the cover-
ing-glass, the markings disappear entirely, as shown in fig. 3 ; and
this, which is true also of the scale of Lepisma, seems to indicate
that the markings are due rather to the plication of the mem-
branous lamellas, than to any structure in the interior of the scale. —
A certain longitudinal continuity may be traced between the ' ex-
clamation-marks' in the ordinary test-scale ; but this is much more
apparent in other scales from the same species (Fig. 366), as well
as in the scales of various allied types, which
Fig. 366. were carefully studied by the late Mr. R.
Beck * In certain other types, indeed, the
scales have very distinct longitudinal parallel
ribs, sometimes with regularly disposed cross-
bars ; these ribs, being confined to one surface
only (that which is in contact with the body) ,
are not subject to any such interference with
their optical continuity as has been shown to
I'BHi^'HIiW'WH occur *n Lepisma ; but more or less distinct
' Ifllllll/illl li'i tW'MwI indications of radiating corrugations often pre-
lilililiitlllft sent themselves. Mr. Joseph Beck thus de-
" Al scribes (op. cit., p. 250) the structure of the
scales in Lepidocyrtiis curvicollis : — "I am
convinced that the scales consist of two mem-
branes ; I have seen them partially separated.
I have satisfied myself that the two exposed
surfaces are totally dissimilar ;f that in all
cases the under surface, or that nearest the
body of the insect, is corrugated ; that in all
cases the upper surface is much less uneven,
and in many is so slight in its irregularities
that it may even be described as smooth ;
whilst I attribute the beaded appearance so
often spoken of and so easily produced, as due to the combination
of the external corrugated structure of the lower membrane and
the internal structure of the upper membrane." The appearance
* ' On the Scales of Lepidocyrtiis ? hitherto termed Podura- scales, and
their value as Tests for the Microscope,' in " Trans, of Microsc. Soc," N.S.,
Vol. x. (1862), p. 83. See also Mr. Joseph Beck in the Appendix to Sir John
Lubbock's " Monograph of the Oollenibola and Thysanura."
f The following is the method of examination adopted by Mr. Joseph Beck : —
" Place the insect from which the scales are to be obtained on a piece of velvet,
and gently press a slip of glass, which we will call No. 1, upon it ; the scales
will be shed on the under surface of the glass, and the surface adhering to the
glass will be the upper or outside surface of the scale. Having obtained a
number of the scales upon No. 1, place a glass No. 2 upon No. 1, and press
them together ; some of the scales on No. 1 will adhere to glass No. 2. The
surface adhering to glass No. 2 will be the under or inside surface of the scale. —
Treat both these glasses exactly alike ; place each in turn on the stage of the
microscope, adjust the object-glass, and breathe gently on the slide. The scales
pn No, 1 [which have their lower surface exposed] will exhibit a most won-
Ordinary scale of
Lepidocyrtiis curvicollis.
STRUCTURE OF TEST-SCALES. 701
of the interrupted ' exclamation marks' Mr. J. Beck (op. cit., p. 254)
considers to be due "to irregular corrugations of the outer
surface of the under membrane, to slight undulations on the outer
surface of the upper membrane, and to structure between the
superposed membranes." The Author has fully satisfied himself by
his own study of the Podura-sc&le, that the 'exclamation-marks'
really represent distinct ribbings or corrugations of one of its mem-
branes ; whilst from an examination of the specimens placed before
him by Mr. Wenham, he is disposed to agree with that observer
that their form is determined, not (as in Lepisma) by optical ' inter-
ruption,' but by the structure of the rib itself, which drops at the
end of each ' note' (!), and then rises again with an increased ex-
panse, as is very clearly shown in the ribs of the scale of Seira
BusJcii, especially when viewed with the black -ground illumination.
Mr. Wenham affirms the truth of this view to be further indicated,
not merely by transverse and longitudinal fractures, but also by a
specimen in which (apparently by a shifting of the covering-glass) the
'notes' are twisted transversely.* That the 'exclamation-marks'
constitute the true optical expression of the ribbed structure of this
scale, further appears from the two unrivalled photographs taken of
it by Col. Dr. Woodward. One of these photographs, taken with a
magnifying power of 2200 diameters, central monochromatic light,
immersion 1-1 6th, and amplifier, shows the ' exclamation-marks'
better than any photographic representation previously obtained ;
and it is clear that Dr. Woodward regards this as the truest
view. "Immediately afterwards," he says, " with the same optical
combination and magnifying power, without any change in the
cover-correction, by simply rendering the illuminating pencil
oblique, and slightly withdrawing the objective from its first focal
position, I obtained a negative which displays the 'bead-like' or
varicose appearance of the ribbing more satisfactorily than I had
previously been able to do."f This photograph, a copy of a portion
of which is given in fig. 3 of Plate XIII. (p. 465), shows — in the
Author's judgment— that besides the arrangement which gives
rise to the 'exclamation-marks,' there is some condition of the
membrane, which produces an appearance of beading alike in
the ' exclamation-marks' and in the intervening spaces ; whilst it
by no means justifies the doctrine of Dr. Eoyston-Pigott, that
derful and beautiful phenomenon ; the moisture from the breath, dropping on
the scales, will run up the furrows in it, and in drying return with the greatest
precision, no running across the scale, no irregularity of action, but steadily
up and down. The scales on No. 2 glass, on being treated in the same manner,
present, on the contrary, a very different appearance : the moisture collects on
the exposed [upper] surface of the scale in minute globules, and when drying
off spreads evenly over the whole surface of the scale, without any apparent
direction being given to it by unevenness in the structure of the scale, save an
indication of a slightly-undulated surface." — ("Monthly Microscopical Journal,"
vol. iv. p. 253.)
* " Monthly Microscopical Journal," Vol. ix. p. 185.
f " Monthly Microscopical Journal," Vol. v. p. 246.
702
INSECTS AND AEACHNIDA.
Fig. 367.
instead of representing longitudinal ribbings of the membrane,
the ' exclamation -marks' are mere optical effects produced by the
mode in which the beads are arranged on the plane surfaces of
the membranous lamellas. And the Author adheres, therefore, to his
previous conclusion — in which the ablest constructors of Objectives,
and the most experienced observers he knows, are in full accor-
dance,— that the sharp and distinct bringing-out of the ' exclama-
tion-marks' of the Podura scale, constitutes, when it co- exists with
the greatest practicable freedom from colour,
and with adequate ' focal depth' or ' penetra-
ting power,' the most valuable proof of the
fitness of an Objective of high power for the
purposes of scientific investigation; while
the only addition made by Dr. Royston-
Pigott to our real knowledge of the structure
of the scale, consists in the indication given
by the 'beading' (which is undoubtedly a
good test of defining power) of corrugation
or interior deposits.*
584. The Hairs of many Insects, and still
more of their larva?, are very interesting
objects for the microscope, on account of their
branched or tufted conformation ; this being
particularly remarkable in those with which
the common hairy Caterpillars are so abun-
dantly beset. Some of these afford very
good tests for the perfect correction of Ob-
jectives. Thus, the hair of the Bee is pretty
sure to exhibit strong prismatic colours, if the
Chromatic aberration should not have been
exactly neutralized ; and that of the larva of
a Dermest.es (commonly but erroneously
termed the 'bacon-beetle') was once thought
a very good test of defining power, and is
still useful for this purpose. It has a cylin-
drical shaft (Fig. 367, b) with closely-set
whorls of spiny protuberances, four or five in
each whorl ; the highest of these whorls is
composed of mere knobby spines : and the hair is surmounted by
a curious circle of six or seven large filaments, attached by
their pointed ends to its shaft, whilst at their free extremities
they dilate into knobs. An approach to this structure is seen in
the hairs of certain Myria/pods (centipedes, gally-worms, &c), of
A, Hair of Myriapod.
B, Hair of Dermestes.
* The successive Volumes of the '' Monthly Microscopical Journal," from
the 2nd (in which Dr. Eoyston-Pigott's views were first promulgated) to the
present date, teem with Papers on this subject from Mr. Jos. Beck, Mr. Mc Entire,
Dr. Maddox, Dr. Koyston-Pigott, Mr. Wenham, and Col. Dr. Woodward, which,
with a Paper by Mr. Slack in "The Student," Vol. v. p. 49, should be
consulted by such as may wish to follow out the inquiry.
MOUNTING OF PAETS OF INTEGUMENT. 703
which an example is shown in Fig. 367, a ; and some minute forms
of this class are most beautiful objects under the Binocular Micro-
scope, on account of the remarkable structure and regular arrange-
ment of their hairs.
585. In examining the Integument of Insects, and its appendages,
parts of the surface may be viewed either by reflected or trans-
mitted light, according to their degree of transparence and the
nature of their covering. The Beetle and the Butterfly tribes
furnish the greater number of the objects suitable to be viewed as
opaque objects ; and nothing is easier than to mount portions of
the elytra of the former (which are usually the most showy parts
of their bodies), or of the wings of the latter, in the manner
described in § 171. The tribe of Curculioiiidce, in which the sur-
face of the body is beset with scales having the most varied and
lustrous hues, is distinguished among Coleoptera for the brilliancy
of the objects it affords ; the most remarkable in this respect being
the well-known Curculio imperialis, or ' diamond-beetle' of South
America, parts of who.se elytra, when properly illuminated and
looked-at with a low power, show like clusters of jewels flashing
against a dark velvet ground. In many of the British Curculionidse,
which are smaller and far less brilliant, the scales lie at the bottom
of little depressions of the surface ; and if the elytra of the ' dia-
mond-beetle' be carefully examined, it will be found that each of the
clusters of scales which are arranged upon it in rows, seems to rise
out of a deep pit which sinks-in by its side. The transition from
Scales to Hairs is extremely well seen by comparing the different
parts of the surface of the diamond-beetle with each other. The
beauty and brilliancy of many objects of this kind are increased by
mounting them in cells in Canada balsam, even though they are
to be viewed with reflected light ; other objects, however, are ren-
dered less attractive by this treatment; and in order to ascertain
whether it is likely to improve or to deteriorate the specimen, it is
a good plan first to test some other portion of the body having
scales of the same kind, by touching it with turpentine, and then
to mount the part selected as an object, either in balsam, or dry,
according as the turpentine increases or diminishes the brilliancy
of the scales on the spot to which it was applied. Portions of the
wings of Lepidoptera are best mounted as opaque objects, without
any other preparation than gumming them flat down to the disk
of the wooden slide (§ 171) ; care being taken to avoid disturbing
the arrangement of the scales, and to keep the objects, when mounted,
as secluded as possible from dust. In selecting such portions, it is
well to choose those which have the brightest and the most con-
trasted colours, exotic butterflies being in this respect usually
preferable to British ; and before attaching them to their slides,
care should be taken to ascertain in what position, with the
arrangement of light ordinarily used, they are seen to the best
advantage, and to fix them there accordingly. — Whenever portions
of the Integument of Insects are to be viewed as transparent
704
INSECTS AND AEACHNIDA.
objects, for tlie display of their intimate structure, they should be
mounted in Canada balsam, after soaking for some time in turpen-
tine ; since this substance has a peculiar effect in increasing their
translucence. Not only the horny casings of perfect Insects of
various orders, but also those of their pupae, are worthy of this
kind of study ; and objects of great beauty (such as the chrysalis
case of the Emperor-moth), as well as of scientific interest, are sure
to reward such as may prosecute it with any assiduity. Further
information may often be gained by softening such parts in potash,
and viewing them in fluid. — The scales of the wings of Lepidoptera,
&c , are best transferred to the slide, by simply pressing a portion
of the wing either upon the slip of glass or upon the cover ; if none
should adhere, the glass may first be gently breathed-on. Some
of them are best seen when examined ' dry,' whilst others are more
clear when mounted in fluid ; and for the determination of their
exact structure, it is well to have recourse to both these methods.
If these scales are to be used as ' test-objects,' it is preferable
to place them between two pieces of thin glass, in the manner
specified in § 170. Hans, on the other hand, are best mounted
in Balsam.
586. Parts of the Head. — The eyes of Insects, situated upon the
upper and outer part of
the head, are usually very
conspicuous organs, and are
frequently so large as to
touch each other in front
(Fig. 368). We find in
their structure a remark-
able example of that mul-
tiplication of similar parts
which seems to be the pre-
dominating ' idea' in the
conformation of Articulated
animals ; for each of the
large protuberant bodies
which we designate as an
eye, is really an aggregate
of many hundred, or even
many thousand minute
eyes, which are designated
ocelli. Approaches to this structure are seen in the Annelida and
Entomostraca ; but the number of ocelli thus grouped- together
is usually small. In the higher Crustacea, however, the ocelli are
very numerous ; their compound eyes being constructed upon the
same general plan as those of Insects, although their shape and
position are often very peculiar (Fig. 436). The individual ocelli are
at once recognized, when the ' compound eyes' are examined under
even a low magnifying power, by the ' facetted ' appearance of the
surface (Fig. 368), which is marked-out by very regular divisions
Head and Compound Eyes of the Bee,
showing the ocelli in situ on one side (a), and
displaced on the other (b) ; a, a, a, stemmata;
b, b, antenna.
COMPOUND EYES OF INSECTS.
70 5
either into hexagons or into sqnares : each facet is the ' cor-
neule' of a separate ocellus, and has a convexity of its own;
hence by counting the facets, we can ascertain the number of
ocelli in each ' compound eye.' In the two eyes of the common
Fly, there are as many as
4000 ; in those of the Cabbage-
Butterfly there are about 17,000 ;
in the Dragon-fly, 24,000 ; and
in the Mordella Beetle, 25,000.
Behind each ' corneule' is a
layer of dark pigment, which
takes the place and serves the
purpose of the ' iris' in the eyes
of Vertebrate animals ; and
this is perforated by a central
aperture or 'pupil,' through
which the rays of light that
have traversed the corneule gain
access to the interior of the eye.
The further structure of these
bodies is best examined by ver-
tical sections (Fig. 369) ; and
these show that the shape of
each ocellus (6) is conical, or
rather pyramidal, the corneule
forming its base (a), whilst its
apex abuts upon the extremity of a fibre
(c) proceeding from the termination of the
optic nerve (d). The details of the structure
of each ocellus are shown in Fig. 370 ; in
which it is shown that each corneule is a
double-convex lens, made up by the junction
of two plano-convex lenses, a a and a' a',
which have been found by Dr. Hicks to pos-
sess different refractive powers ; by this ar-
rangement (it seems probable) the aberra-
tions are diminished, as they are by the combi-
nation of 'humors' in the Human eye. That
each ' corneule' acts as a distinct lens, may be
shown by detaching the entire assemblage by
maceration, and then drying it (flattened-out)
upon a slip of glass ; for when this is placed
under the Microscope, if the point of a knife,
scissors, or any similar object, be interposed
between the mirror and the stage, the image
of this point will be seen, by a proper adjust-
ment of the focus of the microscope, in every
one of the lenses. The focus of each ' cor-
neule' has been ascertained by experiment
z z
\ * t
Section of the Composite Eye o
Melolontha vulgaris (Cockchafer) : — a,
facets of the cornea ; 6, transparent
pyramids surrounded with pigment ;
c, fibres of the optic nerve; d, trunk
of the optic nerve.
Fig. 370.
Minute structure of the
Eye of the Bee: — a a, an-
terior lenses of corneule ;
a' a', its posterior lenses ;
c c, pupillary apertures,
separated by intervening
pigment d d? b 6, pyra-
mids separated by pig-
ment d' d', and abutting
on e e, bulbous extremi-
ties of nerve-fibres.
706 INSECTS AND AEACHNIDA.
to be equivalent to the length of the pyramid behind it ;
so that the image which it produces will fall upon the extremity
of the filament of the optic nerve which passes to the latter. The
pyramids (b, b) consist of a transparent substance, which may be
considered as representing the 'vitreous humour;' and they are
separated from each other by a layer of dark pigment d' d', which
closes-in at d d between their bases and the corneules, leaving a
set of pupillary apertures c, c, for the entrance of the rays which
pass to them from the ' corneules.' After traversing these pyra-
mids, the rays reach the bulbous extremities e, e of the fibres of
the optic nerve, which are surrounded, like the pyramid, by pig-
mentary substance. Thus the rays which have passed through
the several ' corneules ' are prevented from mixing with each other ;
and no rays, save those which pass in the axes of the pyramids,
can reach the fibres of the optic nerve. Hence it is evident, that,
as no two ' ocelli ' on the same side have exactly the same axis,
no two can receive their rays from the same point of an object ;
and thus, as each ' composite eye' is immovably fixed upon the
head, the combined action of the entire aggregate will probably
only afford but a single image, resembling that which we obtain
by means of our single eyes. — Although the foregoing may be
considered as the typical structure of the Eyes of Insects, yet
their are various departures from it (most of them slight) in the
different members of the Class. Thus in some cases the posterior
surface of each ' corneule' is concave ; and a s]3ace is left between
it and the iris-like diaphragm, which seems to be occupied by a
watery fluid or 'aqueous humor;' in other instances again, this
space is occupied by a double-convex body, which seems to repre-
sent the ' crystalline-lens ;' and this body is sometimes found
behind the iris, the number of ocelli being reduced, and each one
being larger, so that the cluster presents more resemblance to that
of Spiders, &c. — Besides their Compound Eyes, Insects usually
possess a small number of rudimentary Single Eyes, resembling
those of the Arachnida ; these are seated upon the top of the head
(Fig. 368, a, a, a), and are termed stemmata. — It is remarkable
that the Larva3 of insects which undergo a complete metamor-
phosis, only possess single eyes ; the compound eyes being deve-
loped, at the same time with the wings and other parts which are
characteristic of the Imago state, during the latter part of Pupal
life.
587. Various modes of preparing and mounting the Eyes of
Insects may be adopted, according to the manner wherein they are
to be viewed. For the observation of their external facetted
surface by reflected light, it is better to lay down the entire head,
so as to present a front-face or a side-face, according to the
position of the eyes ; the former giving a view of both eyes, when
they approach each other so as nearly or quite to meet (as in
Fig. 368) ; whilst the latter will best display one, when the eyes
are situated more at the sides of the head. For the minuter
EYES AND ANTENNAE. 707
examination of the ' corneules,' however, these must be separated
from the hemispheroidal mass whose exterior they form, by pro-
longed maceration ; and the pigment must be carefully washed
away, by means of a fine camel-hair brush, from the inner or
posterior surface. In flattening them out upon the glass -slide, one
of two things must necessarily happen ; either the margin must
tear when the central portion is pressed-down to a level ; or, the
margin remaining entire, the central portion must be thrown into
plaits, so that its corneules overlap one another. As the latter
condition interferes with the examination of the structure much
more than the former does, it should be avoided by making a
number of slits in the margin of the convex membrane before it is
flattened-out. Such preparations may be mounted either in
Liquid, Medium, or Canada balsam ; the latter being preferable
when (as sometimes happens) the membrane is so horny as to be
but imperfectly transparent. Vertical sections, adapted to demon-
strate the structure of the ocelli and their relations to the optic
nerve, can of course be only made when the body of the insect is
fresh ; and these should be mounted in Liquid or in Medium. The
following are. some of the Insects whose eyes are best adapted for
Microscopic preparations : — Coleoptera, Cicindela, Dytiscus, Melo-
lontha (Cockchafer), Lucanus (Stag-beetle) ; — Orthojptera, Acheta
(House and Field Crickets), Locusta ; — Hemiptera, Notonecta
(Boat-fly) ; — Neiirojitera, Libellula (Dragon-fly), Agrion ; — Hijme-
iioptera, Yespidae (Wasps) and Apidae (Bees) of all kinds ; —
Lejjidoptera, Yanessa (various species of Butterflies), Sphinx
ligustri (Privet hawk-moth), Bombyx (Silk- worm moth, and its
allies) ; — Vvptera, Tabanus (Gad-fly), Asilus, Eristalis (Drone-fly),
Tipula (Crane-fly), Musca (House-fly), and many others.
588. The Antennae, which are the two jointed appendages arising
from the upper part of the head of Insects (Fig. 368, b, b), present
a most wonderful variety of conformation in the several tribes of
Insects ; often differing considerably in the several species of one
genus, and even in the two sexes of the same species. Hence the
characters which they afford are extremely useful in classification ;
especially since their structure must almost necessarily be in some
way related to the habits and general economy of the creatures to
which they belong, although our imperfect acquaintance with their
function may prevent us from clearly discerning this relation.
Thus among the Coleoptera we find one large family, including
the Glow-worm, Fire-fly, Skip-jack, &c, distinguished by the
toothed or serrated form of the antennae, and hence called Serri-
comes ; in another, of which the Burying -beetle is the type, the
antennae are terminated by a club-shaped enlargement, so that
these beetles are termed Glavlcornes ; in another, again, of which
the Hydrophilus or large Water-beetle is an example, the antennae
are never longer and are commonly shorter than one of the pairs
of palpi, whence the name of Palpicornes is given to this group ;
in the very large family that includes the Lucani or Stag-beetles
z z 2
708
INSECTS AND AKACHNIDA.
with, the Scarabcei, of which the Cockchafer is the commonest
example, the antennae terminate in a set of leaf -like appendages,
which are sometimes arranged like a fan or the leaves of an open
book (Fig. 371), are sometimes parallel to each other like the teeth
of a comb, and sometimes fold one over the other, thence giving
the name Lamellicornes ; whilst another large family is dis^
tingnished by the appellation Longicomes, from the great length
of the antennae, which are at least as long as the body, and often
longer. Among the Lepidoptera, again, the conformation of the
antenna? frequently enables ns at once to distinguish the group to
which any specimen belongs. As every treatise on Entomology
contains figures and de-
scriptions of the principal
types of conformation of
these organs, there is no
occasion here to dwell
upon them longer than to
specify such as are most
interesting to the Micro-
scopist: — Goleoftera, Bra-
chinus, Calathus, Har-
palus, Dytiscus, Staphy-
linus, Philonthus, Elater,
Lampyris, Silpha, Hydro-
philus, Aphodius, Melo-
lontha, Cetonia, Curculio ;
— Orthoptera, Forfieula
(Earwig), Blatta (Cock-
roach); — Lejndoptera,
Sphinges (Hawk-moths),
and "Nocturna (Moths) of
various kinds, the large
' plumed ' antennae of the
latter being peculiarly
beautiful objects under a
low magnifying power; —
Diptera, Culicidae (Gnats
of various kinds), Tipulidae (Crane-flies and Midges), Tabanus, Eris-
talis, and Muscidae (Flies of various kinds). All the larger
antennae, when not mounted ' dry' as opaque objects, should be
put up in Balsam, after being soaked for some time in turpentine ;
but the small feathery antennae of Gnats and Midges are so liable
to distortion when thus mounted, that it is better to set them up
in fluid, the head with its pair of antennae being thus preserved
together when not too large. — A curious set of organs has been
recently discovered in the antennae of many Insects, which have
been supposed to constitute collectively an apparatus for Hearing.
Each consists of a cavity hollowed out in the horny integument,
sometimes nearly spherical, sometimes flask-shaped, and some-
4
Antenna of Melolontha (Cockchafer).
ANTENNA AND MOUTH.
709
times prolonged into numerous extensions formed by the folding of
its lining membrane ; the month of the cavity seems to be normally
closed-in by a continuation of this membrane, though its presence
cannot always be satisfactorily determined; whilst to its deepest part
a nerve-fibre may be
traced. The expanded Fig. 372.
lamellae of the antennae
of Melolontha present
a great display of these
cavities, which are in-
dicated in Fig. 372, a,
by the small circles
that beset almost their
entire area; their form,
which is very peculiar,
can here be only made
out by vertical sec-
tions ; but in many of
the smaller antennae,
such as those of the
Bee, the cavities can be
seen sideways without any other trouble than that of bleaching
the specimen to render it more transparent.*
589. The next point in the organization of Insects to which the
attention of the Microscopist may be directed, is the structure of
the mouth. Here, again, we find almost infinite varieties in the
details of . conformation ; but these may be for the most part reduced
to a small number of types or plans, which are characteristic of the
different orders of Insects. It is among the Coleoptera, or Beetles,
that we find the several parts of which the mouth is composed, in
their most distinct form ; for although some of these parts are much
more highly developed in other Insects, other parts may be so much
altered or so little developed as to be scarcely recognizable. The
Coleoptera present the typical conformation of the mandibulate
mouth, which is adapted for the prehension and division of solid
substances ; and this consists of the following parts : — 1, a pair of
jaws, termed mandibles, frequently furnished with powerful teeth,
Minute structure of leaf-like expansions of An-
tenna of Melolontha: — A, their internal layer; B,
their superficial layer.
* See the Memoir of Dr. Hicks ' On a new Structure in the Antennae of In-
sects,' in " Trans, of Linn. Soc," Vol. xxii. p. 147 ; and his 'Further Kemarks,'
at p. 383 of the same volume. See also the Memoir of M. Lespes, ' Sur l'Appa-
reil Auditif des Insectes,' in a Ann. des Sci. Nat.," Ser. 4, Zool., Tom. ix. p. 258;
and that of M. Claparede, ' Sur les pre'tendus Organes Auditifs des Cole'opteres
lamellicornes et autres Insectes,' in "Ann. des Sci. Nat.," Ser. 4, Zool., Tom. x.
p. 236. Dr. Hicks lays great stress on the 'bleaching process,' as essential to
success in this investigation ; and he gives the following directions for per-
forming it : — Take of Chlorate of Potass a drachm, and of Water a drachm and
a half ; mix these in a small wide bottle containing about an ounce ; wait five
minutes, and then add about a drachm and a half of strong Hydrochloric Acid.
Chlorine is thus slowly developed ; and the mixture will retain its bleaching
power for some time.
710
INSECTS AND ARACHNID A.
opening laterally on either side of the month, and serving as the
chief instruments of manducation ; 2, a second pair of jaws, termed
maxillce, smaller and weaker than the preceding, beneath which
they are placed, and serving to hold the food, and to convey it to
the back of the month ; 3, an npper lip, or labrum ; 4, a lower lip
or labium; 5, one or two pairs of small jointed appendages termed
palpi, attached to the maxillee, and hence called maxillary palpi ;
Fig. 373.
Tongue of common Fly: — a, lobes of ligula ; 6, portion en-
closing the lancets formed by the metamorphosis of the max-
illa? ; c, maxillary palpi : — A, portion of one of the
tracheae enlarged.
6, a pair of labial palpi. The labinm is often composed of several
distinct parts ; its basal portion being distinguished as the menturn
or chin, and its anterior portion being sometimes considerably pro-
longed forwards, so as to form an organ which is properly designated
the ligula, bnt which is more commonly known as the ' tongue,'
though not really entitled to that designation, the real tongue being
a soft and projecting organ which forms the floor of the mouth,
and which is only found as a distinct part in a comparatively small
number of Insects, as the Cricket. — This ligula is extremely
developed in the Fly kind, in which it forms the chief part of
PEOBOSCIS OF FLY AND BEE.
711
Fig. 374.
what is commonly called the ' proboscis' (Fig. 373) ;* and it also
forms the 'tongne' of the See and its allies (Fig. 374). The ligula
of the common Fry presents a curions modification of the ordinary
tracheal structure (§ 595), the purpose of which is not apparent ; for
instead of its trachea? being kept pervious, after the usual fashion,
by the winding of a continuous spiral fibre through their interior,
the fibre is broken into rings,
and these rings do not sur-
round the whole tube, but
are terminated by a set of
arches that pass from one to
another (Fig. 373, A)."f— In
the D-ijptera or two-winged
Flies generally, the labrum,
maxilla?, mandibles, and the
internal tongue (where it
exists) are converted into
delicate lancet-shaped organs
termed setce, which, when
closed-together, are received
into a hollow on the upper
side of the labium (Fig. 373, &),
but which are capable of being
used to make punctures in
the skin of Animals or the
epidermis of Plants, whence
the juices maybe drawn forth
by the proboscis. Frequently,
however, two or more of
these organs may be want-
ing, so that their number is
reduced from six, to four, a, Parts of the Month of Apis mellifica
three, or two.— In the By- (Honey-bee) :-a mentum ; b, mandibles;
. / D n TT7 c, maxillae ; d, labial palpi ; e, ligula, or
menoptera (Bee and Wasp 'longed labium, commonly termed the
tribe), however, the labrum tongue :— b, portion of the surface of the
and the mandibles (Fig. 374, b) ligula, more highly magnified.
* The representation given in this figure is taken from one of tbe ordinary
preparations of the Fly's proboscis, which is made by slitting it open, flatten-
ing it out, and mounting it in Balsam. For representations of the time relative
positions of the different parts of this wonderful organ, and for minute descrip-
tions of them, the reader is referred to Mr. Suffolk's Memoir ' On the Proboscis
of the Blow-fly,' in " Monthly Microsc. Journ.," Vol. i. p. 381; and to Mr.
Lowne's Treatise on " The Anatomy and Physiology of the Blow-fly," p. 41.
t According to Dr. Anthony ("Monthly Microsc. Journ.," Vol. xi. p. 242),
these 'pseudo-tracheae' are suctorial organs, which can take in liquid alike at
their extremities and through the whole length of the fissure caused by the
interruption of the rings ; the edges of this fissure being formed by an alternat-
ing series of 'ear-like appendages,' connected with the terminal 'arches,' the
closing-together of which converts the pseudo-tracheas into a complete tube.
Dr. A. considers each of these ear-like appendages to be a minute sucker,
" either for the adhesion of the fleshy tongue, or for the imbibition of fluids, or
perhaps for both purposes." — The point is well worthy of further investigation.
712 INSECTS AND ABACHNIDA.
much resemble those of Mandibulate Insects, and are used for cor-
responding purposes ; the maxillae (c) are greatly elongated, and
form, when closed, a tubular sheath for the Ligula or ' tongue,'
through which the honey is drawn up ; the labial palpi (d) also are
greatly developed, and fold together, like the maxillas, so as
to form an inner sheath for the ' tongue ;' while the ' ligula'
itself (e) is a long tapering muscular organ, marked by an immense
number of short annular divisions, and densely covered over its
own length with long hairs (e). It is not tubular, as some
have stated, but is solid ; when actively employed in taking food, it
is extended to a great distance beyond the other parts of the mouth ;
but when at rest, it is closely packed-up and concealed between the
maxillae. " The manner," says Mr. Newport, " in which the honey
is obtained when the organ is plunged into it at the bottom of
a flower, is by ' lapping,' or a constant succession of short and quick
extensions and contractions of the organ, which occasion the fluid
to accumulate upon it and to ascend along its upper surface, until
it reaches the orifice of the tube formed by the approximation of the
maxillaa above, and of the labial palpi and this part of the ligula
below."
590. By the plan of conformation just described, we are led to
that which prevails among the Lepidoptera or Butterfly tribe, and
which, being pre-eminently adapted for suction, is termed the
Fig. 375.
Haustellium (proboscis) of Vanessa.
haustellate mouth. In these Insects, the labrum and mandibles
are reduced to three minute triangular plates ; whilst the maxilla?
are immensely elongated, and are united together along the median
line to form the haustellium or true ' proboscis,' which contains a
*
HAUSTELLIUM OF LEPIDOPTEEA. 713
tube formed by the junction of the two grooves that are channelled
out along their mutually applied surfaces, and which serves to
pump-up the juices of deep cup-shaped flowers, into which the size
of their wings prevents these insects from entering. The length of
this haustellium varies greatly : thus in such Lepidoptera as take
no food in their perfect state, it is a very insignificant organ ; in
some of the white Hawk-moths, which hover over blossoms without
ahghting, it is nearly two inches in length ; and in most Butter-
flies and Moths it is about as long as the body itself. This ' haus-
tellium,' which, when not in use, is coiled-up in a spiral beneath
the mouth, is an extremely beautiful Microscopic object, owing to
the peculiar banded arrangement it exhibits (Fig. 375), which is
probably due to the disposition of its muscles. In many instances,
the two halves may be seen to be locked together by a set of
hooked teeth, which are inserted into little depressions between the
teeth of the opposite side. Each half, moreover, may be ascer-
tained to contain a trachea or air-tube (§ 594) ; and it is probable,
from the observations of Mr. Newport,* that the sucking-up of the
juices of a flower through the proboscis (which is accomplished
with great rapidity) is effected by the agency of the respiratory
apparatus. The proboscis of many Butterflies is furnished, for
some distance from its extremity, with a double row of small pro-
jecting barrel-shaped bpdies (shown in Fig. 375), which are sur-
mised by Mr. Newport (whose opinion is confirmed by the kindred
inquiries of Dr. Hicks, § 588) to be organs of taste. — Numerous
other modifications of the structure of the mouth, existing in the
different tribes of Insects, are well worthy of the careful study
of the Microscopist ; but as detailed descriptions of most of these
will be found in every Systematic Treatise on Entomology, the
foregoing general account of the principal types must suffice.
591. Parts of the Body. — The conformation of the several divi-
sions of the alimentary canal presents such a multitude of diver-
sities, not only in different tribes of Insects, but in different states
of the same individual, that it would be utterly vain to attempt
here to give even a general idea of it ; more especially as it is a
subject of far less interest to the ordinary Microscopist than
to the professed Anatomist. Hence we shall only stop to mention
that the ' muscular gizzard ' in which the oesophagus very commonly
terminates, is often lined by several rows of strong horny teeth for
the reduction of the food, which furnish very beautiful objects,
especially for the Binocular. These are particularly developed
among the Grasshoppers, Crickets, and Locusts, the nature of
whose food causes them to require powerful instruments for its
reduction.
592. The Circulation of Blood may be distinctly watched in
many of the more transparent larvae, and may sometimes be
observed in the perfect insect. It is kept-up, not by an ordinary
* " Cyclopaedia of Anatomy and Physiology," Vol. ii. p. 902.
714 INSECTS AND AKACHNIDA.
heart, but by a ' dorsal vessel ' (so named from the position it
always occupies along the middle of the back), which really consists
of a succession of muscular hearts or contractile cavities, one for
each segment, opening one into another from behind forwards, so
as to form a continuous trunk divided by valvular partitions. In
many larvae, however, these partitions are very indistinct ; and the
walls of the 'dorsal vessel' are so thin and transparent, that
it can with difficulty be made-out, a limitation of the light by the
diaphragm being often necessary. The blood which moves through
this trunk, and which is distributed by it to the body, is a trans-
parent and nearly-colourless fluid, carrying with it a number of
'oat-shaped' corpuscles, by the motion of which its flow can be
followed. The current enters the 'dorsal vessel' at its posterior
extremity, and is propelled forwards by the contractions of the
successive chambers, being prevented from moving in the opposite
direction by the valves between the chambers, which only open
forwards. Arrived at the anterior extremity of the ' dorsal vessel,'
the blood is distributed in three principal channels ; a central one,
namely, passing to the head, and a lateral one to either side,
descending so as to approach the lower surface of the body. It is
from the two lateral currents that the secondary streams diverge,
which pass into the legs and wings, and then return back to the
main stream ; and it is from these also, that, in the larva of the
'Ephemera marginata (Day-fly), the extreme transparence of which
renders it one of the best of all subjects for the observation of In-
sect Circulation, the smaller currents diverge into the gill-like
appendages with which the body is furnished (§ 596). The blood-
currents seem rather to pass through channels excavated among
the tissues, than through vessels with distinct walls ; but it is not
improbable that in the perfect Insect the case may be different. In
many aquatic larvae, especially those of the Gulicidce (Gnat tribe),
the body is almost entirely occupied by the visceral cavity ; and the
blood may be seen to move backwards in the space that surrounds
the alimentary canal, which here serves the purpose of the channels
usually excavated through the solid tissues, and which freely com-
municates at each end with the ' dorsal vessel.' This condition
strongly resembles that found in many Annelida.*
593. The circulation may be easily seen in the wings of many
Insects in their -pupa state, especially in those of the JSTeuroptera
(such as Dragon-flies and Day-flies) which pass this part of their
lives under water in a condition of activity ; the pupa of Agrion
puella, one of the smaller dragon-flies, is a particularly favourable
subject for such observations. Each of the 'nervures' of the
wings contains a 'trachea' or air-tube (§ 594), which branches-ofl2
from the tracheal system of the body ; and it is in a space around
* See the Memoirs on Corethra plumicornis, by Prof. Bymer Jones, in
" Transact, of Microsc. Soc," Vol. xv. (N.S.), P- 99 ; by Mr. E. Eay Laukester,
in the "Popular Science Eeview" for October, 1865; and by Dr. A. Weiss-
mann, in " Siebold and Kolliker's Zeitschrift," Bd. xvi. p. 45.
CIRCULATION AND RESPIRATION.
715
Fig. 376.
the trachea that the blood may be seen to move, when the hard
framework of the nervure itself is not too opaque. The same may
be seen, however, in the wings of pupa3 of Bees, Butterflies, &c,
which remain shnt-np motionless in their cases ; for this condition
of apparent torpor is one of great activity of their nutritive
system, — those organs, especially, which are peculiar to the perfect
Insect, being then in a
state of rapid growth,
and having a vigorous
circulation of blood
through them. In cer-
tain insects of nearly
every order, a movement
of fluid has been seen in
the wings for some little
time after their last me-
tamorphosis ; but this
movement soon ceases,
and the wings dry-up.
The common Fly is as
good a subject for this
observation as can be
easily found; it must
be caught within a few
hours or days of its
first appearance ; and
the circulation may be
most conveniently
brought into view by
enclosing it (without
water) in the aquatic
box, and pressing-down
the cover sufficiently to
keep the body at rest
without doing it any
injury.
594. The Respiratory
apparatus of Insects
affords a very interest-
ing series of Microscopic
objects ; for, with great
uniformity in its general
plan, there is almost in-
finite variety in its de-
tails. The aeration of
the blood in this class is
provided-for, not by the
transmission of the fluid to any special organ representing the lung
of a Yertebrated animal (§ 652) or the gill of a Mollusk (§ 545), but
/
Tracheal system of Xepa (Water-scorpion) :
—a. head; 6, first pair of legs ; c, first segment
of the thorax ; d, second pair of wings ; e, second
pair of legs ; /. tracheal trunk ; g, one of the stig-
mata ; h, air-sac.
716
INSECTS AND ARACHNID A.
by the introduction of air into every part of the body, through a
system of minutely-distributed trachece or air-tubes, which pene-
trate even the smallest and most delicate organs. Thus, as we
have seen, they pass into the haustellium or 'proboscis' of the
Butterfly (§ 590), and they are minutely distributed in the elon-
gated labium or ' tongue' of the Fly (Fig. 373). Their general dis-
tribution is shown in Fig. 376 ; where we see two long trunks (/)
passing from one end of the body to the other, and connected with
each other by a transverse canal in every segment ; these trunks
communicate, on the one hand, by short wide passages, with the
' stigmata,' ' spiracles,' or 'breathing-pores' (g), through which the
air enters and is discharged; whilst they give off branches to
the different segments, which divide again and again into ramifica-
tions of extreme minuteness. They usually communicate also
with a pair of air-sacs (h) which is situated in the thorax ; but the
size of these (which are only found in the perfect Insect, no trace
of them existing in the larvae) varies greatly in different tribes,
being usually greatest in those insects which (like the Bee) can
sustain the longest and most powerful flight, and least in such as
habitually live upon the ground or upon the surface of the water.
The structure of the air-tubes reminds us of that of the ' spiral
vessels ' of Plants, which seem destined (in part at least) to per-
form a similar office (§ 331) ;
Fig. 377. for within the membrane
that forms their outer wall,
an elastic fibre winds round
and round, so as to form
a spiral closely resembling
in its position and func-
tions the spiral wire -spring
of flexible gas-pipes ; within
this again, however, there
is another membranous
wall to the air-tubes, so
that the spire winds be-
tween their inner and outer
coats. — "When a portion of
one of the great trunks with
some of the principal bran-
ches of the tracheal system
has been dissected-out, and
so pressed in mounting
that the sides of the tubes
are flattened against each
other (as has happened in
the specimen represented
in Fig. 377), the spire forms two layers which are brought into close
apposition ; and a very beautiful appearance, resembling that of
watered silk, is produced by the crossing of the two sets of fibres,
^
Portion of a large Trachea of Dytiscus^
some of its principal branches.
V
with
TEACHER AND SPIEACLES.
-17
of which one overlies the other. That this appearance, however,
is altogether an optical illusion, may be easily demonstrated by
carefully following the course of any one of the fibres, which will
be found to be perfectly regular. (See §§ 582, 583.)
595. The ' stigmata ' or ' spiracles ' through which the air enters
the tracheal system, are generally visible on the exterior of the
body of the insect (especially on the abdominal segments) as a
series of pores along each margin of the under surface. In most
larvas, nearly every segment is provided with a pair : but in the
perfect insect, several of them remain closed, especially in the
thoracic region, so that their number is often considerably reduced.
The structure of the spiracles varies greatly in regard to com-
Fig. 378.
Spiracle of Common Fly.
plexity in different insects; and even where the general plan
is the same, the details of conformation are peculiar, so that per-
haps in scarcely any two species are they alike. Generally speak-
ing they are furnished with some
kind of sieve at their entrance, Fig. 379.
by which particles of dust, soot,
&c, which would otherwise
enter the air-passages, are fil-
tered out ; and this sieve may
be formed by the interlacement
of the branches of minute ar-
borescent growths from the
border of the spiracle, as in the
common Fly (Fig. 378), or in the
Dytiscus ; or it may be a mem-
brane perforated with minute
holes, and supported upon a
framework of bars that is pro-
longed in like manner from the
thickened margin of the aper-
ture (Fig. 379), as in the larva of the Melolontlia (Cockchafer).
:it
Spiracle of Larva of Cockchafer.
718 INSECTS AND ARACHNID A.
'Not unfrequently,the centre of the aperture is occupied by an imper-
vious disk, from which radii proceed to its margin, as is well seen
in the spiracle of Tvpula (Crane-fly). — In those aquatic Larvae
which breathe air, we often find one of the spiracles of the last
segment of the abdomen prolonged into a tube, the mouth of
which remains at the surface while the body is immersed ; the
larvae of the Gnat tribe may frequently be observed in this
position.
596. There are many aquatic Larva3, however, which have an
entirely-different provision for respiration ; being furnished with
external leaf -like or brush-like appendages into which the tracheae
are prolonged, so that, by absorbing air from the water that bathes
them, they may convey this into the interior of the body. We
cannot have a better example of this than is afforded by the larva
of the common Ephemera (Day-fly), the body of which is furnished
with a set of branchial appendages resembling the 'fin-feet' of
Branchiopods (§ 563), whilst the three-pronged tail also is fringed
with clusters of delicate hairs which appear to minister to the
same function. In the larva of the Libellida (Dragon-fly), the
extension of the surface for aquatic respiration takes-place within
the termination of the intestine ; the lining membrane of which is
folded into an immense number of plaits, each containing a minutely
ramified system of tracheae ; the water, slowly drawn-in through
the anus for bathing this surface, is ejected with such violence
that the body is impelled in the opposite direction ; and the air
taken-up by its tracheae is carried, through the system of air-tubes
of which they form part, into the remotest organs. This apparatus
is a peculiarly interesting object for the Microscope, on account of
the extraordinary copiousness of the distribution of the tracheae in
the intestinal folds.
597. The main trunks of the tracheal system, with their prin-
cipal ramifications, may generally be got- out with little difficulty,
by laying-open the body of an Insect or Larva under water in a
Dissecting-trough (§ 150), and removing the whole visceral mass,
taking care to leave as many as possible of the branches which
will be seen proceeding to this from the two great longitudinal
tracheae, to whose position these branches will serve as a guide.
Mr. Quekett recommends the following as the most simple method
of obtaining a perfect system of tracheal tubes from a larva : — a
small opening having been made in its body, this is to be placed in
strong acetic acid, which will soften or decompose all the viscera ;
and the tracheae may then be well-washed with the syringe, and
removed from the body with the greatest facility, by cutting away
the connections of the main tubes with the spiracles by means of
fine pointed scissors. In order to mount them, they should be
floated upon the slide, on which they should then be laid-out in
the position best adapted for displaying them. If they are to be
mounted in Canada balsam, they should be allowed to dry upon
the slide, and should then be treated in the usual way ; but their
PREPARATION OF TRACHEAE: — WINGS. 719
natural appearance is best preserved by mounting thern in fluid
(weak spirit or G-oadby's solution), using a shallow cell to prevent
pressure. The finer ramifications of the tracheal system may
generally be seen particularly well in the membranous wall of the
stomach or intestine ; and this, having been laid-out and dried
upon the glass, may be mounted in balsam so as to keep the
trachea? full of air (whereby they are much better displayed), if
care be taken to use balsam that has been previously thickened, to
drop this on the object without liquefying it more than is abso-
lutely necessary, and to heat the slide and the cover (the heat
may be advantageously applied directly to the cover, after it has
been put-on, by turning-over the slide so that its upper face shall
look downwards) only to such a degree as to allow the balsam to
spread and the cover to be pressed-down. — The spiracles are easily
dissected-out by means of a pointed knife or a pair of fine scissors ;
they should be mounted in Fluid or Medium when their texture is
soft, and in Balsam when the integument is hard and horny.
598. Wings. — These organs are essentially composed of an ex-
tension of the external, membranous layer of the integument, over
a framework formed by prolongations of the inner horny layer,
within which prolongations trachea? are nearly always to be found,
whilst they also include channels through which blood circulates
during the growth of the wing and for a short time after its
completion (§ 593). This is the simple structure presented to
us in the Wings of Neuroptera (Dragon-flies, &c), Hymenoptera
(Bees and Wasps), Diptera (two-winged-Flies), and also of many
Homoptera (Cicadas and Aphides) ; and the principal interest of
these wings as Microscopic objects lies in the distribution of their
' veins' or ' nervures' (for by both names are the ramifications of
their skeleton known), and in certain points of accessory structure.
The venation of the wings is most beautiful in the smaller
Neuroptera ; since it is the distinguishing feature of this order
that the veins, after subdividing, reunite again, so as to form a
close network ; whilst in the Hymenoptera and Diptera such re-
unions are rare, especially towards the margin of the wings, and
the areola? are much larger. Although the membrane of which
these wings are composed appears perfectly homogeneous when
viewed by transmitted light, even with a high magnifying power,
yet, when viewed by light reflected obliquely from their surfaces,
an appearance of cellular areolation is often discernible ; this is
well seen in the common Fly, in which each of these areola? has a
hair in its centre. In order to make this observation, as well as
to bring-out the very beautiful iridescent hues which the wings of
many minute Insects (as the Aphides) exhibit when thus viewed,
it is convenient to hold the wing in the Stage-forceps for the sake
of giving it every variety of inclination ; and when that position
has been found which best displays its most interesting features, it
should be set up as nearly as possible in the same. For this pur-
pose it should be mounted on an opaque slide ; but instead of
720 INSECTS AND AEACHNIDA.
being laid down npon its surf ace, the wing should be raised a little
above it, its ' stalk' being held in the proper position by a little
cone of soft wax, in the apex of which it may be imbedded. — The
wings of most Hymenoptera are remarkable for the peculiar
apparatus by which those of the same side are connected together,
so as to constitute in flight but one large wing ; this consists of a
row of curved hooklets on the anterior margin of the posterior
wing, which lay hold of the thickened and doubled-down posterior
edge of the anterior wing. These hooklets are sufficiently apparent
in the wings of the common Bee, when examined with even a low
magnifying power ; but they are seen better in the Wasp, and
better still in the Hornet. — The peculiar scaly covering of the
wings of the Lepidoptera has already been noticed (§ 581) ; but it
may here be added that the entire wings of many of the smaller
and commoner insects of this order, such as the Tineidce or
' clothes-moths,' form very beautiful opaque objects for low
powers ; the most beautiful of all being the divided wings of
the Fissipennes or ' plumed moths,' especially those of the genus
Pterophorus.
599. There are many Insects, however, in which the Wings are
more or less consolidated by the interposition of a layer of horny
substance between the two layers of membrane. This plan of
structure is most fully carried-out in the Coleoj)tera (Beetles), whose
anterior wings are metamorphosed into elytra or 'wing-cases ;' and
it is upon these that the brilliant hues by which the integument
of many of these insects is distinguished, are most strikingly dis-
played. In the anterior wings of the Forficuliclce or Earwig-tribe
(which form the connecting link between this order and the Orthop-
tera), the cellular structure may often be readily distinguished
when they are viewed by transmitted light, especially after having
been mounted in Canada balsam. The anterior wings of the
Ortlio^tera (Grasshoppers, Crickets, &c), although not by any
means so solidified as those of Coleoptera, contain a good deal
of horny matter; they are usually rendered sufficiently trans-
parent, however, by Canada balsam, to be viewed with trans-
mitted light; and many of them are so coloured as to be very
showy objects (as are also the posterior fan-like wings) for the
Electric or Gas-microscope, although their large size, and the absence
of any minute structure, prevent them from affording much inte-
rest to the ordinary Microscopist. — We must not omit to mention,
however, the curious Sound-producing apparatus which is possessed
by most insects of this order, and especially by the common House-
cricket. This consists of the ' tympanum' or drum, which is a
space on each of the upper wings, scarcely crossed by veins, but
bounded externally by a large dark vein provided with three or
four longitudinal ridges; and of the 'file' or 'bow,' which is a
transverse horny ridge in front of the tympanum, furnished with
numerous teeth : and it is believed that the sound is produced by
the rubbing of the two bows across each other, while its intensity
WIXGS AXD FEET. 721
is increased by the sound-board action of the tympanum. — The
wings of the Fulgoriclce (Lantern-flies) have mnch the same texture
with those of the Orthoptera, and possess about the same value as
Microscopic objects ; differing considerably from the purely mem-
branous wings of the Cicadee and Aphides, which are associated
with them in the order Homoptera. In the order Hemiptera, to
which belong various kinds of land and water Insects that have a
suctorial mouth resembling that of the common bug, the wings of
the anterior pair are usually of parchmenty consistence, though
membranous near their tips, and are often so richly coloured as to
become very beautiful objects, when mounted in Balsam and
viewed by transmitted light ; this is the case especially with the
terrestrial vegetable-feeding kinds, such as the Pentatoma and its
allies, some of the tropical forms of which rival the most brilliant
of the Beetles. The British species are by no means so interesting ;
and the aquatic kinds, which, next to the bed-bugs, are the most
common, always have a dull brown or almost black hue : even among
these last, however, — of which the Notonecta (water-boatman) and
the Nepa (water-scorpion) are well-known examples, — the wings
are beautifully variegated by differences in the depth of that hue.
The halter es of the Diptera, which are the representatives of the
posterior wings, have been shown Dr. J. B. Hicks to present a
very curious structure, which is found also in the elytra of Coleop-
tera and in many other" situations ; consisting in a multitude of
vesicular projections of the superficial membrane, to each of which
there proceeds a nervous filament, that comes to it through an
aperture in the tegumentary wall on which it is seated. Ararious
considerations are stated by Dr. Hicks, which lead him to the
belief that this apparatus, when developed in the neighbourhood of
the spiracles or breathing-pores, essentially ministers to the sense
of smell, whilst, when developed upon the palpi and other organs
in the neighbourhood of the mouth, it ministers to the sense of
taste*
600. Feet. — Although the feet of Insects are formed pretty
much on one general plan, yet that plan is subject to considerable
modifications, in accordance with the habits of life of different
species. The entire limb usually consists of five divisions, namely.
the coxa or hip, the trochanter, the femur or thigh, the tibia
or shank, and the tarsus or foot ; and this last part is made up
of several successive joints. The typical number of these joints
seems to be five ; but that number is subject to reduction ; and the
vast order Coleoptera is subdivided into primary groups, accord-
ing as the tarsus consists of five, four, or three segments. The
last joint of the tarsus is usually furnished with a pair of strong
* See his Memoir ' On a new Organ in Insects,' in " Journal of Linnsean
Society," Vol. i. (1856), p. 136 ; his l Further Kemarks on the Organs found on
the bases of the Halteres and Wings of Insects,' in " Transact, of the Linn.
Soc," Vol. xxii. p. 141 ; and his Memoir ' On certain Sensory Organs in In-
sects hitherto undescribed,1 in " Transact, of Linn. 6oc," Vol. xxiii. p. 189.
3 A
722
INSECTS AND ABACHNIDA.
/
hooks or claws (Figs. 380, 381) ; and these are often serrated
(that is, furnished with saw -like teeth), especially near the base.
The nnder- surface of the other joints is frequently beset with tufts
of hairs, which are arranged in various modes, sometimes forming
a complete ' sole ;' this is especially the case in the family Cur-
culioniclce ; so that a pair of the feet of the 'diamond-beetle,'
mounted so that one shows the upper surface made resplendent by
its jewel-like scales, and the other the hairy cushion beneath, is a
very interesting object. In many Insects, especially of the fly
kind, the foot is furnished with a pair of membranous expansions
termed pulvilli (Fig. 380) ; and these are beset with numerous
hairs, each of which has a minute disk at its extremity^ This
structure is evidently
Fig. 380. connected with the power
which these Insects pos-
sess, of walking over
smooth surfaces in oppo-
sition to the force of gra-
vity ; yet there is still
considerable uncertainty
as to the precise mode in
which it ministers to this
faculty. Some believe
that the disks act as
suckers, the Insect being
held-up by the pressure
of the air against their
upper surface, when a
vacuum is formed be-
neath ; whilst others
maintain that the adhe-
sion is the result of the
secretion of a viscid liquid
from the under side of the foot. The careful observations of Mr.
Hepworth have led him to a conclusion which seems in harmony
with all the facts of the case ; namely, that each hair is a tube
conveying a liquid from a glandular sacculus situated in the
tarsus ; and that when the disk is applied to a surface, the
pouring-forth of this liquid serves to make its adhesion perfect.
That this adhesion is not produced by atmospheric pressure alone,
is proved by the fact that the feet of Flies continue to hold
on to the interior of an exhausted receiver ; whilst, on the other
hand, that the feet pour-forth a secreted fluid, is evidenced by the
marks left by their attachment on a clean surface of glass.
Although when all the hairs have the strain put upon them
equally, the adhesion of their disks suffices to support the insect,
yet each row may be detached separately by the gradual raising of
the tarsus and pulvilli, as when we remove a piece of adhesive
plaster by lifting it from the edge or corner. Flies are often
Foot of Fly.
FEET AND SUCKEES.
'23
found adherent to window-panes in the autumn, their _ reduced
strength not being sufficient to enable them to detach their tarsi*
— A similar apparatus, on a far larger scale, presents itself on the
foot of the Dytiscus (Fig. 381, a). The first joints of the tarsus of
this insect are widely expanded, so as to form a nearly-circular
Fig. 381.
A, Foot of Dytiscus. showing its apparatus of suckers; a, ft,
large suckers; c, ordinary suckers : — B, one of the ordinary
suckers more highly magnified.
plate ; and this is provided with a very remarkable apparatus of
suckers, of which one disk (a) is extremely large, and is furnished
with strong radiating fibres, a second (b) is a smaller one formed
on the same plan (a third, of the like kind, being often present),
whilst the greater number are comparatively small tubular club-
shaped bodies, each having a very delicate membranous sucker at
its extremity, as seen on a larger scale at b. These all have essen-
tially the same structure ; the large suckers being furnished, like
the hairs of the Fly's foot, with secreting sacculi, which pour-forth
fluid through the tubular footstalks that carry the disks, whose
adhesion is thus secured ; whilst the small suckers form the con-
necting link between the larger suckers and the hairs of many
beetles, especially CurcuUonidce.f The leg and foot of the Dytiscus,
* See Mr. Hepworth's communications to the " Quart. Journ. of Microsc.
Science," Vol. ii. (1854), p. 158, and Vol. iii. (1855), p. .412. See also Mr. Tuffen
West's Memoir ' On the Foot of the Fly,' in " Transact, of Linn. Society," Vol.
xxii. p. 393, and Mr. Lowne's "Anatomy of the Blow fly," p. 19.
t See Mr. Lowne ' On the so-called Suckers of Dytiscus and the Pulvilli of
Insects,' in " Monthly Microscopical Journal," Vol. v. p. 267.
3 a 2
724 INSECTS AND AKACHNIDA.
if mounted without compression, furnish a peculiarly beautiful
object for the Binocular Microscope. — The Feet of Caterpillars
differ considerably from those of perfect Insects. Those of the first
three segments, which are afterwards to be replaced by true legs,
are furnished with strong horny claws ; but each of those of the
other segments which are termed ' pro-legs,' is composed of a
circular series of comparatively slender curved hooklets, by which
the Caterpillar is enabled to cling to the minute roughnesses of the
surface of the leaves, &c, on which it feeds. This structure is well
seen in the pro-legs of the common Silk-worm.
601. Stings and Ovipositors. — The Insects of the order Hyme-
noptera are all distinguished by the prolongation of the last
segment of the abdomen into a peculiar organ, which in one
division of the order is a ' sting,' and in the other is an ' ovipositor'
or instrument for the deposition of the eggs, which is usually also
provided with the means of boring a hole for their reception. The
former group consists of the Bees, Wasps, Ants, &c. ; the latter
of the Saw-flies, Gall-flies, Ichneumon-flies, &c. These two sets of
instruments are not so unlike in structure, as they are in function.
— The ' sting' is usually formed of a pair of darts, beset with
barbed teeth at their points, and furnished at their roots with
powerful muscles, whereby they can be caused to project from
their sheath, which is a horny case formed by the prolongation of
the integument of the last segment, slit into two halves, which
separate to allow the protrusion of the sting ; whilst the peculiar
' venom' of the sting is due to the ejection, by the same muscular
action, of a poisonous liquid, from a bag situated near the root of
the sting, which passes down a canal excavated between the darts,
so as to be inserted into the puncture which they make. The
stings of the common Bee, "Wasp, and Hornet, may all be made to
display this structure without much difficulty in the dissection. — ■
The ' ovipositor' of such insects as deposit their eggs in holes
ready-made, or in soft animal or vegetable substances (as is the
case with the Ichneumonidce), is simply a long tube, which is
enclosed, like the sting, in a cleft sheath. In the Gall-flies
(Cynipidce), the extremity of the ovipositor has a toothed edge^ so
as to act as a kind of saw whereby harder substances may be
penetrated ; and thus an aperture is made in the leaf, stalk, or bud
of the plant or tree infested by the particular species, in which the
egg is deposited, together with a drop of fluid that has a peculiarly
irritating effect upon the vegetable tissues, occasioning the pro-
duction of the ' galls,' which are new growths that serve not only
to protect the larvae, but also to afford them nutriment. The oak
is infested by several species of these Insects, which deposit their
eggs in different parts of its fabric ; and some of the small ' galls'
which are often found upon the surface of oak-leaves, are ex-
tremely beautiful objects for the lower powers of the Microscope.
It is in the Tenthredinidce, or ' saw-flies,' and in their allies the
Siricidce, that the ovipositor is furnished with the most powerful
OYIPOSITOES AND EGGS. 725
apparatus for penetration ; and some of these Insects can bore by
its means into bard timber. Their ' saws' are not unlike the
' stings' of Bees, &c, but are broader, are toothed for a greater
length, and are made to slide along a firm piece that supports each
blade, like the ' back' of a carpenter's ' tenon-saw ;' they are worked
alternately (one being protruded while the other is drawn back)
with great rapidity ; and when the perforation has been made, the
two blades are separated enough to allow the passage of the eggs
between them. — Many other insects, especially of the order Dvptera,
have very prolonged ovipositors, by means of which they can insert
their eggs into the integuments of animals, or into ether situations
in which the larvae will obtain appropriate nutriment. A remark-
able example of this is furnished by the Gad-fly (Tabanns), whose
ovipositor is composed of several joints, capable of being drawn
together or extended like those of a telescope, and is terminated by
boring instruments ; and the egg being conveyed by its means, not
only into but through the integument of the Ox, so as to be im-
bedded in the tissue beneath, a peculiar kind of inflammation is
set-up there, which (as in the analogous case of the gall-fly) forms
a nidus appropriate both to the protection and to the nutrition of
the larva. Other insects which deposit their eggs in the ground,
such as the Locusts, have their ovipositors so shaped as to
answer for digging holes for their reception. — The preparations
which serve to display the foregoing parts, are best seen when
mounted in Balsam ; save in the case of the muscles and poison-
apj)aratus of the sting, which are better preserved in Fluid or in
Medium.
602. The Sexual organs of Insects furnish numerous objects of
extreme interest to the Anatomist and Physiologist; but as an
account of them would be unsuitable to the present work, a refe-
rence to a copious source of information respecting one of their most
curious features, and to a list of the Species that afford good
illustrations, must here suffice.* The eggs of many Insects are
objects of great beauty, on account of the regularity of their form,
and the symmetry of the markfngs on their surface (Fig. 382).
The most interesting belong for the most part to the order
Lepidoptera ; and there are few among these that are not worth ex-
amination, some of the commonest (such as those of the Cabbage
butterfly, which are found covering large patches of the leaves of
that plant) being as remarkable as any. Those of the Puss-moth
(Gerura vimda), the Privet hawk-moth (Sphinx ligustri), the
small Tortoise-shell butterfly (Vanessa urtiam), the Meadow-brown
butterfly (Hipparchia janira), the Brimstone-moth (Rumia crato3-
gata), and the Silk-worm (Boinbyx mori), may be particularly
* See the Memoirs of M. Lacaze-Duthiers, <Sur l'armure genitale des In-
sectes,' in " Ann. des Sci. Nat," Ser. 3, Zool., Tomes xii., xiv., xvii., xviii.,
xix. ; and M. Ch. Eobin's "Memoire sur les Objets qui peuvent etre conserve's
en Preparations Microscopiques " (Paris, 1856), which is peculiarly full in the
enumeration of the objects of interest afforded by the Class of Insects.
726
INSECTS AND AEACHNLDA.
specified ; and from other orders, those of the Cockroach (Blatta
orientalis), Field cricket {Acheta campestris), Water- scorpion
{Nepa ranatra), Bug {Gimex ledularius), Cow-dung-fly (Scato-
phaga stercoraria), and Blow-fly (Musca vomitoria) . In order to
Eggs of Insects, magnified; — A, Pontia napi ; B, Vanessaurticce ; c, Hipparcliia
tithous; D, Argynnis Lathonia.
preserve these eggs, they should be mounted in fluid in a cell ;
since they will otherwise dry up and may lose their shape. — They
are very good objects for the 'conversion of relief effected by
Nachet's Stereo-pseudoscopic Binocular (§ 35).
603. The remarkable mode of Eeproduction that exists among
the Aphides must not pass unnoticed here, from its curious
connection with the non-sexual reproduction of Entomostraca
(§ 568) and Botifera (§ 412), as also of Hydra (§ 472) and
Zoophytes generally, all of which fall specially, most of them
exclusively, under the observation of the Microscopist. The
Aphides which may be seen in the spring and early summer, and
which are commonly but not always wingless, are all of one sex,
and give birth to a brood of similar Aphides, which come into the
world alive, and before long go through a like process of multipli-
cation. As many as from seven to ten successive broods may thus
be produced in the course of a single season ; so that from a single
Aphis, it has been calculated that no fewer than ten thousand
million millions may be evolved within that period. In the latter
part of the year, however, some of these viviparous Aphides attain
their full development into males and females ; and these perform
the true Generative process, whose products are eggs, which, when
hatched in the succeeding spring, give origin to a new viviparous
brood that repeat the curious life-history of their predecessors. It
appears from the observations of Prof. Huxley,* that the broods of
viviparous Aj)hides originate in ova which are not to be distin-
guished from those deposited by the perfect winged female.
Nevertheless, this non-sexual or agamic reproduction must be
considered analogous rather to the ' gemmation' of other Animals
and Plants, than to their sexual ' generation ; ' for it is favoured,
* ' On the Agamic Eeproduction and Morphology of Aphis,' in " Transact,
of Linn. Soc," Vol. sxii. p. 193.
REPRODUCTION AND DEVELOPMENT. 727
like the gemmation of Hydra, by warmth and copious suste-
nance, so that by appropriate treatment the viviparous repro-
duction may be caused to continue (as it would seem) indefinitely,
without any recurrence to the sexual process. Further, it seems
now certain that this mode of reproduction is not at all peculiar
to the Aphides, but that many other Insects ordinarily multiply
by ' agamic' propagation, the production of males and the per-
formance of the true generative act being only occasional pheno-
mena ; and the researches of Prof. Siebold have led him to conclude
that even in the ordinary economy of the Hive-bee the same
double mode of reproduction occurs. The queen, who is the only
perfect female in the hive, after impregnation by one of the drones
(or males), deposits eggs in the 'royal' cells, which are in due
time developed into young queens ; others in the drone-cells,
which become drones ; and others in the ordinary cells, which
become workers or neuters. It has long been known that these
last are really undeveloped females, which, under certain conditions,
might become queens ; and it has been observed by bee-keepers
that worker-bees, in common with virgin or unimpregnated queens,
occasionally lay eggs, from which eggs none but drones are ever
produced. From careful Microscopic examination of the drone
eggs laid even by impregnated queens, Siebold drew the conclu-
sion that they have not received the fertilizing influence of the
male fluid, which is communicated to the queen-eggs and worker
eggs alone ; so that the products of sexual generation are always
female, the males being developed from these by a process which
is essentially one of gemmation.*
604. The embryonic development of Insects is a study of peculiar
interest, from the fact that it may be considered as divided (at
least in such as undergo a ' complete metamorphosis') into two
stages that are separated by the whole active life of the larva ;
that, namely, by which the Larva is produced within the egg, and
that by which the Imago or perfect insect is produced within the
body of the Pupa. Various circumstances combine, however, to
render the study a very difficult one ; so that it is not one to be
taken up by the inexperienced Microscopist. The following sum-
mary of the history of the process in the common Blow-fly, however,
will probably be acceptable. — A gastrula with two membranous
lamellge (§ 468) having been evolved in the first instance, the outer
lamella very rapidly shapes itself into the form of the larva, and
shows a well-marked segmental division. The alimentary canal, in
like manner, shapes itself from the inner lamella ; at first being
straight and very capacious, including the whole yolk ; but gradually
becoming narrow and tortuous, as additional layers of cells are
developed between the two primitive lamellse, from which the other
internal organs are evolved. When the larva comes forth from
the egg, it still contains the remains of the yolk ; it soon begins,
* See Prof. Siebold's Memoir " On true Parthenogenesis in Moths and Bees,"
translated by W. S. Dallas ; London, 1857.
728 INSECTS AND ARACHNIDA.
however, to feed voraciously ; and in no long period it grows to many
thousand times its original weight, without making any essential
progress in development, but simply accumulating material for
future use. An adequate store of nutriment (analogous to the
; supplemental yolk' of Purpura, § 543) having thus been laid up
within the body of the larva, it resumes (so to speak) its embryonic
development ; its passage into the pupa state, from which the imago
is to come forth, involving a degeneration of all the larval tissues ;
whilst the tissues and organs of the imago " are re-developed from
cells which originate from the disintegrated parts of the larva, under
conditions similar to those appertaining to the formation of the
embryonic tissues from the yolk." The development of the
segments of the head and body in Insects generally proceeds from
the corresponding larval segments ; but, according to Dr. Weismann,
there is a marked exception in the case of the Diptera and other
Insects whose larvae are unfurnished with legs, — their head and
thorax being newly formed from ' imaginal disks,' which adhere
to the nerves and tracheae of the anterior extremity of the larva ;*
and, strange as this assertion may seem, it has been confirmed by
the subsequent investigations of Mr. Lowne.
605. Arachnida. — The general remarks which have been made
in regard to Insects, are equally applicable to this Class ; which
includes, along with the Spiders and Scorpions, the tribe of Acarida,
consisting of the Mites and Tides. Many of these are parasitic,
and are popularly associated with the wingless parasitic Insects,
to which they bear a strong general resemblance, save in having
eight legs instead of six. The true 'mites' (Acarince) generally
have the legs adapted for walking, and some of them are of
active habits. The common cheese-mite, as seen by the naked
eye, is familiar to every one ; yet few who have not seen it under
a Microscope have any idea of its real conformation and move-
ments ; and a cluster of them, cut out of the cheese they infest,
and placed under a magnifying power sufficiently low to enable
a large number to be seen at once, is one of the most amusing
objects that can be shown to the young. There are many other
species, which closely resemble the Cheese-mite in structure and
habits, but which feed upon different substances ; and some of
these are extremely destructive. To this group belongs a small
species, the Sar copies scabiei, whose presence appears to be the
occasion of one of the most disgusting diseases of the skin — the
itch, — and which is hence commonly termed the ' itch-insect.' It
is not found in the pustule itself, but in a burrow which passes-off
from one side of it, and which is marked by a red line on the
surface ; and if this burrow be carefully examined, the creature
will very commonly, but not always, be met- with. It is scarcely
visible to the naked eye ; but when examined under the microscope,
it is found to have an oval body, a mouth of conical form, and
* See his ' Entwickelnng der Dipteren,' in "Kolliker and Siebold's Zeit-
schrift," Bande xiv.-xvi. ; and Mr. Lowne's Monograph, pp. 6-9, 113-121.
PARASITIC ACARIDA. 729
eight feet, of which the four anterior are terminated by small
suckers, whilst the four posterior end in very prolonged bristles.
The male is only about half the size of the female. The Ricinice
or ' ticks' are usually destitute of eyes, but have the mouth pro-
vided with lancets, that enable them to penetrate more readily
the skins of animals whose blood they suck. They are usually of
a flattened, round, or oval form ; but they often acquire a very
large size by suction, and become distended like a blown bladder.
Different species are parasitic upon different animals ; and they
bury their suckers (which are often furnished with minute recurved
hooks) so firmly in the skins of these, that they can hardly be
detached without pulling away the skin with them. It is probably
the young of a species of this group, which is commonly known as
the ' harvest-bug,' and which is usually designated as the Acarus
autumnalis ; this is very common in the autumn upon grass or
other herbage, and insinuates itself into the skin at the roots of
the hair, producing a painful irritation ; like other Acarida, it
possesses only six legs for some time after its emersion from the egg
(the other pair being only acquired after the first moult), so that
its resemblance to parasitic Insects becomes still stronger. — It is
probable that to this group also belongs the Lemoclex folliculorum,
a creature which is very commonly found parasitic in the sebaceous
follicles of the Human skin, especially in those of the nose. In
order to obtain it, pressure should be made upon any one of these
that appears enlarged and whitish with a terminal black spot ;
the matter forced-out will consist principally of the accumulated
sebaceous secretion, having the parasites with their eggs and young
mingled with it. These are to be separated by the addition of oil,
which will probably soften the sebaceous matter sufficiently to set
free the animals, which may be then removed with a pointed brush ;
but if this mode should not be effectual, the fatty matter may be
dissolved-away by digestion in a mixture of alcohol and ether.
The pustules in the skm of a Dog affected with the ' mange' have
been found by Mr. Topping to contain a Demodex, which seems
only to differ from that of the human sebaceous follicles in its
somewhat smaller size ; and M. Gruby is said to have given to a
dog a disease resembling the mange, if not identical with it, by
inoculating it with the Human parasite. — The Acarida are best
preserved as Microscopic objects by mounting in one or other of the
'media' described in § 181.
606. The number of objects of general interest furnished to the
Microscopist by the Sjjider tribe, is by no means considerable.
Their eyes exhibit a condition intermediate between that of Insects
and Crustaceans, and that of Vertebrata ; for they are single, like
the ' stemmata' of the former (§ 586), usually number from six to
eight, are sometimes clustered-together in one mass, but are some-
times disposed separately ; while they present a decided approach
in internal structure to the type characteristic of the visual organs
of the latter. — The structure of the Mouth is always mandibulate,
'30
INSECTS AND AKACHNIDA.
and is less complicated than that of the ' mandibnlate' insects. —
The Respiratory apparatus, which, where developed at all among
the Acarida, is tracheary like that of Insects, is here constructed
upon a very different plan ; for the ' stigmata,' which are usually
four in number on
Fig. 883. each side, open into a
. like number of respira-
tory sacculi, each of
which contains a series
of leaf -like folds of its
lining membrane, upon
which the blood is dis-
tributed so as to
afford a large surface
to the air. — In the
structure of the limbs,
the principal point
worthy of notice is
the peculiar appen-
dage with which they
usually terminate ; for
the strong claws, with
a pair of which the last joint of the foot is furnished, have their
edges cut into comb-like teeth (Fig. 383), which seem to be used by
the animal as cleansing-instruments.
607. One of the most curious parts of the organization of the
Spiders, is the ' spinning-apparatus' by means of which they
fabricate their elaborately constructed webs. This consists of the
' spinnerets,' and of the glandular organs in which the fluid that
hardens into the thread is elaborated. The usual number of the
spinnerets, which are situated at the posterior extremity of the
body, is six ; they are little teat-like prominences, beset with hairy
appendages ; and it is through a certain set of these appendages,
which are tubular and terminate in fine-drawn points, that the
glutinous secretion is forced-out in a multitude of streams of
extreme minuteness. These streams harden into fibrils imme-
diately on coming into contact with the air ; and the fibrils pro-
Fig. 384.
Foot, with comb-like claws of the common
Spider (Epeira).
Ordinary thread (A), and viscid thread (b), of the
common Spider.
ceeding from all the apertures of each spinneret coalesce into a
single thread. It is doubtful, however, whether all the spinnerets
SPINNERETS OF SPIDERS. 731
are in action at once, or whether those of different pairs may not
have dissimilar functions ; for whilst the radiating threads of a
spider's web are simple (Fig. 384, a), those which lie across these,
forming its concentric circles, or rather polygons, are studded at
intervals with viscid globules (b), which appear to give to these
threads their peculiarly adhesive character ; and it does not seem
by any means unlikely that each kind of thread should be pro-
duced by its own pair of spinnerets. It was observed by Mr. R.
Beck, that these viscid threads are of uniform thickness when first
spun ; but that undulations soon appear in them, and that the
viscid matter then accumulates in globules at regular intervals. —
The total number of spinning-tubes varies greatly, according to
the species of the Spider, and the sex and age of the individual ;
being more than 1000 in some cases, and less than 100 in others.
The size and complexity of the secreting glandulas vary in like
manner : thus in the Spiders which are most remarkable for the
large dimensions and regular construction of their webs, they occupy
a large portion of the abdominal cavity, and are composed of slender
branching tubes, whose length is increased by numerous convolu-
tions ; whilst in those which have only occasional use for their
threads, the secreting organs are either short and simple follicles,
or undivided tubes of moderate length.
CHAPTER XVIII,
YEETEBRATED ANIMALS.
608. We are now arrived at the highest division of the Animal
Kingdom, in which the bodily fabric attains its greatest develop-
ment, not only as to completeness, but also as to size ; and it is in
most striking contrast with the Class we have been last considering.
Since not only the entire bodies of Yertebrated animals, but,
generally speaking, the smallest of their integral parts, are far too
large to be viewed as Microscopic objects, we can study their
structure only by a separate examination of their component
elements ; and it seems, therefore, to be a most appropriate course
to give under this head a sketch of the microscopic characters of
those Primary Tissues of which their fabric is made-up, and which,
although they may be traced with more or less distinctness in the
lower tribes of Animals, attain their most complete development in
this group.* — For some time after Schwann first made public the
remarkable results of his researches, it was very generally believed
that all the Animal tissues are formed, like those of Plants, by a
metamorphosis of cells ; an exception being taken, however, by
some Physiologists in regard to the ' simple fibrous' tissues (§ 628).
There can be no longer any doubt, however, that this doctrine must
be greatly modified ;f so that, whilst the Vegetable Physiologist
may rightly treat the most highly organized Plant as a mere aggre-
gation of cells, analogous in all essential particulars to those which
singly constitute the ' unicellular' Protopliytes (§ 203), the Animal
Physiologist does wrong in seeking a like cellular origin for the com-
* This sketch is intended, not for the professional student, but ouly for the
amateur Microscopist, who wishes to gain some general idea of the elementary-
structure of his own body and of that of Vertebrate animals generally. Those
who wish to go more deeply into the inquiry are referred to the following as
the most recent and elaborate Treatises that have appeared in this country : —
The Translation of Strieker's " Manual of Histology," published by the New
Sydenham Society; the "Handbook for the Physiological Laboratory," by Drs.
Burdon-Sanderson, Michael Foster, Brunton, and Klein; the translation of
the 4th Edition of Prof. Frey's "Histology and Histo-chemistry of Man ;" and
the ' General Anatomy' of the Eighth Edition of " Quain's Anatomy" (1874).
t The important 'Beview of the Cell-Theory,' by Prof. Huxley, in the
"Brit, and For. Med.-Chir. Beview," Vol. xii. (Oct. 1853), p. 285, may be con-
sidered the starting-point of many later inquiries.
GEEMINAL MATTEE AND FOEMED MATERIAL. 733
ponent parts of the Animal fabric ; and that lie may best interpret
the phenomena of tissne-formation in the most complicated
organisms, by the study of the behaviour of that apparently-homo-
geneous 'protoplasm' of which the simplest Protozoa are made
up, and by tracing the progressive ' differentiation' which presents
itself as we pass from this through the ascending series of Animal
forms.*
609. Although there would at first sight appear but little in
common between the simple body of those humble Rhizopods which
constitute the lowest types of the Animal series (§ 369), and the
complex fabric of Man or other Vertebrates, yet it appears from
recent researches, that in the latter, as in the former, the process of
' formation' is essentially carried-on by the instrumentality of pro-
toplasmic substance, universally diffused through it in such a
manner as to bear a close resemblance to the pseudopodial net-
work of the Ehizopod (Fig. 250) ; whilst the tissues produced by its
agency lie, as it were, on the outside of this, bearing the same kind
of relation to it as the Foraminiferal shell (Fig. 266) does to the
sarcodic substance which fills its cavities and extends itself over
its surface. For it appears that the smallest living ' elementary
part' of every organized fabric is composed of organic matter in two
states ; the one, which may be termed germinal matter, possessing
the power of selecting pabulum from the blood, and of transforming
this either into the material of its own extension, or into some
product which it elaborates ; whilst the other, which may be termed
formed material, may present every gradation of character from
a mere inorganic deposit to a highly organized structure, but is in
every case altogether incapable of self-increase. A very definite
line of demarcation can be generally drawn between these two
substances by the careful use of the staining-process (§ 161) ; but
there are many instances in which there is the same gradation
between the one and the other, as we have formerly noticed between
the ' endosarc' and the ' ectosarc' of the Amoeba (§ 376). — Thus it
is on the ' germinal matter' that the existence of every form of
Animal organization essentially depends ; since it serves as the
instrument by which the nutrient material furnished by the blood
is converted into the several forms of tissue. Like the sarcodic
substance of the Bhizopods, it seems capable of indefinite extension ;
and it may divide and subdivide into independent portions, each of
which may act as the instrument of formation of an ' elementary
part.' Two principal forms of such elementary parts present them-
* The study of Comparative Histology, prosecuted on this basis, promises
to be exceedingly fertile in results of this most interesting character. Thus
Dr. N. Kleinenberg, in his admirable "Anatomische entwickhmsgeschichteliche
Untersuchung " (1872), on Hydra, gives strong reason for regarding a par-
ticular set of cells in the body of that animal as combining the functions of
Nerve and Muscle. And the Author has been led by his study of Comatula to
recognize the most elementary type of Nerve-trunk in a simple protoplasmic
cord, not yet separated into distinct fibres with insulating sheaths (§ 6il).
734 VEBTEBEATED ANIMALS.
selves in the fabric of the higher Animals, — namely, cells and
fibres -, and it will be desirable to give a biief notice of the£e, before
proceeding to describe these more complex tissues which are the
products of a higher elaboration.*
610. The cells of which many Animal tissues are essentially com-
posed consist, when fully and completely formed, of the same parts
as the typical cell of the Plant (§ 200) ; — viz., a definite ' cell-wall,'
enclosing ' cell -contents' (of which the nature may be very diverse),
and also including a ' nucleus,' which is the seat of its formative
activity. It is of such cells, retaining more or less of their charac-
teristic spheroidal shape, that every mass of fat, whether large or
small, is chiefly made up (§ 634). And the internal cavities of the
body are lined by a layer of epithelium-cells (§ 633), which, although
of flattened form, present the like combination of components. But
there is a large number of cases in which the cell shows itself in
a form of much less complete development ; the ' elementary part'
being a corpuscle of protoplasm or ' germinal matter,' of which the
exterior has undergone a slight consolidation, like that which con-
stitutes the ' primordial utricle' of the Vegetable cell (§ 201) or the
' ectosarc' of the Amoeba (§ 376), but in which there is no proper
distinction of ' cell-wall,' ' cell-contents,' or 'nucleus.' This condition,
which is characteristically exhibited by the nearly-globular colourless
corpuscles of the Blood (§ 62t?), appears to be common to all cells in
the incipient stage of their formation ; and the progress of their
development consists in the gradual differentiation of their parts,
the ' cell-wall' and ' cell-contents' being separated (as ' formed
material') from the 'germinal-matter,' which last usually remains
as the ' nucleus,' — generally, however, contracting, and sometimes
(when its work has been completely done) disappearing altogether.
The large flattened red corpuscles of the Blood of the Frog and
other Oviparous Yertebrata (§ 625) appear to be generated from
the colourless by the production of a layer of ' formed material'
(paraglobulin coloured by Haemoglobin) around the original proto-
plasmic particles. For corpuscles are met with, which seem to
constitute an intermediate stage between the Wo kinds ; their
* The doctrine above stated is that to which the Author has been led by the
comparison of the results of the recent inquiries of several British and Conti-
nental Histologists, especially Prof. Beale and Prof. Max. Schultze, with those
of his own study of the Khizopod and Echinodeim types. Prof. Beale's views
are most systematically expounded in his lectures "On the Structure of the
simple Tissues of the Human Body," 1661 ; in his " How to work with the
Microscope," 4th Edition, 1868; and in the Introductory portion of his new
Edition of uTodd and Bowman's Physiological Anatomy," 1867. The principal
results of the inquiries of German Histologists on this point are well stated in
a Paper by Dr. Duffin on ' Protoplasm, and the part it plays in the actions of
Living Beings,' in " Quart. Journ. of Microsc. Science," Vol. hi, N.S. (1863),
p. 251.— The Author feels it necessary, however, to express his dissent from
Prof. Beale's views in one important particular, — viz., his denial of 'vital'
endowments to the ' formed material ' of any of the tissues ; since it seems to
him illogical to designate contractile muscular fibre (for example) as 'dead,'
merely because it has not the power of self -reparation.
CELLULAE AND FIBEOUS TISSUES. 735
form being still globular, but their size being greater than that of
the colourless corpuscles ; whilst their peripheral portion shows a
distinct layer of ' formed material,' which is beginning to assume the
characteristic hue of the red disk, but which is not tinged by the
carmine- solution that deeply dyes the central or nuclear portion. This
' formed material,' however, does not seem ever to acquire a distinct
membranous envelope or cell- wail; the changes of shape which the red
corpuscles spontaneously undergo under favourable circumstances,
being such as could scarcely occur if their form were thus limited.
In Cartilage (§ 636), on the other hand, the ' nucleus' and the
' cell-contents' are completely differentiated from the ' cell- wall ;'
but the ' cell-wall' itself cannot be separated from the ' intercellular
substance' which usually constitutes the principal portion of
this tissue in its mature condition. And it would appear from
the history of its development (which has been carefully studied
by Dr. Beale), that the ' intercellular substance,' ' cell-wall,' and
' cell- contents,' are all to be regarded in the light of layers of
' formed material,' successively exuded from the corpuscle of
' germinal matter' wherein the cell originated, a portion of which
remains as the ' nucleus.'
611. A large part of the fabric of the higher Animals, however,
is made up of fibrous tissues, which serve to bind together the other
components, and which, when consolidated by calcareous deposit,
constitute the substance- of the skeleton. In these, the relation of
the ' germinal matter' and the 'formed material' presents itself under
an aspect which seems at first sight very different from that just
described. A careful examination, however, of those ' connective-
tissue-corpuscles' (Fig. 406) that have long been distinguished in
the midst of the fibres of which these tissues are made up, shows
that they are the equivalents of the corpuscles of ' germinal matter,'
which in the previous instance came to constitute cell-nuclei ; and
that the fibres hold the same relation to them, that the ' walls' and
' contents' of cells do to their germinal corpuscles. The transition
from the one type to the other is well seen in Fibro-cartilage,
in which the so-called ' intercellular substance' is often as fibrous
as tendon. The difference between the two types, in fact, seems
essentially to consist in this, — that, whilst the segments of ' germinal
matter' which form the cell- nuclei in cartilage (Fig. 415) and in
other cellular tissues, are completely isolated from each other, each
being completely surrounded by the product of its own elaborating
action, those which form the ' connective-tissue-corpuscles' are
connected together by radiating prolongations (Fig. 407) that pass
between the fibres, so as to form a continuous network closely
resembling that formed by the pseudopodia of theEhizopod (§ 869).
Of this we have a most beautiful example in Bone ; for whilst its
solid substance may be considered as connective tissue solidified by
calcareous deposit, the ' lacunas' and ' canaliculi ' which are excavated
in it (Fig. 386) give lodgment to a set of radiating corpuscles
closely resembling those just described ; and these are centres of
736 VEETEBEATED ANIMALS.
' germinal matter,' which appear to have an active share in the
formation and subsequent nutrition of. the osseous texture. In
Dentine (or tooth -substance) we seem to have another form of the
same thing ; the walls of its ' tubuli' and the ' intertubular sub-
stance' (§ 615) being the ' formed material' that is produced from
thread-like prolongations of ' germinal matter' issuing from its
pulp, and continuing during the life of the tooth to occupy its
tubes ; just as in the Foraminifera we have seen a minutely-tubular
structure to be formed by a process of exudation around the
individual threads of sarcode which proceeded from the body of the
contained animal (Figs. 266, 282). — Although there still remains
much to be made out, in order to give completeness to the doctrine
which has been thus sketched, it may be stated with considerable
confidence that the tendency of all recent inquiry has been to show,
that the bodies of even the highest Animals are everywhere pene-
trated by that sarcodic substance of which those of the lowest and
simplest are entirely composed; and that this substance, which
forms a continuous network through almost' every portion of the
fabric, is the instrument of the Formation and Nutrition of the
more specialized or differentiated Tissues. As it is the purpose of
this work, not to instruct the professional student in Histology (or
the Science of the Tissues), but to supply scientific information of
general interest to the ordinary Microscopist, no attempt will here
be made to do more than describe the most important of those
distinctive characters, which the principal tissues present when
subjected to Microscopic examination ; and as it is of no essential
consequence what order is adopted, we may conveniently begin with
the structure of the skeleton* which gives support and protection
to the softer parts of the fabric.
612. Bone. — The Microscopic characters of osseous tissue may
sometimes be seen in a very thin natural plate of bone, such as in
that forming the scapula (shoulder-blade) of a Mouse; but they
are displayed more perfectly by artificial sections, the details of the
arrangement being dependent upon the nature of the specimen
selected, and the direction in which the section is made. Thus
when the shaft of a ' long' bone of a Bird or Mammal is cut-across
in the middle of its length, we find it to consist of a hollow cylinder
of dense bone, surrounding a cavity which is occupied by an oily
marrow ; but if the section be made nearer its extremity, we find
the outside wall gradually becoming thinner, whilst the interior,
instead of forming one large cavity, is divided into a vast number
of small chambers, partially divided by a sort of ' lattice- work' of
osseous fibres, but communicating with each other and with the
cavity of the shaft, and filled, like it, with marrow. In the bones
of Eeptiles and Fishes, on the other hand, this ' cancellated' struc-
* This term is used in its most general sense, as including not only the proper
vertebral or internal skeleton, but also the hard parts protecting the exterior
of the body, which form the dermal skeleton.
STEUCTUEE OF BONE.
73;
ture usually extends throughout the shaft, which is not so com-
pletely differentiated into solid bone and medullary cavity as it is
in the higher Vertebrata. In the most developed kinds of ' flat'
bones, again, such as those of the head, we find the two surfaces to
be composed of dense plates of bone, with a ' cancellated' structure
between them ; whilst in the less perfect type presented to us in the
lower Vertebrata, the whole thickness is usually more or less
' cancellated,' that is, divided-up into minute medullary cavities.
When we examine, under a low magnifying power, a longitudinal
section of a long bone, or a section of a flat bone parallel to its
surface, we find it traversed by numerous canals, termed Haversian
after their discoverer Havers, which are in connection with the
central cavity, and are filled, like it, with marrow: in the shafts of
'long' bones these canals usually run in the direction of their
length, but are connected here and there by cross branches ; whilst
in the ' flat' bones they form an irregular network. — On applying a
higher magnifying power to a thin transverse section of a long
bone, we observe that each of the canals whose orifices present
themselves in the field of view (Fig. 385), is the centre of a rod of
Fig. 385.
Minute structure of Bone, as seen in transverse section :—
1 a rod surrounding an Haversian canal, 3, showing the
concentric arrangement of the lamella} ; 2, the same, with the
lacunae and canaliculi ; 4, portions of the lamellae parallel with
the external surface.
bony tissue (1), usually more or less circular in its form, which is
arranged around it in concentric rings, resembling those ot an
Exogenous stem (Fig. 229). These rings are marked out and
divided by circles of little dark spots ; which, when closely ex-
amined (2), are seen to be minute flattened cavities excavated in
the solid substance of the bone, from the two flat sides ot which
pass-forth a number of extremely minute tubules, one set extending
1 3b
738
VERTEBKATED ANIMALS.
Fig. 386.
inwards, or in the direction of the centre of the system of rings, and
the other outwards, or in the direction of its circumference ; and by
the inosculation of the tubules (or canaliculi) of the different rings
with each other, a continuous communication is established between
the central Haversian canal and the outermost part of the bony rod
that surrounds it, which doubtless ministers to the nutrition of the
texture. Blood-vessels are traceable into the Haversian canals,
but the 'canaliculi' are far too minute to carry blood- corpuscles ;
they are occupied, however, in the living bone, by threads of sar-
codic substance, which bring into communication with the walls of
the blood-vessels the segments of ' germinal matter' contained in
the lacunas.
613. The minute cavities or lacunae (sometimes, but erroneously
termed 'bone-corpuscles,' as if they were solid bodies), from which
the canaliculi proceed (Fig. 386), are highly characteristic of the
true osseous structure ;
being never deficient in the
minutest parts of the bones
of the higher Yertebrata,
although those of fishes are
occasionally destitute of
them. The dark appear-
ance which they present in
sections of a dried bone is
not due to opacity, but is
simply an optical effect, de-
pendent (like the blackness
of air-bubbles in liquids)
upon the dispersion of the
rays by the highly-refracting substance that surrounds them (§ 142).
The size and form of the lacunas differ considerably in the several
Classes of Yertebrata, and even in some instances in the Orders ;
so as to allow of the determination of the tribe to which a bone
belonged, by the Microscopic examination of even a minute frag-
ment of it (§ 665). The following are the average dimensions of
the lacunas, in characteristic examples drawn from the four principal
Classes, expressed in fractious of an inch : —
Lacunae of Osseous substance
cavity; 5, its ramifications
«, central
Long Diameter.
Man 1-1440 to 1-2400
Ostrich .... 1-1333 to 1-2250
Turtle .... 1-375 to 1-1150
Conger-eel . . . 1-550 to 1-1135
SJiort Diameter.
1-4000 to 1-8000
1-5425 to 1-9650
1-4500 to 1-5840
1-4500 to 1-8000
The lacunas of Birds are thus distinguished from those of Mam-
mals by their somewhat greater length and smaller breadth ; but
they differ still more in the remarkable tortuosity of their canaliculi,
which wind backwards and forwards in a very irregular manner,
There is an extraordinary increase in length in the lacunas of
Reptiles, without a corresponding increase in breadth ; and this is
also seen in some Fishes, though in general the lacunas of the
LACUNiE AND CANALICULI OF BONE.
739
latter are remarkable for their angularity of form and the fewness
of their radiations, — as shown in Fig. 387, which represents the
lacunae and canaliculi in the bony scale of the Lepidosteus ('bony
pike' of the North American lakes and rivers), with which the
Fig. 38;
Section of the Bony Scale of Lepidosteus : — a, showing the
regular distribution of the lacunae and of the connecting cana-
liculi ; &, small portion more highly magnified.
bones of its internal skeleton perfectly agree in structure. The
dimensions of the lacunas in any bone do not bear any relation
to the size of the animal to which it belonged ; thus there is little
or no perceptible difference between their size in the enormous
extinct Iguanodon and in the smallest Lizard now inhabiting the
earth. But they bear a close relation to the size of the Blood-
corpuscles in the several Classes ; and this relation is particularly
obvious in the ' perennibranchiate' Batrachia, the extraordinary
size of whose blood-corpuscles will be presently noticed (§ 625) : —
Long Diameter. Short Diameter.
Proteus 1-570 to 1-980 ... 1-885 to 1-1200
Siren 1-290 to 1-480 ... 1-510 to 1-975
Menopoma .... 1-450 to 1-700 ... 1-1300 to 1-2100
Lepidosiren . . . 1-375 to 1-494 ... 1-980 to 1-2200
Pterodactyls . . . 1-445 to 1-1185 ... 1-4000 to 1-5225*
614. In preparing Sections of Bone, it is important to avoid the
penetration of the Canada balsam into the interior of the lacunae
and canaliculi; since, when these are filled by it, they become
almost invisible. Hence it is preferable not to employ this cement
at all, except it may be, in the first instance ; but to rub-down
the section beneath the finger, guarding its surface with a slice of
cork or a slip of gutta-percha (§ 157) ; and to give it such a polish
that it may be seen to advantage even when mounted dry. As
the polishing, however, occupies much time, the benefit which is
* See Prof. J. Quekett's Memoir on this subject, in the " Transact, of the
Microsc. Soc," Ser. 1, Vol. ii. ; and his more ample illustration of it in the
" Illustrated Catalogue of the Histological Collection in the Museum of the
Pioyal College of Surgeons," Vol. ii.
3 b 2
710 VEETEBEATED ANIMALS.
derived from covering the surfaces of the specimen with Canada
balsam may be obtained, without the injury resulting from the
penetration of the balsam into its interior, by adopting the follow-
ing method : — a quantity of Balsam proportioned to the size of the
specimen is to be spread upon a glass slip, and to be rendered
stiffer by boiling, until it becomes nearly solid when cold; the
same is to be done to the thin-glass cover ; next, the specimen being
placed on the balsamed surface of the slide, and being overlaid by
the balsamed cover, such a degree of warmth is to be applied as
will suffice to liquefy the balsam without causing it to now freely ;
and the glass cover is then to be quickly pressed-down, and the
slide to be rapidly cooled, so as to give as little time as possible
for the penetration of the liquefied balsam into the lacunar
system. — The same method may be employed in making sections of
Teeth.* — The study of the organic basis of Bone (commonly, but
erroneously, termed cartilage) should be pursued by macerating a
fresh bone in dilute Nitro-hydrochloric acid, then macerating it
for some time in pure water, and then tearing thin shreds from
the residual substance, which will be found to consist of an imper-
fectly-flbriliated material, allied in its essential constitution to the
' white fibrous' tissue (§ 628).
615. Teeth. — The intimate structure of the Teeth in the several
Classes and Orders of Vertebrata, presents differences which are no
less remarkable than those of their external form, arrangement,
and succession. It will obviously be impossible here to do more
than sketch some of the most important of these varieties.- — -The
principal part of the substance of all teeth is made-up of a solid
tissue that has been appropriately termed dentine. In the Shark
tribe, as in many other Fishes, the general structure of this den-
tine is extremely analogous to that of bone; the tooth being
traversed by numerous canals, which are continuous with the
Haversian canals of the subjacent bone, and receive blood-vessels
from them (Fig. 388) ; and each of these canals being surrounded
by a system of tubuli (Fig. 389), which radiate into the surrounding
solid substance. These tubuli, however, do not enter lacunas,
nor is there any concentric annular arrangement around the medul-
lary canals ; but each system of tubuli is continued onwards
through its own division of the tooth, the individual tubes some-
times giving-off lateral branches, whilst in other instances their
trunks bifurcate. This arrangement is peculiarly well displayed,
when sections of teeth constructed upon this type are viewed
as opaque objects (Fig. 390). — In the teeth of the higher Yer-
tebrata, however, we usually find the centre excavated into a single
cavity (Fig. 391), and the remainder destitute of vascular canals ;
but there are intermediate cases (as in the teeth of the great fossil
Sloths) in which the inner portion of the dentine is traversed by
* Some -useful hmts on the mode of making these preparations will be found
in the "Quart. Journ. of Microsc. Science," Vol. vii. (1859), p. 258.
STRUCTURE OF TEETH.
741
prolongations of this cavity, conveying blood-vessels, which do not
pass into the exterior layers. The tubuli of the 'non-vascular'
Fig. 388.
Fig. 389.
H
7" ^-" -- __^^_ > 12?
Fig. 388. Perpendicular section of Tooth of Lamna, mode-
rately enlarged, showing network of medullary canals.
Fig. 389. Transverse section of portion of Tooth of Prist Is,
more highly magnified, showing orifices of medullary canals,
with systems of radiating and inosculating tubuli.
dentine, which exists by itself in the teeth of nearly all Mammalia,
and which in the Elephant is known as ' ivory,' all radiate from
the central cavit}r, and
pass towards the surface
of the tooth in a nearly
parallel course. Their di-
ameter at their largest
part averages l-10,000th
of an inch ; their smallest
branches are immeasur-
ably fine. The tubuli in
their course present greater
and lesser undulations ;
the former are few in
number ; but the latter
are numerous, and as they
occur at the same part of
the course of several con-
tiguous tubes, they give
rise to the appearance of
lines concentric with the
centre of radiation. These
Fig. 390.
Transverse Section of Tooth of Mylidbates
(Eagle Fiay) viewed as an opaque object.
742
VEETEBEATED ANIMALS.
Fig. 391.
' secondary curvatures ' probably indicate, in dentine, as in the
Crab's shell (§ 573), successive stages of calcification. — The tubuli
are occupied, during the life of the tooth, by delicate threads of
protoplasmic substance, extending into them from the central pulp
(§ 611).
616. In the Teeth of Man and most other Mammals, and in
those of many Reptiles and some Fishes, we find two other sub-
stances, one of them harder, and the other softer, than dentine ; the
former is termed enamel; and the latter cementum or crusta
petrosa. — The enamel is composed of long prisms, closely resem-
bling those of the 'prismatic' Shell-substance formerly described
(§ 522), but on a far more minute scale ;
the diameter of the prisms not being
more in Man than l-5600th of an inch.
The length of the prisms corresponds
with the thickness of the layer of
enamel ; and the two surfaces of this
layer present the ends of the prisms,
the form of which usually approaches
the hexagonal. The course of the enamel-
prisms is more or less wavy; and they
are marked by numerous transverse striae,
resembling those of the prismatic shell-
substance, and probably originating in
the same cause,- — the coalescence of a
series of shorter prisms to form the
lengthened prism. In Man and in Car-
nivorous animals the enamel covers the
crown of the tooth only, with a simple
cap or superficial layer of tolerably uni-
form thickness (Fig. 391, a) which fol-
lows the surface of the dentine in all its
inequalities ; and its component prisms
are directed at right angles to that sur-
face, their inuer extremities resting in
slight but regular depressions on the ex-
terior of the dentine. In the teeth of
many Herbivorous animals, however, the
enamel forms (with the cementum) a
series of vertical plates, which dip
down into the substance of the dentine, and present their edges
alternately with it, at the grinding surface of the tooth ; and there
is in such teeth no continuous layer of enamel over the crown.
This arrangement provides, by the unequal wear of these three
substances (of which the enamel is the hardest, and the cementum
the softest), for the constant maintenance of a rough surface,
adapted to triturate the tough vegetable substances on which these
animals feed. The enamel is the least constant of the dental
tissues. It is more frequently absent than present in the teeth of
Vertical Section of Human
Molar Tooth: — a, enamel ; b,
cementum or crusta petrosa;
c, dentine or ivory ; d, osse-
ous excrescence, arising from
hypertrophy of cementum ; e,
pulp-cavity ;/, osseous lacu-
nae at outer part of dentine.
TEETH ;— SCALES OF FISH. 743
Fishes ; it is entirely wanting in the teeth of Serpents ; and it
forms no part of those of the Edentata* (sloths, &c.) and Cetacea
(whales) amongst Mammals. — The cementum, or crusta petrosa, has
the characters of true bone; possessing its distinctive stellate
lacunas and radiating canaliculi. Where it exists in small amount,
we do not find it traversed by medullary canals ; but, like dentine,
it is occasionally furnished with them, and thus resembles bone in
every particular. The?e medullary canals enter its substance from
the exterior of the tooth, and consequently pass towards those
which radiate from the central cavity in the direction of the surface
of the dentine, where this possesses a similar vascularity, — as was
remarkably the case in the teeth of the great extinct Megatherium. In
the Human tooth, however, the cementum has no such vascularity ;
but forms a thin layer (Fig. 391, b), which envelopes the root of the
tooth, commencing near the termination of the capping of enameL
In the teeth of many herbivorous Mammals, it dips clown with the
enamel to form the vertical plates of the interior of the tooth ; and
in the teeth of the Edentata, as well as of many Eeptiles and
Fishes, it forms a thick continuous envelope over the whole surface,
until worn-away at the crown.
617. Dermal Skeleton. — The Skin of Fishes, of most Eeptiles,
and of a few Mammals, is strengthened by plates of a horny, car-
tilaginous, bony, or even enamel-like texture ; which are some-
times fitted-together at their edges, so as to form a continuous box-
like envelope ; whilst more commonly they are so arranged as
partially to overlie one another, like the tiles on a roof; and it is in
this latter case that they are usually known as scales. Although
we are accustomed to associate in our minds the ' scales' of Fishes
with those of Eeptiles, yet they are essentially-different structures ;
the former being developed in the substance of the true skin, with
a layer of which in addition to the epidermis they are always
covered, and bearing a resemblance to cartilage and bone in their
texture and composition ; whilst the latter are formed upon the
surface of the true skin, and are to be considered as analogous to
nails, hoofs, &c, and other ' epidermic appendages.' In nearly all
the existing Fishes, the scales are flexible, being but little con-
solidated by calcareous deposit ; and in some species they are so
thin and transparent, that, as they do not project obliquely from
the surface of the skin, they can only be detected by raising the
superficial layer of the skin, and searching beneath it, or by tearing
off the entire thickness of the skin, and looking for them near its
under surface. This is the case, for example, with the common
Eel, and with the viviparous Blenny ; of either of which fish the
skin is a very interesting object when dried and mounted in Canada
balsam, the scales being seen imbedded in its substance, whilst its
* It has been shown by ]VIr. Chas. Tomes, however, that the ' enamel organ '
is originally present within the tooth-capsule of the Armadillo, though it
undergoes an early degeneration ; a fact of no little interest in connection with
the general doctrine of " Unity of Type."
■u
YEETEBEATED ANIMALS.
„?vr,r,.
Portion of Skin of Sole, viewed as an opaque
object.
outer surface is studded with pigment- cells . Generally speaking,
however, the posterior extremity of each scale projects obliquely
from the general surface, carrying before it the thin membrane that
encloses it, which is
-Frc. 392. studded with pigment-
cells ; and a portion of
the skin of almost any
Fish, but especially of
such as have scales of
the ctenoid kind (that is,
furnished at their pos-
terior extremities with
comb-like teeth, Fig.
393), when dried with its
scales in situ, is a very
beautiful opaque object
for the low powers of
the Microscope (Fig.
392), especially with the
Binocular arrangement. Care must be taken, however, that the
light is made to glance upon it in the most advantageous manner ;
since the brilliance with which it is reflected from the comb-like
projections entirely depends upon the angle at which it falls upon
them. The only appearance of structure
exhibited by the thin flat scale of the
Eel, when examined microsco|3ically, is
the presence of a layer of isolated
spheroidal transparent bodies, imbedded
in a plate of like transparence ; these,
from the researches of: Prof. Williamson
upon other scales, appear not to be
cells (as they might readily be supposed
to be), but to be concretions of Carbo-
nate of Lime. When the scale of the
Eel is examined by Polarized light, its
surface exhibits a beautiful St. An-
drew's cross ; and if a plate of Selenite
be placed behind it, and the analyzing
prisni be made to revolve, a remarkable
play of colours is presented.
618. In studying the structure of
the more highly developed scales, we
may take as an illustration that of the
Carp ; in which two very distinct layers
can be made-out by a vertical section,
with a third but incomplete layer in-
terposed between them. The outer
layer is composed of several concentric laminae of a structureless
transparent substance, like that of cartilage ; the outermost of
Fig. 398.
:M
Scale of Sole, viewed as
transparent object.
SCALES OF FISH. 745
these laminae is the smallest, and the size of the plates increases
progressively from without inwards, so that their margins appear
oa the surface as a series of concentric lines ; and their surfaces
are thrown into ridges and furrows, which commonly have a
radiating direction. The inner layer is composed of numerous
lamina? of a fibrous structure, the fibres of each lamina being in-
clined at various angles to those of the lamina above and below it.
Between these two layers is interposed a stratum of calcareous
concretions, resembling those of the scale of the Eel; these are
sometimes globular or spheroidal, but more commonly ' lenticular,'
that is, having the form of a double- convex lens. The scales which
resemble those of the Carp in having a form more or less circular,
and in being destitute of comb-like prolongations, are called
cycloid; and such are the characters of those of the Salmon,
Herring. Roach, &c. The structure of the ctenoid scales (Fig. 393),
which we find in the Sole, Perch, Pike, &c, does not differ essen-
tially from that of the cycloid, save as to the projection of the
comb-like teeth from the posterior margin ; and it does not appear
that the strongly-marked division which Prof. Agassiz has at-
tempted to establish between the ' cycloid' and the ' ctenoid' Orders
of Fishes, on the basis of this difference, is in harmony with their
general organization. Scales of either kind may become con-
solidated to a considerable extent by the calcification of their soft
substance ; but still they never present any approach to the true
Bony structure, such as is shown in the two Orders to be next
adverted-to.
619. In the ganoid Scales, on the other hand, the whole sub-
stance of the scale is composed of a substance which is essentially
bony in its nature : its intimate structure being always comparable
to that of one or other of the varieties which present themselves
in the bones of the Vertebrate skeleton ; and being very frequently
identical with that of the bones of the same fish, as is the case with
the Lepidosteus (Fig. 387), one of the few existing representatives
of this order, which, in former ages of the Earth's history, compre-
hended a large number of important families. Their name (from
yavos, splendour) is bestowed on account of the smoothness, hard-
ness, and high polish of the outer surface of the scales ; which is due
to th e presence of a peculiar layer that has been likened (though
erroneously) to the enamel of teeth, and is now distinguished as
ganoin. The scales of this order are for the most part angular in
their form ; and are arranged in regular rows, the posterior edges
of each slightly overlapping the anterior ones of the next, so as to
form a very complete defensive armour to the body. — The scales of
the placoid type, which characterizes the existing Sharks and Eays,
with their fossil allies, are irregular in their shape, and very com-
monly do not come into mutual contact; but are separately imbedded
in the skin, projecting from its surface under various forms. In the
Kays each scale usually consists of a flattened plate of a rounded
shape, with a hard spine projecting from its centre ; in the Sharks
716 VEETEBEATED ANIMALS.
(to which tribe belongs the ' dog-fish' of our own coast) the scales
have more of the shape of teeth. This resemblance is not confined
to external form ; for their intimate structure strongly resembles
that of dentine, their dense substance being traversed by tubuli,
which extend from their centre to their circumference in minute
ramifications, without any trace of osseous lacunee. These tooth-
like scales are often so small as to be invisible to the naked eye ;
but they are well seen by drying a piece of the skiu to which they
are attached, and mounting it in Canada balsam ; and they are
most brilliantly shown by the assistance of polarized light. — A
like structure is found to exist in the ' spiny rays' of the dorsal
fin, which, also, are parts of the dermal skeleton ; and these
rays usually have a central cavity filled with medulla, from which
the tubuli radiate towards the circumference. This structure is
very well seen in thin sections of the fossil ' spiny rays,' which, with
the teeth and scales, are often the sole relics of the vast multitudes
of Sharks that must have swarmed in the ancient seas, their
cartilaginous internal skeletons having entirely decayed away. —
In making sections of bony Scales, Spiny rays, &c, the method
must be followed which has been already detailed under the head of
Bone (§ 614).*
620. The scales of Eeptiles, ijke feathers of Birds, and the hairs,
hoofs, nails, claivs, and horns (when not bony) of Mammals, are
all epidermic appendages; that is, they are produced upon the
surface, not within the substance, of the true skin, and are allied
in structure to the Epidermis (§ 631) ; being essentially composed
of aggregations of cells filled with horny matter, and frequently
much altered in form. This structure may generally be made-out
in horns, nails, &c, with little difficulty, by treating thin sections
of them with a dilute solution of soda ; which after a short time
causes the cells that had been flattened into scales, to resume their
globular form. The most interesting modifications of this structure
are presented to us in hairs and in feathers ; which forms of
clothing are very similar to each other in their essential nature, and
are developed in the same manner, — namely, by an increased pro-
duction of epidermic cells at the bottom of a flask-shaped follicle,
which is formed in the substance of the true Skin, and which is sup-
plied with abundance of blood by a special distribution of vessels to
its walls. When a hair is pulled-out ' by its root,' its base ex-
hibits a bulbous enlargement, of which the exterior is tolerably
firm, whilst its interior is occupied by a softer substance, which
is known as the ' pulp ;' and it is to the continual augmentation
of this pulp in the deeper part of the follicle, and to its conversion
into the peculiar substance of the hair when it has been pushed
* The structure of the Scales of Fishes has been most elaborately described
by Prof. Williamson in his Memoirs ' On the Microscopic Structure of the Scales
and Dermal Teeth of some Ganoid and Placoid Fish,' in " Philos. Transact.,"
1849, and ' Investigations into the Structure and Development of the Scales
and Bones of Fishes,' in " Philos. Transact.," 1851.
STRUCTURE OF FEATHERS AND HAIRS.
747
upwards to its narrow neck, that the growth of the hair is due. —
The same is true of feathers, the stems of which are but hairs on a
larger scale ; for the ' quill ' is the part contained within the
follicle, answering to the ' bulb' of the hair ; and whilst the outer
part of this is converted into the peculiarly-solid horny substance
forming the ' barrel' of the quill, its interior is occupied, during the
whole period of the growth of the feather, with the soft pulp, only
the shrivelled remains of which, however, are found within it after
the quill has ceased to grow.
Fig. 394.
Fig. 395.
Fig. 394. Hair of -Sable, showing large rounded cells in its
interior, covered by imbricated scales or flattened cells.
Fig. 395. Hair of Musk-deer, consisting almost entirely of
polygonal cells.
621. Although the hairs
of different Mammals differ
greatly in the appear-
ances they present, we may
generally distinguish in
them two elementary parts ;
namely, a cortical or invest-
ing substance, of a dense
horny texture, and a medul-
lary or pith-like substance,
usually of a much softer tex-
ture, occupying the interior.
The former can sometimes be
distinctly made-out to con-
sist of flattened scales ar-
ranged in an imbricated
manner, as in some of the
hairs of the Sable (Fig. 394) ;
whilst, in the same hairs, the
medullary substance is com-
posed of large spheroidal cells.
In the Mush-deer,onthe other
hand, the cortical substance
Fig. 396.
A, Small Hair of Squirrel: — B, Large Hair
of Squirrel: — C, Hair of Indian Bat.
748 VEETEBEATED ANIMALS.
is nearly undistingnishable ; and almost the entire hair seems made
up of thin- walled polygonal cells (Fig. 395). The hair of the Rein-
deer, though much larger, has a very similar structure ; and its cells,
except near the root, are occupied with hair alone, so as to seem
black by transmitted light, except when penetrated by the fluid in
which they are mounted. In the hair of the Mouse, Squirrel, and
other small Rodents (Fig. 396, a, b), the cortical substance forms
a tube, which we see crossed at intervals by partitions that are
sometimes complete, sometimes only partial; these are the walls of
the single or double line of cells, of which the medullary substance
is made-up. The hairs of the Bat tribe are commonly distinguished
by the projections on their surface, which are formed by extensions
of the component scales of the cortical substance : these are par-
ticularly well seen in the hairs of one
Fig. 397. of the Indian species, which has a set
of whorls of long narrow leaflets (so
to speak) arranged at regular intervals
on its stem (c). In the hair of the
Pecari(¥ig. 397), the cortical envelope
sends inwards a set of radial prolon-
gations, the interspaces of which are
occupied by the polygonal cells of the
Transverse section of Hair of medullary^ substance ; and this, on a
Pecari. larger scale, is the structure of the
' quills' of the Porcupine ; the radiating
partitions of which, when seen through the more transparent parts
of the cortical sheath, give to the surface of the latter a fluted
appearance. The hair of the Ornithorhyncus is a very curious
object ; for whilst the lower part of it resembles the fine hair of the
Mouse or Squirrel, this thins away and then dilates again into a
very thick fibre, having a central portion composed of polygonal
cells, enclosed in a flattened sheath of a brown fibrous substance.
622. The structure of the human Hair is in certain respects
peculiar. When its outer surface is examined, it is seen to be
traversed by irregular lines (Fig. 398, a), which are most strongly
marked in foetal hairs ; and these are the indications of the imbri-
cated arrangement of the flattened cells or scales which form the
cuticular layer. This layer, as is shown by transverse sections
(c, d), is a very thin and transparent cylinder ; and it encloses the
peculiar fibrous substance that constitutes the principal part of the
shaft of the hair. The constituent fibres of this substance, which
are marked-out by the delicate strias that may be traced in
longitudinal sections of the hair (b), may be separated from each
other by crushing the hair, especially after it has been macerated
for some time in sulphuric acid ; and each of them, when com-
pletely isolated from its fellows, is found to be a long spindle-
shaped cell. In the axis of this fibrous cylinder there is very
commonly a band which is formed of spheroidal cells ; but this
• medullary' substance is usually deficient in the fine hairs scattered
STEUCTUEE OF HUMAN HAIE.
749
over the general surface of the body, and is not always present in
those of the head. The hue of the Hair is due, partly to the
presence of pigmentary granules, either collected into patches, or
diffused through its substance ; but partly also to the existence
of a multitude of minute air-spaces, which cause it to appear
dark by transmitted and white by reflected light. The cells of the
Fig. 398.
Structure of Human Hair: — A, external surface of the shaft, showing
the transverse striae and jagged boundary caused by the imbrications
of the cuticular layer ; B, longitudinal section of the shaft, showing
the fibrous character of the cortical substance, and the arrangement
of the pigmentary matter ; c, transverse section, showing the distinc-
tion between the cuticular envelope, the cylinder of cortical substance,
and the medullary centre ; D, another transverse section, showing
deficiency of the central cellular substance.
medullary axis in particular, are very commonly found to contain
air, giving it the black appearance shown at c. The difference
between the blackness of pigment and that of air-spaces may be
readily determined by attending to the characters of the latter
as already laid-down (§§ 142, 143) ; and by watching the effects
of the penetration of Oil of Turpentine or other liquids, which
do not alter the appearance of pigment- spots, but obliterate all
the markings produced by air-spaces, these returning again as
the hair dries. — In mounting Hairs as Microscopic preparations,
they should in the first instance be cleansed of all their fatty
matter by maceration in ether ; and they may then be put up
either in weak Spirit or in Canada balsam, as may be thought
preferable, the former menstruum being well adapted to display the
characters of the finer and more transparent hairs, while the latter
allows the light to penetrate more readily through the coarser and
more opaque. Transverse sections of Hairs are best made by
gluing or gumming several together, and then putting them into
the Section-instrument ; those of Human hair may be easily ob-
tained, however, by shaving a second time, very closely, a part of
the surface over which the razor has already passed more lightly,
750 VEKTEBRATED ANIMALS.
and by picking- out from the lather, and carefully washing, the
sections thus taken-off.
623. The stems of feathers exhibit the same kind of structure as
Hairs ; their cortical portion being the horny sheath that envelopes
the shaft, and their medullary portion being the pith-like substance
which that sheath includes. In small feathers, this may usually
be made very plain by mounting them in Canada balsam; in large
feathers, however, the texture is sometimes so altered by the drying
up of the pith (the cells of which are always found to be occupied
by air alone), that the cellular structure cannot be demonstrated
save by boiling thin slices in a dilute solution of potass, and not
always even then. In small feathers, especially such as have a
downy character, the cellular structure is very distinctly seen in
the lamince or ' barbs,' which are sometimes found to be composed
of single files of pear-shaped cells, laid end-to-end ; but in larger
feathers it is usually necessary to increase the transparence of the
barbs, especially when these are thick and but little pervious to
light, either by soaking them in Turpentine, mounting them in
Canada balsam, or boiling them in a weak solution of Potass. In
the feathers which are destined to strike the air with great force in
the act of flight, we find the barbs . fringed on each side with hair-
like filaments or jpinnce ; on one side of each barb these filaments
are toothed on one edge, whilst on the other side they are furnished
with curved hooks ; and as the two sets of pinnae which spring from
two adjacent barbs cross one another at an angle, and each hooked
pinna on one locks into the teeth of several of the toothed pinnae
arising from the other, the barbs are connected together very firmly
by this apparatus of ' hooks and eyes,' which remind us of that
already mentioned as observable on the wings of Hymen opterous
Insects (§ 598). — Feathers or portions of feathers of Birds distin-
guished by the splendour of their plumage are very good objects
for low magnifying powers, when illuminated on an opaque ground ;
but care must be taken that the light falls upon them at the angle
necessary to produce their most brilliant reflection into the axis of
the Microscope; since feathers which exhibit the most splendid
metallic lustre to an observer at one point, may seem very dull to
the eye of another in a different position. The small feathers of
Humming-birds, portions of the feathers of the Peacock, and others of
a like kind, are well worthy of examination; and the scientific
Microscopist who is but little attracted by mere gorgeousness, may
well apply himself to the discovery of the peculiar structure which
imparts to these objects their most remarkable character.
624. Sections of horns, hoofs, claws, and other like modifications
of Epidermic structure, — which may be made by the Section-instru-
ment (§ 152), the substance to be cut having been softened, if
necessary, by soaking in warm water, — do not in general afford any
very interesting features when viewed in the ordinary mode ; but
there are no objects on which Polarized light produces more remark-
able effects, or which display a more beautiful variety of colours
HOKNY TISSUES ;— BLOOD.
751
when a plate of selenite is placed behind them and the analyzing
prism is make to rotate. A curious modification of the ordinary
structure of Horn is presented in the appendage borne by the
Rhinoceros upon its snout, which in many points resembles a bundle
of hairs, its substance being arranged in minute cylinders around a
number of separate centres, which have probably been formed by
independent papillae (Fig. 399). When transverse sections of these
cylinders are viewed by
polarized light, each of Fig. 399.
them is seen to be marked
by a cross, somewhat re-
sembling that of Starch-
grains (§ 327) ; and the
lights and shadows of
this cross are replaced by
contrasted colours, when
the Selenite plate is inter-
posed. The substance
commonly but erroneously
termed whalebone, which
is formed from the surface
of the membrane that
lines the mouth of the
Whale, and has no rela-
tion to its true bony
skeleton, is almost iden-
tical in structure with
Rhinoceros horn, and is
similarly affected by polarized light. The central portion of each
of its component threads,- like the medullary substance of Hairs,
contains cells that have been so little altered as to be easily
recognized ; and the outer or cortical portion also may be shown
to have a like structure, by macerating it in a solution of potass,
and then in water. — Sections of any of the Horiry tissues are best
mounted in Canada balsam.
625. Blood. — Carrying our Microscopic survey, now, to the
elementary parts of which those softer tissues are made up, that
are subservient to the active life of the body rather than to its
merely-mechanical requirements, we shall in the first place notice
the isolated floating cells contained in the Blood, and known as the
Blood-corpuscles. These are of two kinds ; the 'red' and the 'white'
or ' colourless.' The red present, in every instance, the form of a
flattened disk, which is circular in Man and most Mammalia (Fig.
401), but is oval in Birds, Eeptiles (Fig. 400), and Fishes, as also in
a few Mammals (all belonging to the Camel tribe). In the one form,
as in the other, these corpuscles seem to be flattened cells, the walls
of which, however, are not distinctly differentiated from the viscid
substance they contain ; as appears from the changes of form which
(as shown by Dr. Beale) they spontaneously undergo when kept at
U
Transverse section of Horn of Rhinoceros,
viewed by Polarized Light.
752
VEETEBEATED ANIMALS.
a temperature of about 100°, and from the effects of pressure in
"breaking them up. The red corpuscles in the blood of Oviparous
Vertebrata are distinguished by the presence of a central spot or
nucleus, which appears to be composed of an aggregation of minute
granules ; this is most distinctly brought into view by treating the
blood-disks with Acetic acid, which renders the remaining portion ex-
tremely transparent, while it increases the opacity of the nucleus
(Fig. 400, d). It is remarkable, however, that the red corpuscles
of the blood of Mammals should possess no obvious nucleus ; the
dark spot which is seen in their centre (Fig. 401, b) being merely
an effect of refraction, consequent upon the double-concave form of
the disk. When the corpuscles are treated with water, so that
their form becomes first
Fig. 400. flat, and then double-con-
vex, the dark spot disap-
pears ; whilst, on the
other hand, it is made
more evident when the
concavity is increased by
the partial shrinkage of
the corpuscles, which may
be brought about by treat-
ing them with fluids of
greater density than their
own substance. The size
of the red corpuscles is
not altogether uniform in
the same blood; thus it
varies in that of Man from
about the 1 -4000th to the 1 -2800th of an inch. But we generally
find that there is an average size, which is pretty constantly
maintained among the dif-
Fig. 401. ferent individuals of the
same species ; that of Man
may be stated at about
l-3200thof an inch. _ The fol-
lowing Table* exhibits the
average dimensions of some
of the most interesting ex-
amples of the red blood- cor-
puscles in the four classes of
Yertebrated Animals, ex-
pressed in fractions of an
inch. Where two measure-
ments are given, they are
the long and the short dia-
meters of the same cor-
puscles. (See also Fig. 402.)
* These measurements are chiefly selected from those given by Mr. Gulliver
in his edition of Hewson's Works, p. 286 et seq.
Eed Corpuscles of Frog's Blood : — a a, their
flattened face ; b, particle turned nearly edge-
ways ; c, colourless corpuscle ; d, red cor-
puscles altered by dilute acetic acid.
Jfe
Eed Corpuscles of Human Blood ; repre-
sented at a, as they are seen when rather
within the focus of the Microscope, and
at b as they appear when precisely in the
focus.
COEPUSCLES OF BLOOD.
753
Man . . . . 1-3200
Dog 1-3542
Whale 1-3099
Elephant 1-2745
Mouse 1-3814
Camel .... 1-3254, 1-5921
Llama .... 1-3361, 1-6294
Java Musk-Deer . . 1-12325
Caucasian Goat .... 1-7045
Two-toed Sloth. . . . 1-2865
Golden Eagle
Owl. . . .
Crow . . .
Blue-Tit . .
Parrot . . .
1-1812, 1-3832
1-1830, 1-3400
1-1961, 1-4000
1-2313, 1-4128
1-1898, 1-4000
Ostrich. . .
Cassowary .
Heron . . .
Fowl . . .
Gull . . .
. 1-1649, 1-3000
. 1-1455, 1-2800
. 1-1913, 1-3491
. 1-2102, 1-3466
. 1-2097, 1-4000
REPTILES.
Turtle . . .
Crocodile . .
Green Lizard
Slow- worm .
Viper . . .
1-1231, 1-1882
1-1231, 1-2286
1-1555, 1-2743
1-1178, 1-2666
1-1274, 1-1800
FISl
Frog . . .
Water-Newt .
Siren . . .
Proteus . .
Lepido siren .
iES.
. 1-1108, 1-1821
. 1-814, 1-1246
. 1-420, 1-760
. 1-400, 1-727
. 1-570, 1-941
Perch . . .
Carp . . .
Gold-Fish .
. 1-2099, 1-2824
1-2112, 1-3429
. 1-1777, 1-2824
Pike . . .
Eel ... .
Gymnotus
. 1-2000, 1-3555
. 1-1745,1-2842
. 1-1745, 1-2599
Tims it appears that the smallest red corpuscles known are those of
the Mush-deer ; whilst the largest are those of that curious group
of Batrachian (frog-like) Eeptiles which retain their gills through
the whole of life ; and one of the oval blood-disks of the Proteus,
being more than 30 times as long and 17 times as broad as those of
the Musk-deer, would cover no fewer than 510 of them. — According
to the estimate of Yierordt, a cubic inch of Human Blood contains
upwards of eighty millions of red corpuscles, and nearly a quarter
of a million of the colourless.
626. The white or 'colourless' corpuscles are more readily distin-
guished in the blood of Eeptiles than in that of Man ; being in the
former case, of much smaller size, as well as having a circular out-
line (Fig. 400, c) ; whilst in the latter their size and contour
are nearly the same, so that, as the red corpuscles themselves wlieo
seen in a single layer have but a very pale hue, the deficiency
of colour does not sensibly mark their difference of nature. It is
remarkable that, notwithstanding the great variations in the sizes
of the red corpuscles in different species of Yertebrated animals,
the size of the white is extremely constant throughout, their
diameter being seldom much greater or less than 1 -3000th of an
inch in the warm-blooded classes, and l-2500th in Eeptiles. Their
ordinary form is globular ; but their aspect is subject to consider-
able variations, which seem to depend in great part upon their
phase of development. Thus in their early state, in which they
seem to be identical with the corpuscles found floating in clujle and
lymph, they seem to be nearly homogeneous particles of proto-
3 c
754
YEETEBEATED ANIMALS.
plasmic substance ; but in their more advanced condition a differen-
tiation is observable, analogous to that which, exists between
the 'ectosarc' and 'endosarc' of Ehizopods (§ 369); and the
isolated particles of the latter are often to be seen executing an
active molecular niove-
Fig. 402. ment within the for-
mer, which continues
when they are dis-
charged by the burst-
ing of the corpuscle,
consequent upon the
addition of a solution
of potass. These cor-
puscles are occasionally
seen to exhibit very
curious changes of
form (Fig. 403), which
reminds us of those of
the Amoeba (§ 376) ; a
protrusion taking place
from some portion of
the ectosarc, the form
of which seems quite
indeterminate ; and
this being soon suc-
ceeded by another from
some different part, the
first being either
drawn-in again, or re-
maining as it was.
Such changes have
been observed, not
only in the white cor-
puscles of the blood
of various Yertebrated
animals, but also in
the corpuscles floating
in the circulating fluid
of the higher Inverte-
brata, as the Crab,
which resemble the ' white ' corpuscles of Yertebrated blood rather
than its 'red' corpuscles, — these last, in fact, being altogether pecu-
liar to the circulating fluid of Yertebrated animals.
627. In examining the Blood microscopically, it is, of course,
important to obtain as thin a stratum of it as possible, so that the
corpuscles may not overlie one another. This is best accomplished
by selecting a piece of thin glass of perfect flatness, and then,
having received a small drop of Blood upon a glass side, to lay the
thin-glass cover not upon this, but with its edge just touching the
Comparative sizes of Bed Blood-Corpuscles : —
1. Man; 2. Elephant; 3. Musk-Deer; 4. Drome-
dary; 5. Ostrich ; 6. Pigeon ; 7. Humming Bird:
8. Crocodile ; 9. Python ; 10. Proteus ; 11. Perch ;
12. Pike ; 13. Shark.
CORPUSCLES OF BLOOD.
755
edge of the drop ; for the blood will then be drawn-in by capillary
attraction, so as to spread in a uniformly-thin layer between
the two glasses. The inexperienced observer will be surprised at
the very pale hue which the red corpuscles exhibit beneath the Mi-
croscope, when seen in a single stratum ; but this surprise need no
longer be felt, when it is borne in mind that the thickness of the
film of colouring fluid which they contain is probably not more than
1 -20,000th of an inch ; and if a drop of ink, or of almost any
coloured liquid, however dark, be pressed-out between two glasses
into an equally thin film, its hue will be lightened in the same
degree. The red hue of the corpuscles, however, becomes obvious
enough, when two or more layers of them are seen-through at once.
The white corpuscles in Human blood are usually not more than
1 : 350 of the Eed, so
that no more than one or JFIG. 403.
two are likely to be in the
field at once; and these
may generally be recog-
nized most readily by
their standing-apart from
the rest ; for whilst the
red corpuscles have a ten-
dency to adhere to each
other by their discoidal
surfaces, the white show
no such disposition. The
prolongation of their
active condition essen-
tially depends upon their
being subjected to a con-
tinuance of a temperature
approaching that of the
living Human body. — Thin films of blood may be preserved
in the liquid state, with little change, by applying Gold-size
or Asphalte round the edge of the thin-glass cover before evapora-
tion has had time to take-place; but it is in some respects pre-
ferable to dilute the liquid with a small quantity of Goadby's solu-
tion, its strength being so adjusted as not to produce any endos-
motic change of form in the corpuscles. But it is far simpler to
allow such films to dry, without any cover, and then merely to cover
them for protection ; and in this condition the general characters of
the corpuscles can be very well made-out, notwithstanding that
they have in some degree shrivelled by the desiccation they have
undergone. And this method is particularly serviceable, as afford-
ing a fair means of comparison, when the assistance of the Micro -
scopist is sought in determining, for Medico-legal purposes, the
source of suspicious blood-stains ; the average dimensions of the
dried blood-corpuscles of the several domestic animals being suffi-
ciently different from each other and from those of Man, to allow
3c2
Altered White Corpuscle of Blood, an hour
after having been drawn from the finger.
756
VEETEBRATED ANIMALS.
Fig. 404
the nature of any specimen to be pronounced-upon with a high
degree of probability.
628. Simple Fibrous Tissues. — A very beautiful example of a
tissue of this kind is furnished by the membrane of the common
Fowl's egg ; which (as may be seen by examining an egg whose
shell remains soft for want of consolidation by calcareous par-
ticles), consists of two principal layers, one serving as the basis of
the shell itself, and the other forming that lining to it which
is known as the membrana putaminis. The latter may be sepa-
rated by careful tearing with needles and forceps, after prolonged
maceration iu water, into several matted lamella? resembling that
represented in Fig. 404; and similar lamellae may be readily
obtained from the shell itself, by dissolving away its lime by dilute
acid.* — The simply-fibrous struc-
tures of the body generally, how-
ever, belong to one of two very defi-
nite kinds of tissue, the ' white ' and
the ' yellow,' whose appearance,
composition, and properties are very
different. The white fibrous tis-
sue, though sometimes apparently
composed of distinct fibres, more
commonly presents the aspect of
bands, usually of a flattened form,
and attaining the breadth oil
1 -500th of an inch, which are
marked by numerous longitudinal
streaks, but can seldom be torn-
up into minute fibres of determinate size. The fibres and bands
are occasionally somewhat wavy
in their direction ; and they
have a peculiar tendency to
fall into undulations, when it is
attempted to tear them apart
from each other (Fig. 405). This
tissue is easily distinguished from
the other by the effect of Acetic
acid, which swells it up and ren-
ders it transparent, at the same
time bringing into view certain
oval nuclear particles of ' germinal
matter/ which are known as ' con-
nective-tissue-corpuscles' (§ 611).
These are relatively much larger,
and their connections more dis-
tinct, in the earlier stages of the
formation of this tissue (Fig. 406).
Fibrous membrane from Egg-shell.
Fig. 405
'Mi M
White Fibrous Tissue from Liga-
ment.
* For an account of the curious manner in which the Carbonate of Lime is
disposed in the Egg-shell, see § 66y.
WHITE AND YELLOW FIBROUS TISSUES.
757
Fig. 406.
It is perfectly inelastic; and we find it in such parts as tendons,
ordinary ligaments, fibrous capsules, &c , whose functions it is to
resist tension without yielding to it. It constitutes, also, the
organic basis or matrix of bone ; for although the
substance which is left when a bone has been
macerated sufficiently long in dilute acid for
all its Mineral components to be removed, is
commonly designated as cartilage, this is shown
by careful Microscopic analysis not to be a cor-
rect description of it; since it does not show
any of the characteristic structure of cartilage,
but is capable of being torn into lamellae, in
which, if sufficiently thin, the ordinary struc-
ture of a fibrous membrane can be distinguished.
— The yelloiv fibrous tissue exists in the form
of long, single, elastic, branching filaments, with
a dark decided border ; which are disposed to curl
when not put on the stretch (Fig. 407), and fre-
quently anastomose, so as to form a network.
They are for the most part between l-5000th and
1 -10,000th of an inch in diameter ; but they are
often met with both larger and smaller. This
tissue does not undergo any change, when
treated with Acetic acid. It exists alone (that
is, without any mixture of the white) in parts
which require a peculiar elasticity, such as the
middle coat of the arteries, the ' vocal cords,'
the 'ligamentum nuchas' of Quadrupeds, the
elastic ligament which holds together the valves
of a Bivalve shell, and that by which the claws of the Feline
tribe are retracted when
not in use ; and it enters
largely into the composi-
tion of areolar or con-
nective tissue.
629. The tissue for-
merly known to Anato-
mists as ' cellular,' but
now more properly desig-
nated connective or areo-
lar tissue, consists of a
network of minute fibres
and bands, which are
interwoven in every di-
rection, so as to leave
innumerable areolae or
little spaces that com-
municate freely with
one another. Of these fibres, some are of the ' yellow' or elastic
Portion of young
Tendon showing the
corpuscles of Ger-
minal Matter, with
their stellate prolon-
gations, interposed
among; its fibres.
Fig. 407.
Fellow Fibrous Tissue from Ligamentum
Nuchas of Calf.
758 VEETEBEATED ANIMALS.
kind, but the majority are composed of the ; white' fibrous tissue ;
and, as in that form of elementary structure, they frequently pre-
sent the condition of broad flattened bands or membranous shreds
in which no distinct fibrous arrangement is visible. The propor-
tion of the two forms varies, according to the amount of elasticity,
or of simple resisting power, which the endowments of the part
may require. We find this tissue in a very large proportion of
the bodies of higher Animals ; thus it binds together the ultimate
muscular fibres into minute fasciculi, unites these fasciculi into
larger ones, these again into still larger ones which are obvious
to the eye, and these into the entire muscle ; whilst it also forms
the membranous . divisions between distinct muscles. In like
manner it unites the elements of nerves, glands, &c, binds to-
gether the fat-cells into minute masses, these into large ones,
and so on ; and in this way penetrates and forms part of all
the softer organs of the body. But whilst the fibrous struc-
tures of which the 'formed tissue' is composed have a purely
mechanical function, there is good reason to regard the ' con-
nective-tissue-corpuscles' which are everywhere dispersed among
them, as having a most important f unction in the first production
and subsequent maintenance of the more definitely organized
portions of the fabric (§ 610). In these corpuscles distinct move-
ments, analogous to those of the sarcodic extensions of Rhizopods,
have lately been recognized in transparent parts, such as the
cornea of the eye and the tail of the young Tadpole, by observa-
tions made on these parts whilst living. — For the display of the
characters of the fibrous tissues, small and thin shreds may be cut
with the curved scissors from any part that affords them ; and
these must be torn asunder with needles under the Simple Micro-
scope, until the fibres are separated to a degree sufficient to enable
them to be examined to advantage under a higher magnifying
power. The difference between the ' white' and the ' yellow'
components of connective tissue is at once made apparent
by the effect of acetic acid ; whilst the ' connective-tissue-
corpuscles' are best distinguished by the staining-process (§ 161),
especially in the early stage of the formation of these tissues
(Fig. 406).
630. Shin, Mucous, and Serous Membranes. — The Skin which
forms the external envelope of the body, is divisible into two prin-
cipal layers ; the cutis vera or ' true skin,' which usually makes up
by far the larger part of its thickness, and the ' cuticle,' ' scarf-
skin,' or epidermis, which covers it. At the mouth, nostrils, and
the other orifices of the o^en cavities and canals of the body, the
skin passes into the membrane that lines these, which is dis-
tinguished as the mucous membrane, from the peculiar glairy secre-
tion of mucus by which its surface is protected. But those great
closed cavities of the body, which surround the heart, lungs,
intestines, &c, are lined by membranes of a different kind ; which,
as they secrete only a thin serous fluid from their surfaces, are
known as serous membranes. Both Mucous and Serous membranes
STEUCTUKE OF SKIN.
759
Fig. 408.
consist, like the Skin, of a proper membranous basis, and of a
thin cuticular layer, which, as it differs in many points from the
epidermis, is distinguished as the Epithelium (§ 633). — The sub-
stance of the 'true skin' and of the 'mucous' and 'serous' mem-
branes is principally composed of the fibrous tissues last described;
but the skin and the mucous membranes are very copiously
supplied with Blood-vessels and with Glandulas of various kinds ;
and in the skin we also find abundance of Nerves and Lymphatic
vessels, as well as, in some parts, of Hair-follicles. The general
appearance ordinarily presented by a thin vertical section of the
skin of a part furnished with numerous sensory papillce (§ 642), is
shown in Fig. 408 : where we see in the deeper layers of the cutis
vera little clumps of fat-cells, /, and
the perspiratory glandulee d, d,
whose ducts, e, e, pass upwards ;
whLst on its surface we distin-
guish the vascular papilla?, p, sup-
plied with loops of blood-vessels
from the trunk, g, and a tactile
papilla, t, with its nerve twig. The
spaces between the papillse are
filled-up by the soft Malpighian
layer, m, of the epidermis, a, in
whioh its colouring matter is chiefly
comained, whilst this is covered
by the horny layer, h, which is
traversed by the spirally-twisted
continuations of the perspiratory
ducts, opening at s upon the sur-
face, which presents alternating
depressions, a, and elevations b. —
The distribution of the blood-vessels
in the skin and mucous membranes,
which is one of the most interesting
features in their structure, and
which is intimately connected with
their several functions, will come
under our notice hereafter (Figs.
424, 427, 428). In serous mem-
branes, on the other hand, whose
function is simply protective, the
supply of Blood-vessels is more
scanty.
631. Epidermic and Epithelial
Cell-layers. — The Epidermis or
' cuticle' covers the whole exterior of the body, as a thin semi-
transparent pellicle, which is shown by Microscopic examination to
consist of a series of layers of cells, that are continually wearing-off
at_ the external surface, and renewed at the surface of the true
skin ; so that the newest and deepest layers gradually become the
Vertical Section of Skin of Fin-
ger : — A, epidermis, the surface of
which shows depressions «, a, be-
tween the eminences &, 6, on which
open the perspiratory ducts s ; at
m is seen the deeper layer of the
epidermis, or stratum Malpighii :
— B, cutis vera, in which are im-
bedded the perspiratory glands a\
with their ducts e, and aggrega-
tions of fat-cells/; g, arterial twig
supplying the vascular papillae p ;
t, one of the tactile papillae with
its nerve.
760 VERTEBEATED ANIMALS.
oldest and most superficial* and are at last thrown-off by slow
desquamation. In their progress from the internal to the external
surface of the epidermis, the cells undergo a series of well marked
changes. When we examine the innermost layer, we find it soft
and granular; consisting of germinal corpuscles in various stages
of development into cells, held-together by a tenacious semi-fluid
substance. This was formerly considered as a distinct tissue, and
was supposed to be the peculiar seat of the colour of the skin ; it
received the designation of Malpighian layer or rete mucosum.
Passing outwards, we find the cells more completely formed; at
first nearly spherical in shape, but becoming polygonal where they
are flattened one against another. As we proceed further towards
the surface, we perceive that the cells are gradually more and more
flattened until they become mere horny scales, their cavity being
obliterated ; their origin is indicated, however, by the nucleus in the
centre of each. This change in form is accompanied by a change
in the Chemical composition of the tissue, which seems to be due to
the metamorphosis of the contents of the cells into a horny sub-
stance identical with that of which hair, horn, nails, hoofs, &c, are
composed. — Mingled with the epidermic cells, we find others which
secrete colouring matter instead of horn ; these, which are teimed
' pigment-cells,' are especially to be noticed in the epidermis of the
Negro and other dark races, and are most distinguishable in the
Malpighian layer, their colour appearing to fade as they pass
towards the surface. — The most remarkable development of
pigment-cells in the higher animals, however, is on the inner
surface of the choroid coat of the eye, where they have a very
regular arrangement, and form several layers, known as the fig-
mentum nigrum. When examined separately, these cells are found
Y1G 409 to have a polygonal form (Fig. 409, a),
and to have a distinct nucleus (&)
i in their interior. The black colour
■<#^lk> ^*». *s ^verL by the accumulation, within
the cell, of a number of flat rounded
or oval granules, of extreme minute-
^^^^^/y ness, which exhibit an active movement
when set-free from the cell, and even
WW^^^I^ whilst enclosed within it. The pig-
' . '' r • ment-cells are not always, however,
.. of this simply rounded or polygonal
^^ ^Hfpp form ; they sometimes present remark-
able stellate prolongations, under which
Cells from Pigmentum M- form they are well seen in the skin of
grum:— a, pigmentary granules faQ ji rpis. 423, C, c). The gradual
concealing the nUcieuS . &J the formation f thege prolongations may
nucleus distinct. , . , .^ : ° ■.-. c ,■, j
be traced m the pigment-cells ol the
Tadpole during its metamorphosis (Fig. 410) Similar varieties of
form are to be met-with in the pigmentary cells of Fishes and small
Crustacea, which also present a great variety of hues ; and these
EPIDERMIS : — PIGMENT-CELLS.
761
Fig. 410.
seem to take the colour of the bottom over which the animal may-
live, so as to serve for its conceal-
ment.
632. The structure of the Epidermis
may be examined in a variety of ways.
If it be removed b}r maceration from
the true Skin, the cellular nature of its
under surface is at once recognized,
when it is subjected to a magnifying
power of 200 or 300 diameters, by light
transmitted through it, with this sur-
face uppermost ; and if the epidermis
be that of a Xegro or any other dark-
skinned race, the pigment-cells will be
very distinctly seen. This under-surface
of the epidermis is not flat, but is
excavated into pits and channels for
the reception of the papillary elevations
of the true Skin ; an arrangement which
is shown on a large scale in the thick
cuticular covering of the Dog's foot, the
subjacent pa pillas being large enough
to be distinctly seen (when injected) with
the naked eye. The cellular nature of
the newly- forming layers is best seen by
examining a little of the soft film that is of recent
found upon the surface of the true Skin,
after the more consistent layers of the assumed,
cuticle have been raised by a blister. The
alteration which the cells of the external layers have undergone,
tends to obscure their character ; but if any fragment of epidermis
be macerated for a little time in a weak solution of Soda or
Potass, its dry scales become softened, and are filled-out by im-
bibition into rounded or polygonal cells. The same mode of treat-
ment enables us to make out the cellular structure in warts and
corns, which are epidermic growths from the surface of papillae
enlarged by hypertrophy.
633. The Epithelium may be designated as a delicate cuticle,
covering all the free internal surfaces of the body, and thus
lining all its cavities, canals, <fcc. Save in the mouth and other
parts in which it approximates to the ordinary cuticle both in locality
and in nature, its cells (Fig. 41 1) usually form but a single layer ;
and are so deficient in tenacity of mutual adhesion, that they cannot
be detached in the form of a continuous membrane. Their shape
varies greatly ; for sometimes they are broad, flat, and scale-like,
and their edges approximate closely to each other, so as to form
what is termed a ' pavement' or ' tesselated' epithelium ; such cells
are observable on the web of a Frog's foot, or on the tail of a Tadpole ;
for, though covering an external surface, the soft moist cuticle of
Pigment-cells from tail of
Tadpole : — «, o, simple forms
origin ; 6, &, more
complex forms subsequently
762
VERTEBEATED ANIMALS.
Fig. 411.
from
mouth.
these parts has all the characters of an epithelium. In other cases,
the cells have more of the form of cylinders, standing erect side-by-
side, one extremity of each cylinder forming part of the free surface,
whilst the other rests upon the mem-
brane to which it serves as a covering.
If the cylinders be closely pressed to-
gether, their form is changed into
prisms ; and such epithelium is often
known as ' prismatic' On the other
hand, if the surface on which it rests
be convex, the bases or lower ends of
the cylinders become smaller than their
free extremities ; and thus each has
the form of a truncated cone rather
than of a cylinder, and such epi-
Detached Epithelium-cells ; a, thelium (of which that covering the
with nuclei 6, and nucleoli c v^ 0f the intestine, Fig. 424, is a pecu-
Mucous Membrane of v -, , -. \ • < i <r
harly-good example) is termed coni-
cal.' But between these primary forms
of epithelial cells, there are several intermediate gradations ; and one
often passes almost insensibly into the other. — Any of these forms
of epithelium may be furnished with cilia ; but these appendages
are more commonly found attached to the elongated, than to the
flattened forms of epithelial cells (Fig. 412). Ciliated epithelium
is found upon the lining membrane of the air-passages in all air
breathing Vertebrata ; and it also presents itself in many other
situations, in which a propulsive power is needed to prevent an
accumulation of mucous or other secretions. Owing to the very
slight attachment that usually
Fig. 412. exists between the epithelium
and the membranous surface
whereon it lies, there is usually
no difficulty whatever in exami-
ning it ; nothing more being ne-
cessary than to scrape the sur-
face ' of the membrane with a
knife, and to add a little water
to what has been thus removed.
The ciliary action will generally
be found to persist for some
hours or even days after death, if the animal has been previously
in full vigour j* and the cells that bear the cilia, when detached
from each other, will swim freely about in water. If the thin fluid
that is copiously discharged from the nose in the first stage of an
Ciliated Epithelium; a, nucleated
cells resting on their smaller extremi-
ties ; b, cilia.
* Thus it has been observed in the lining of the windpipe of a decapitated
criminal, as much as seven days after death ; and in that of the river Tortoise
it has been seen fifteen days after death, even though putrefaction had already
far advanced.
ADLTOSE TISSUE; FAT-CELLS. 763
ordinary 'cold in the head,' be subjected to microscopic exami-
nation, it will commonly be fonnd to contain a great nnmber of
ciliated epithelium-cells, which have been thrown-offfrom the lining
membrane of the nasal passages.
634. Fat. — One of the best examples which the bodies of higher
animals afford, of a tissue composed of an aggregation of cells, is
presented by Fat ; the cells of which are distinguished by their
power of drawing into themselves oleaginous matter from the
blood. Fat-cells are sometimes dispersed in the interspaces of
areolar tissue ; whilst in other cases they are aggregated in distinct
masses, constituting the proper Adipose substance. The individual
fat- cells always present a nearly spherical or spheroidal form ;
sometimes, however, when they are closely pressed together, they
become somewhat polyhedral, from the flattening of their walls
against each other (Fig. 413). Their intervals are traversed by a
minute network of blood-vessels (Fig.
425), from which they derive their secre-
tion ; and it is probably by the con stant
moistening of their walls with a
watery fluid, that their contents are .
retained without the least transudation,
although these are quite fluid at the
temperature of the living body. Fat-
cells, when filled with their characteristic
contents, have the peculiar appearance
which has been already described as ap-
pertaining to oil-globules (§ 143), being
very bright in their centre, and very dark y-4
towards their margin, in consequence of l—-'*-"-C:-f.\^-\
their high refractive power ; but if, as
often happens in preparations that have
been long mounted the oily contents Areolar and Adipose tissue;
should have escaped, they then look a, a, fat-cells; 6, &, fibres of
like any other cells of the same form, areolar tissue.
Although the fatty matter which fills
these cells (consisting of a solution of Stearine or Margarine in
Oleine) is liquid at the ordinary temperature of the body of a warm-
blooded animal, yet its harder portion sometimes crystallizes on
cooling ; the crystals shooting from a centre, so as to form a star-
shaped cluster. — In examining the structure of adipose tissue, it is
desirable, where practicable, to have recourse to some specimen in
which the fat-cells lie in single layers, and in which they can be
observed without disturbing or laying them open ; such a con-
dition is found, for example, in the mesentery of the Mouse ; and
it is also occasionally met with in the fat-deposits which present
themselves at intervals in the connective tissues of the muscles,
joints, &c. Small collections of fat-cells exist in the deeper layers
of the true skin, and are brought into view by vertical sections of
it (Fig. 408. /). And the structure of large masses of fat may be
764 VEETEBEATED ANIMALS.
examined by thin sections, these being placed under water in thin
cells, so as to take-off the pressure of the glass-cover from their
surface, which would cause the escape of the oil-particles. No
method of mounting (so far as the Author is aware) is successful in
causing these cells permanently to retain their contents.
635. Cartilage. — In tne ordinary forms of Cartilage, also, we
have an example of a tissue essentially composed of cells ; but
these are commonly separated
Fig. 414. from each other by an ' intercel-
wmmfmmmr^mwwJmvm lular substance,' which is so closely
WM adherent to the outer walls of the
III cells as not to be separable from
m them (§ 610). The thickness of
^1 this substance differs greatly in
fS\ different kinds of cartilage, and
^K3H even in different stages of the
growth of any one. Thus in the
XtftoJUtie&aUSS&mhifa cartilage of the external ear of a
Cellu'ar Cartilage of Mouse's-ear. Bat or Mouse (Fig. 414), the cells
are packed as closely together as
are those of an ordinary Vegetable parenchyma (Fig. 211, a) ; and
this seems to be the early condition of most cartilages that are
afterwards to present a different aspect. In the ordinary cartilages,
however, that cover the extremities of the bones, so as to form smooth
surfaces for the working of the joints, the amount of intercellular
substance is usually con-
Fig. 415. siderable ; and the car-
tilage-cells are commonly
found imbedded in this
in clusters of two, three,
or four (Fig. 415), which
,,'V V;, it '- -~_^\ are evidently formed by a
ll^A . X"' ""'^.. :.;-r-V\/^Vvw J^>r d process of 'binary sub-
**>-~^-~ '-■' /^pV^ £) \ W<^~^ ' i ^% division' analogous to that
1 ^^^^F^^^y^^' . 'xj^ '*"'■■ v- by which fhe multiplica-
'tf'&^A, ' >.\ ,'"" }' ■ /^H> tion of cells takes place in
^r^€>\'% .■"■.' _>"">. / °~\\ the Vegetable Kingdom
^ ^ ~ : &y (§ 264).' The substance
f - :Ji. „ "' - . ® ) , rj^"1" of these cellular cartilages
■ -. :■> ^-— .;„-- "' ig entirely destitute of
"■'<:.LZ-~r\ ' - ■■■"' blood-vessels ; being nour-
ished solely by imbibition
Section of the branchial Cartilage of 'Tadpole: from the blood "brought
— «, group of four cells, separating from each , ,i ■,
other ■ b, pair of cells in apposition ; c, c, nuclei *? the membrane covering
of cartilage-cells ; d, cavity containing three their surface. Hence they
cells. may be compared, in re-
gard to their grade of
organization, with the larger Algse ; which consist, like them, of ag-
gregations of cells held together by intercellular substance, without
vessels of any kind, and are nourished by imbibition through their
A
CAETILAGE. — STEUCTUEE OF GLANDS. 765
whole surface. — There are many cases, however, in which the struc-
tureless intercellular substance is replaced by bundles of fibres,
sometimes elastic, but more commonly non -elastic ; such combina-
tions, which are termed _/i&ro-cartilages, are interposed in certain
joints, wherein tension as well as pressure has to be resisted, as for
example, between the vertebrae of the spinal column, and the bones
of the pelvis. — In examining the structure of Cartilage, nothing more
is necessary than to make very thin sections with a sharp razor or
scalpel, or with a Valentin's knife (§ 152), or, if the specimen be
large and dense (as the cartilage of the ribs), with the Section-
instrument (§ 153). These sections may be mounted in weak
Spirit, in Goadby's solution, or in Glycerine-jelly ; but in what-
ever way they are mounted, they undergo a gradual change by
the lapse of time, which renders them less fit to display the charac-
teristic features of their structure.
636. Structure of the Glands. — The various Secretions of the body
(as the saliva, bile, urine, &c.) are formed by the instrumentality of
organs termed Glands ; which are, for the most part, constructed
on one fundamental type, whatever be the nature of their product.
The simplest idea of a gland is that which we gain from an exami-
nation of the ' follicles' or little bags imbedded in the wall of the
stomach ; some of which secrete mucus for the protection of its
surface, and others gastric juice. These little bags are filled with cells
of a spheroidal form, which may be considered as constituting their
epithelial lining ; these cells, in the progress of their development,
draw into themselves from the blood the constituents of the
particular product they are to secrete ; and they then seem to
deliver it up, either by the bursting or by the melting-away of
their walls, so that this product may be poured-forth from the
mouth of the bag into the cavity in which it is wanted. The
Liver itself, in the lowest animals wherein it is found, presents
this condition. Some of the cells that form the lining of the
stomach in the Hydra and Actinia, seem to be distinguished from
the rest by their power of secreting bile, which gives them a
brownish-yellow tinge ; in many Polyzoa, Compound Tunicata, and
Annelida, these biliary cells can be seen to occupy follicles in the
walls of the stomach ; in Insects these follicles are few in number,
but are immensely elongated so as to form biliary tubes, which lie
loosely within the abdominal cavity, frequently making many con-
volutions within it, and discharge their contents into the com-
mencement of the intestinal canal ; whilst in the higher Mollusca,
and in Crustacea, the follicles are vastly multiplied in number, and
are connected with the ramifications of gland-ducts, like grapes
upon the stalks of their bunch, so as to form a distinct mass which
now becomes known as the Liver. The examination of the biliary
tubes of the Insect, or of the biliary follicles of the Crab, which
may be accomplished with the utmost facility, is well adapted to
give an idea of the essential nature of glandular structure. Among
Vertebrated animals, the Salivary glands, the Pancreas (sweet-
bread), and the Mammary glands, are well adapted to display the
766 VEETEBEATED ANIMALS.
lollicular structure (Fig. 416) ; nothing more being necessary than
to make sections of these organs, thin enough to be viewed as
transparent objects. The Liver of Yertebrata, however, presents
certain peculiarities of structure, which are not yet fully understood ;
for although it is essentially composed, like other glands, of secret-
ing cells, yet it has not yet been determined beyond doubt whether
these ceils are contained within any kind
Fig. 416. of membranous investment. The Kidneys
of Vertebrated animals are made-up of
elongated tubes, which are straight and
are lined with a pavement epithelium
in the inner or ' medullary' portion of
the kidney, whilst they are convoluted and
filled with a spheroidal epithelium in
the outer or ' cortical.' Certain flask-
shaped dilatations of these tubes include
TTU. „ ir , e ,T curious little knots of blood-vessels, which
Ultimate Follicles of Mam- -, ,, (11- , ■ , . , ',. , „
mary Gland, with their secret- are known as the Malpighian bodies of
ing cells a, a, containing nu- the kidney ; these are well displayed in
c]ei b, b. injected preparations. — For such a full
and complete investigation of the struc-
ture of these organs as the Anatomist and Physiologist require,
various methods must be put in practice which this is not the
place to detail. It is perfectly easy to demonstrate the cellular
nature of the surface of the Liver, by simply scraping a portion
of its cut surface ; since a number of its cells will be then
detached. The general arrangement of the cells in the lobules
may be displayed by means of sections thin enough to be trans-
parent ; whilst the arrangement of the blood-vessels can only
be shown by means of Injections (§ 647). Fragments of the
tubules of the Kidney, sometimes having the Malpighian capsules
in connection with them, may also be detached by scraping its cut
surface ; but the true relations of these parts can only be shown by
thin transparent sections, and by injections of the blood-vessels and
tubuli. The simple follicles contained in the walls of the Stomach
are brought into view by vertical sections ; but they may be still
better examined by leaving small portions of the lining membrane
for a few days in dilute nitric acid (one part to four of water),
whereby the fibrous tissue will be so softened, that the clusters of
glandular epithelium lining the follicles (which are but very little
altered) will be readily separated.
637. Muscular Tissue. — Although we are accustomed to speak
of this tissue as consisting of ' fibres,' yet the ultimate structure of
the ' muscular fibre' is very different from that of the ' simple
fibrous tissues' already described. When we examine an ordinary
muscle (or piece of ' flesh') with the naked eye, we observe that it
is made-up of a number of fasciculi or bundles of fibres, which are
arranged side-by-side with great regularity in the direction in which
the muscle is to act, and are united by areolar tissue. These
STEIATED MUSCULAE FIBEE ;— FIBEILLiE. 767
fasciculi may be separated into smaller parts, which appear like
simple fibres ; but when these are examined by the Microscope, they
are found to be themselves fasciculi, composed of minuter fibres
bound together by delicate filaments of connective tissue. By care-
fully separating these, we may obtain the ultimate muscular fibre.
This fibre exists under two forms, the striated and the non-striated.
The former is chiefly distinguished by the transversely-striated ap-
pearance which it presents (Fig. 417), and which is due to an altera-
tion of light and dark spaces along its whole extent ; the breadth and
distance of these striae vary, however, in different
fibres, and even in different parts of the same fibre, Fig. 417.
according to its state of contraction or relaxa- <:;. . : .;
tion. Longitudinal striae are also frequently o] j : ; -
visible, which are due to a partial separation be- \:\
tween the component fibrillar into which the fibre u %\'
may be broken up. — When a fibre of this kind is ||| g 'iW&
more closely examined, it is seen to consist of a ; . '■■
delicate tubular sheath, quite distinct on the one m'tiV^M^J'Jt
hand from the connective tissue which binds \'-.
the fibres into fasciculi, and equally distinct from \\} I ' ;;
the internal substance of the fibre. This mem-
branous tube, which has been termed the sarco- [., I lj| j f'iiv
lemma, is not perforated by capillary vessels, V -.
which therefore lie outside the ultimate elements
of the muscular substance; whether it is pene- _
trated by the ultimate fibrils of nerves, is a point afe<ZMuscular Fibre"
not yet certainly ascertained. — The diameter of the skewing at a the
fibres varies greatly in different kinds of Ver- transverse stri0e,and
tebrated animals. Its average is greater in at 6 its junction with
Eeptiles and Fishes than in Birds and Mam- tne Tendon,
mals, and its extremes also are wider ; thus its
dimensions vary in the Frog from l-100th to 1 -1000th of an inch,
and in the Skate from l-65th to 1 -300th; whilst in the Human
subject the average is about 1 -400th of an inch, and the extremes
about l-200th and 1 -600th.
638. The elements of Muscular Fibre appear to be very minute
cylindrical particles with flattened faces of nearly uniform size,
adherent to each other both longitudinally and laterally. The
former adhesion is usually the more powerful ; and causes the
substance of the fibre, when it is broken up, to present itself in the
form of delicate fibrittce, each of which is composed of a single row
of the primitive particles (Fig. 418). Sometimes, however, the
lateral adhesion is the stronger, so that the fibre tends to cleave
transversely into disks, each of which is composed of a layer of
the primitive particles arranged side by side. When the fibrillas are
separately examined under a magnifying power of from 250 to 400
diameters, they are seen to present a cylindrical or slightly -beaded
form ; and they show the same alternation of light and dark spaces,
as when the fibrillge are united into fibres or into small bundles.
768 VEETEBEATED ANIMALS.
The dark and light spaces are nearly of equal length ; hut each light
space is usually divided by a fine dark transverse line, which,
under a sufficient magnifying power, may be resolved into a row of
dark points. The number of these alternations in a given length is
extremely variable, and appears to depend in part upon the state of
contraction or relaxation of the fibre ; a converse variation showing
itself in the diameter of the fibrillae. The ordinary length of each
space may be stated at about 1-1 7,000th of an inch, so that there
Fig. 418.
Striated Muscular Fibre, separating into fibrillas.
would be eight or nine dark spaces, and as many light, in the length
of 1 -1000th of an inch ; but not unfrequently there are double that
number of alterations in the same length. The average diameter
of the fibrillae seems to be tolerably uniform in different animals,
being for the most part about l-10,000th of au inch : it has been
observed, however, as high as 1 -5000th of an inch, and as low as
1 -20,000th, even when the fibre was not put upon the stretch. In
the ' anterior adductor' muscles, which draw together the valves of
the shells of Terehratulce, the fibrillae (Fig. 418), which are so easily
separable that they can scarcely be bound together by a proper
sarcolemma, have a diameter of 1- 7500th of an inch.
639. In the examination of Muscular tissue, a small portion may
be cut-out with the curved scissors ; this should be torn up into its
component fibres ; and these, if possible, should be separated into
their fibrillae, by dissection with a pair of needles under the Simple
Microscope. The general characters of the striated fibre are
admirably shown in the large fibres of the Frog ; and by selecting
a portion in which these fibres spread themselves out to unite
with a broad tendinous expansion, they may often be found
so well displayed in a single layer, as not only to exhibit all their
characters without any dissection, but also to show their mode of
connection with the ' simple fibrous' tissue of which that expansion
is formed. As the ordinary characters of the fibre are but little
altered by boiling, this process may be had-recourse-to for their
more ready separation, especially in the case of the tongue. The
separation of the fibres into their fibrillae is only likely to be
accomplished, in the higher Yertebrata, by repeated attempts, of
MUSCULAE FIBEE. 769
which the greater number are likely to be unsuccessful ; but it may
be accomplished with much greater facility in the Eel and other Fish,
the tenacity of whose muscular tissue is much less. The characters
of the fibrillEe are not nearly so well pronounced, however, in the
Fish, as in the warm-blooded Yertebrata. Dr. Beale recommends
Glycerine for the preparation, and Grlycerine-media for the preser-
vation, of objects of this class ; and states that the alternation of
light and dark spaces in the fibrillas is rendered more distinct by
such treatment. The fibrillar are often more readily separable
when the muscle has been macerated in a weak solution of Chromic
acid. — The shape of the fibres can only be properly seen in cross
sections ; and these are best made by either partially drying, or by
freezing a piece of muscle, so that very thin slices can be cut with
a sharp instrument, which, on being moistened again, will resume
in great part their original characters. — Striated fibres, separable
with great facility into their component fibrillaB, are readily obtain-
able from the limbs of Crustacea and of Insects ;* and their presence
is also readily distinguishable in the bodies of "Worms, even of very
low organization ; so that it may be regarded as characteristic of the
Articulated series generally. On the other hand, the Molluscous
classes are for the most part distinguished by the non-striation of
their fibre; there are, however, two remarkable exceptions, strongly
striated fibre having been found in the Terebratula and other
Brachiopods (where, however, it is limited to the adductor muscles
of the shell), and also in many Polyzoa. Its presence seems related
to energy and rapidity of movement ; the non-striated presenting
itself where the movements are slower and feebler in their character.
640. The ' smooth' or non-striated form of Muscular fibre, which
is especially found in the walls of the stomach, intestines, bladder,
and other similar parts, is composed of flattened bands whose
diameter is usually between 1 -2000th and I-3000th of an inch;
and these bands are collected into fasciculi, which do not lie parallel
with each other, but cross and interlace. By macerating a portion
of such muscular substance, however, in dilate nitric acid (about
one part of ordinary acid to three parts of water) for two or three
days, it is found that the bands just mentioned may be easily
separated into elongated fusiform cells, not unlike ' woody fibre' in
shape ; each distinguished, for the most part, by the presence of a
long staff-shaped nucleus, brought into view by the action of
acetic acid. These cells, in which the distinction between cell- wall
and cell-contents can by no means be clearly seen, are composed of
a soft yellow substance often containing small pale granules, and
* The careful study of the structure of the muscular tissue of Dytiscics,
recently prosecuted by Mr. E. A. Schafer, has led him to a view of its nature
very different from that above given. He considers that the fibre is made up
of a homogeneous ' ground-substance,' in which are imbedded parallel series of
'muscle-rods' arranged longitudinally; the enlarged ends of which give the
appearance of transverse lines of dots, and produce by diffraction a relatively-
bright appearance in their immediate neighbourhood, thus giving rise to the
bright bauds. (See his Memoir in " Phil. Trans.," 1873.)
3 D
770
VERTEBEATED ANIMALS.
Fig. 419.
sometimes yellow globules of fatty matter. In the coats of the
blood-vessels are found cells having the
same general characters, but shorter
and wider in form ; and although some
of these approach very closely in their
general appearance to epithelium-cells,
yet they seem to have quite a different
nature, being distinguished by their
elongated nuclei, as well as by their
contractile endowments.
641. Nerve-substance. — "Wherever a
distinct Nervous System can be made
out, it is found to consist of two very
different forms of tissue ; namely, the
cellular, which are the essential com-
ponents of the ganglionic centres, and
the fibrous, of which the connecting
trunks consist. The typical form of
the nerve-cells or ' ganglion- globules '
highly magnified ; c, a similar may be regarded as globular ; but they
cell treated with acetic acid. often present an extension into one or
more long processes, which give them a
' caudate ' or a ' stellate ' aspect. These processes have been traced
into continuity, in some instances, with the axis -cylinders of nerve-
tubes (Fig. 420); whilst in other cases they seem to inosculate
The cells, which do not seem to pos-
sess a definite cell-wall, are for the
most part composed of a finely-
granular substance, which extend.3
into its prolongations ; and in the
midst of this is usually to be seen a
large well-defined nucleus. They
also generally contain pigment-
granules, which give them a reddish
or yellowish-brown colour, and thus
impart to collections of ganglionic
cells in the warm-blooded Yerte-
brata that peculiar hue, which
causes it to be known as the cine-
ritious or grey matter; they are
commonly absent, however, among
the lower animals. — Each of the
Nerve-tubes, on the other hand, of
which the trunks are composed,
consists, in its most completely-de-
veloped form, of a delicate mem-
branous sheath, within which is a
hollow cylinder of a material known
as the 'white substance of Schwann,'
Structure of non-striated Mus^
cular Fibre : — A, portion of tis
sue showing fusiform cells a, a
with elongated nuclei b, b ; — B
a single cell isolated and more
with those of other vesicles.
Fig. 420.
Ganglion cells and Nerve-fibres,
from a ganglion of Lamprey.
whose outer and inner boundaries are marked out by two distinct
NEEYE-FIBEES. 771
lines, giving to each margin of the nerve-tube what is described as a
' double contour.' The contents of the membranous envelope are very
soft, yielding to slight pressure ; and they are so quickly altered by
the contact of water or of any liquids which are foreign to their
nature, that their characters can only be properly judged-of when
they are quite fresh. The centre or axis of the tube is then found
to be occupied by a transparent substance which is known as the
' axis-cylinder :' and there is reason to believe that this last, which
is a protoplasmic substance, is the essential component of the
nerve-fibre, and that the hollow cylinder which surrounds it, and
which is composed of a combination of fat and albuminous matter,
serves, like the tubular sheath, for the insulation which is essential
to its functional action. For every nerve-fibre, like the individual
wires bound up in the suspended cords of the District Telegraph,
establishes a distinct communication between two remote points, —
as, in the case of a nerve of common sensation, between a certain
spot of the skin and a certain point of the central sensorium ; or,
in the case of a motor nerve, between a certain point of the motor
nerve-centre, and a certain muscular fasciculus. And it is in
virtue of the insulation of the nerve-fibres (as of the telegraphic
wires) from one another, that each does its own work without dis-
turbance from the rest. But in some of the lower tribes of
animals, whose parts are mere repetitions of each other, and all
whose movements are of the same kind, it seems that the nerve-
trunks consist of ztwinsulated fibrils. Thus the Author has
found in each of the arms of Comatula (Fig. 324) a trunk
sending off pairs of branches to the successive pairs of muscles
by the contraction of which the arm is coiled-up ; and the
fibrils into which this trunk can be torn longitudinally are
not separated by any intermediate substance, and show no de-
finite structure. When the central organ is irritated, from which
all the trunks radiate, all the arms are immediately coiled up into
spirals by the contraction of their muscles ; and when by the with-
drawal of the irritation the muscles relax, the arms are straightened
out again by the elasticity of the ligaments which connect their
successive segments. — Even in the highest animals, there are nerve -
fibres which do not show the complete structure of the proper
' nerve-tubes.' These, which are known as ' gelatinous,' are consi-
derably smaller than the preceding, and do not exhibit any differen-
tiation of parts (Fig. 421). They are flattened, soft, and homo-
geneous in their appearance, and contain numerous nuclear
particles which are brought into view by acetic acid. They can
sometimes be seen to be continuous with the axis -cylinders of the
ordinary fibres, and also with the radiating prolongations of the
ganglion-cells ; so that their nervous character, which has been
questioned by some anatomists, seems established beyond doubt.
642. The ultimate distribution of the Nerve-fibres is a subject on
which there has been great divergence of opinion, and which can
only be successfully investigated by observers of great experience.
3d2
VEETEBEATED ANIMALS.
Fig. 421.
. m
Gelatinous Nerve-fibres, from
Olfactory Nerve.
The Author believes that it may be stated as a general fact, that in
both the motor and the sensory nerve-tubes, as they approach their
terminations in the muscles and in the
skin respectively, the protoplasmic axis-
cylinder is continued beyond its enve-
lopes; often then breaking-up into very
minute fibrillae, which inosculate with
each other so as to form a network closely
resembling that formed by the pseudopo-
dial threads of Wvizopods (Fig. 250).
Recent observers have described the
fibrillas of motor nerves as terminating in
' motorial end-plates ' seated upon or in the
muscular fibres ; and these seem analo-
gous to the little 'islets' of sarcodic
substance, into which those threads often
dilate. — Where the Skin is specially en-
dowed with tactile sensibility, we find a
special papillary apparatus, which in the
skin may be readily made out in thin ver-
tical sections treated with solution of
soda (Fig. 422). It was formerly sup-
posed that all the cutaneous papillae are
furnished with nerve-fibres, and minister to sensation : but is now
known that a large proportion (at any rate) of those furnished with
loops of blood-vessels (Figs. 408p, 428), being destitute of nerve
fibres, must have for their special office the production of the
Epidermis ; whilst
those which, possess-
ing nerve-fibres, have
sensory functions, are
usually destitute of
blood-vessels. The
greater part of the
interior of each sen-
sory papilla (Fig.
422, c, c) of the skin
is occupied by a pe-
culiar ' axile body,'
which seems to be
merely a bundle of
ordinary connective
tissue, whereon the
nerve-fibre appears
to terminate. The
nerve-fibres are more
readily seen, however,
in the ' fungiform '
papillae of theTongue,
Fig. 422.
jSsA, i^kMMb%
mi
Vertical Section of the Skin of the Finger, show-
ng the branches of the cutaneous nerves, a, 6, inos-
culating to form a plexus, of which the ultimate
fibres pass into the cutaneous papillas, c, c.
EXAMINATION OF NERVE-SUBSTANCE. 773
to each of winch several of them proceed ; these bodies, which are
very transparent, may be well seen by snipping-off minute portions
of the tongue of the Frog ; or by snipping- off the papillae them-
selves from the surface of the living Human tongue, which can be
readily done by a dexterous use of the curved scissors, with no
more pain than the prick of a pin would give. The transparence
of these papillas also is increased by treating them with a weak
solution of soda. — Nerve-fibres have also been found to terminate
on sensory surfaces in minute ' end-bulbs' of spheroidal shape and
about 1 -600th of an inch in diameter ; each of them being com-
posed of a simple outer capsule of connective tissue, filled with
clear soft matter, in the midst of which the nerve-fibre, after losing
its dark border, ends in a knob. The ' Pacinian corpuscles,' which
are best seen in the mesentery of the Cat, and are from 1-loth to
l-10th of an inch long, seem to be more developed forms of these
' end-bulbs.'
643. For the sake of obtaining a general acquaintance with the
Microscopic characters of these principal forms of Nerve-substance,
it is best to have recourse to minute nerves and ganglia. The
small nerves which are found between the skin and the muscles of
the back of the Frog, and which become apparent when the former
is being stripped-off, are extremely suitable for this purpose ; but
they are best seen in the Hyla or ' tree-frog,' which is recom-
mended by Dr. Beale as being much superior to the common Frog
for the general purposes of minute histological investigation. If
it be wished to examine the natural appearance of the nerve-fibres,
no other fluid should be used than a little blood- serum ; but if
they be treated with strong acetic acid, a contraction of their tubes
takes place, by which the axis-cylinder is forced-out from their
cut extremities, so as to be made more apparent than it can be in
any other way. On the other hand, by immersion of the tissue in
a dilute solution of Chromic acid (about one part of the solid
crystals to two hundred of water), the nerve-fibres are rendered
firmer and more distinct. Again, the axis-cylinders are brought
into distinct view by the Staining-process (§ 161), being dyed much
more quickly than their envelopes ; and they may thus be readily
made-out by reflected light, in transverse sections of nerves that
have been thus treated. The gelatinous fibres are found in the
greatest abundance in the Sympathetic nerves ; and their characters
may be best studied in the smaller branches of that system. — So,
for the examination of the ganglionic cells, and of their relation to
the nerve-tubes, it is better to take some minute ganglion as a
whole (such as one of the sympathetic ganglia of the Frog, Mouse,
or other small animal), than to dissect the larger ganglionic masses,
whose structure can only be successfully studied by such as are
proficient in this kind of investigation. The nerves of the orbit of
the eyes of Fishes, with the ophthalmic ganglion and its branches,
which may be very readily got-at in the Skate, and of which
the components may be separated without much difficulty, form
774 VEETEBKATED ANIMALS.
one of the most convenient objects for the demonstration of the
principal forms of nerve-tissue, and especially for the connec-
tion of nerve-fibres and ganglion -cells. — For minute inquiries,
however, into the ultimate distribution of the nerve-fibres in
Muscles and Sense-organs, certain special methods must be fol-
lowed, and very high magnifying powers must be employed. Those
who desire to follow out this inquiry should acquaint themselves
with the methods which have been found most successful in the
hands of the able Histologists whose works have been already
referred to.
644. Circulation of the Blood. — One of the most interesting
spectacles that the Microscopist can enjoy, is that which is fur-
nished by the Circulation of the Blood in the capillary blood-
vessels, which distribute the fluid through the tissues it nourishes.
This, of course, can only be observed in such parts of Animal
bodies as are sufficiently thin and transparent to allow of the trans-
mission of light through them, without any disturbance of their
ordinary structure ; and the number of these is very limited. The
web of the Frog's foot is perhaps the most suitable for ordinary
purposes, more especially since this animal is to be easily obtained
in almost every locality ; and the following is the arrangement
which the Author has found most convenient for the purpose. A
piece of thin Cork is to be obtained, about 9 inches long and 3
inches wide (such pieces are prepared by Cork-cutters, as soles),
and a hole about 3-8ths of an inch in diameter is to be cut at about
the middle of its length, in such a position that, when the cork is
secured upon the stage, this aperture may correspond with the axis
of the Microscope. The body of the Frog is then to be folded in a
pieca of wet calico, one leg being left free, in such a manner as to
confine its movements, but not to press too tightly upon its body ;
and being then laid down near one end of the cork plate, the free
leg is to be extended, so that the foot can be laid over the central
aperture. The spreading-out of the foot over the aperture is to be
accomplished, either by passing pins through the edge of the web
into the cork beneath, or by tying the ends of the toes with threads
to pins stuck into the cork at a small distance from the aperture ;
the former method is by far the least troublesome, and it may be
doubted whether it is really the source of more suffering to the
animal than the latter, the confinement being obviously that which
is most felt. A few turns of tape, carried loosely around the
calico bag, the projecting leg, and the cork, serve to prevent any
sudden start ; and when all is secure, the cork-plate is to be laid
down upon the stage of the Microscope, where a few more turns of
the tape will serve to keep it in place. The web being moistened
with water (a precaution which should be repeated as often as the
membrane exhibits the least appearance of dryness), and an
adequate light being reflected through the web from the mirror,
this wonderful spectacle is brought into view on the adjustment of
the focus (a power of from 75 to 100 diameters being the most
CAPILLAEY CIRCULATION IN LIVING FROG.
775
suitable for ordinary purposes), provided that no obstacle to the
movement of the blood be produced by undue pressure upon the
body or leg of the animal. It will not unfrequently be found,
however, that the current of blood is nearly or altogether stagnant
for a time ; this seems occasionally due to the animal's alarm at
its new position, which weakens or suspends the action of its heart,
the movement recommencing again after the lapse of a few minutes,
although no change has been made in any of the external con-
ditions. But if the movement should not renew itself, the tape
Fig. 423.
Capillary Circulation in a portion of the web of a Frog's foot
vein; 6, b, its branches; c, c, pigment-cells.
trunk of
which passes over the body should be slackened; and if this does
not produce the desired effect, the calico envelope also must be
loosened. When everything has once been properly adjusted, the
animal will often lie for hours without moving, or will only give an
occasional twitch. Even this may be avoided by previously sub-
jecting the animal to the influence of chloroform, which may be
renewed from time to time whilst it is under observation. — The
movement of the Blood will be distinctly seen by that of its
corpuscles (Fig. 423), which course after one another through the
776 VEETEBEATED ANIMALS.
network of Capillaries that intervenes between the smallest arteries
and the smallest veins : in those tubes which pass most directly
from the veins to the arteries, the current is always in the same
direction ; but in those which pass-across between these, it may not
unfrequently be seen that the direction of the movement changes
from time to time. The larger vessels with which the capillaries
are seen to be connected, are almost always veins, as may be
known from the direction of the flow of blood in them from the
branches (b, b) towards their trunks (a)-, the arteries, whose
ultimate subdivisions discharge themselves into the capillary net-
work, are for the most part restricted to the immediate borders of
the toes. When a power of 200 or 250 diameters is employed, the
visible area is of course greatly reduced ; but the individual vessels
and their contents are much more plainly seen ; and it may then be
observed that whilst the 'red' corpuscles (§ 625) flow at a very
rapid rate along the centre of each tube, the 'white' corpuscles
(§ 626) which are occasionally discernible, move slowly in the clear
stream near its margin.
645. The Circulation may also be displayed in the tongue of the
Frog, by laying the animal (previously chloroformed) on its back,
with its head close to the hole in the cork-plate, and, after securing
the body in this position, drawing-out the tongue with the forceps,
and fixing it on the other side of the hole with pins. So, again,
the circulation may be examined in the lungs — where it affords a
spectacle of singular beauty — or in the mesentery of the living
Frog, by laying open its body, and drawing forth either organ ; the
animal having previously been made insensible by chloroform. The
tadpole of the Frog, when sufficiently young, furnishes a good dis-
play of the capillary circulation in its tail ; and the difficulty of keep-
ing it quiet during the observation may be overcome by gradually
mixing some warm water with that in which it is swimming, until it
becomes motionless ; this usually happens when it has been raised to
a temperature between 100° and 1 10° ; and notwithstanding that the
muscles of the body are thrown into a state of spasmodic rigidity by
this treatment, the heart continues to pulsate, and the circulation is
maintained.* — The larva of the Water-nevjt, when it can be
obtained, furnishes a most beautiful display of the circulation,
both in its external gills and in its delicate feet. It may be
enclosed in a large Aquatic-box or in a shallow cell, gentle pressure
being made upon its body so as to impede its movements without
stopping the heart's action. — The circulation may also be seen in
the tails of small Fish, such as the minnow or the stichleback, by
confining these animals in tubes, or in shallow cells, or in a large
Aquatic-box ;f but although the extreme transparence of these
* A special form of Live-box for the observation of living Tadpoles, &c,
contrived by F. E. Schultze, of Eostock, is described and figured in the " Quart.
Journ. of Microsc. Science," N.S., Vol. vii. (1867), p. 261.
f A convenient Trough for this purpose is described in the " Quart. Journ.
of Microsc. Science," Vol. vii. (1859), p. llo.
CIECULATION IN FISH AND TADPOLE. 777
parts adapts them well for this purpose in one respect, yet the
comparative scantiness of their blood-vessels prevents them from
being as suitable as the Frog's web in another not less important
particular. — One of the most beautiful of all displays of the cir-
culation, however, is that which may be seen upon the yolk-bag of
young Fish (such as the Salmon or Trout) soon after they have been
hatched ; and .as it is their habit to remain almost entirely motion-
less at this stage of their existence, the observation can be made
with the greatest facility by means of the Zoophyte-trough, provided
that the subject of it can be obtained. Now that the artificial
breeding of these Fish is largely practised for the sake of stocking
rivers and fish-ponds, there can seldom be much difficulty in pro-
curing specimens at the proper period. The store of yolk which
the yolk-bag supplies for the nutrition of the embryo, not being
exhausted in the Fish (as it is in the Bird), previously to the
hatching of the egg, this bag hangs-down from the belly of the
little creature on its emersion ; and continues to do so until its
contents have been absorbed into the body, which does not happen
for some little time afterwards. And the blood is distributed over
it in copious streams, partly that it may draw into itself fresh
nutritive material, and partly that it may be subjected to the
aerating influence of the surrounding water.
646. The Tadpole serves, moreover, for the display, under proper
management, not only of the capillary but of the general Circula-
tion ; aud if this be studied under the Binocular Microscope, the
observer not only enjoys the gratification of witnessing a most
wonderful spectacle, but may also obtain a more accurate notion
of the relations of the different parts of the circulating system
than was previously possible.* The Tadpole, as every Naturalist is
aware, is essentially a Fish in the early period of its existence,
breathing by gills alone, and having its circulating apparatus
arranged accordingly : but as its limbs are developed and its tail
becomes relatively shortened, its lungs are gradually evolved in
preparation for its terrestrial life, and the course of the blood is
considerably changed. In the tadpole as it comes forth from the
egg, the gills are external, forming a pair of fringes hanging at the
sides of the head (Plate XXIV., fig. I) ; and at the bases of these,
concealed by opercula or gill-flaps resembling those of Fishes, are
seen the rudiments of the internal gills, which soon begin to be
developed in the stead of the preceding. The external gills reach
their highest development on the fourth or fifth day after emersion ;
and they then wither so rapidly, whilst at the same time being drawn-
in by the growth of the animal, that by the end of the first week
only a remnant of the right gill can be seen under the edge of the
* See Mr. TVhitney's account of 'The Circulation in the Tadpole,' in
" Transact, of Microsc. Soc," N.S., Vol. x (1862), p. 1, and his subsequent
paper 'On the Changes which accompany the Metamorphosis of the Tadpole'
in the same Transactions, Vol. xv. p. 43. — In the first of these Memoirs Mr. W.
described the internal gills as lungs, an error which he corrected in the second.
778 VEETEBEATED ANIMALS.
operculum (fig. 2, c), though the left gill (b) is somewhat later in
its disappearance. Concurrently with this change, the internal gills
are undergoing rapid development ; and the beautiful arrangement
of their vascular tufts, which originate from the roots of the
arteries of the external gills, as seen at g, fig. 5, is shown in fig. 4.
It is requisite that the tadpole subjected to observation should not
be so far advanced as to have lost its early transparence of skin ;
and it is further essential to the tracing-out the course of the
abdominal vessels, that the creature should have been kept without
food for some days, so that the intestine may empty itself. This
starving process reduces the quantity of red corpuscles, and thus
renders the blood paler ; but this, although it makes the smaller
branches less obvious, brings the circulation in the larger trunks
into more distinct view. " Placing the tadpole on his back," says
Mr. Whitney, " we look, as through a pane of glass, into the chamber
of the chest. Before us is the beating heart, a bulbous-looking
cavity, formed of the most delicate transparent tissues, through
which are seen the globules of the blood, perpetually, but alternately,
entering by one orifice and leaving it by another. The heart,
(Plate XXIV., fig. 3 a) appears to be slung, as it were, between two
arms or branches, extending right and left. From these trunks (b)
the main arteries arise. The heart is enclosed within an envelope
or pericardium (c), which is, perhaps, the most delicate, and is,
certainly, the most elegant beauty in the creature's organism.
Its extreme fineness makes it often elude the eye under the single
Microscope, but under the Binocular its form is distinctly revealed.
Then it is seen as a canopy or tent, enclosing the heart, but of such
extreme tenuity that its folds are really the means by which its
existence is recognized. Passing along the course of the great
vessels to the right and left of the heart, the eye is arrested by a
large oval body (d) of a more complicated structure and dazzling
appearance. This is the internal gill, which, in the tadpole, is a
cavity formed of most delicate transparent tissue, traversed by
certain arteries, and lined by a crimson network of blood-vessels,
the interlacing of which, with their rapid currents and dancing
globules, forms one of the most beautiful and dazzling exhibitions
of vitality." Of the three great arterial trunks which arise on
each side from the truncus arteriosus, b, the first or cephalic, e, is
distributed entirely to the head, running first along the upper edge
of the gill, and giving off a branch, /, to the thick fringed lip which
surrounds the mouth, after which it suddenly curves upwards and
backwards, so as to reach the upper surface of the head, where
it dips between the eye and the brain. The second main trunk, h,
seems to be chiefly distributed to the gill, although it freely com-
municates by a network of vessels both with the first or cephalic
and with the third or abdominal trunk. The latter also enters the
gill and gives off branches ; but it continues its course as a large
trunk, bending downwards and curving towards the spine, where it
meets its fellow to form the abdominal aorta, i, which, after
PLATE XXIV.
tax
--- "
^
<\
.£.-
Cieculatiox iif Tadpole.
[To face p. 778.
GENEEAL CIRCULATION IN TADPOLE. 779
giving-off branches to the abdominal viscera, is continued as the
caudal artery, h, to the extremity of the tail. The blood is
returned from the tail by the caudal vein, I, which is gradually
increased in size by its successive tributaries as it passes towards
the abdominal cavity ; here it approaches the kidney, m, and sends
off a branch which encloses that organ on one side, while the main
trunk continues its course on the other, receiving tributaries from
the kidney as it passes. (This supply of the kidney by venous
blood is a peculiarity of the lower Vertebrata.) The venous blood
returned from the abdominal viscera, on the other hand, is collected
into a trunk, _p, known as the portal vein, which distributes it
through the substance of the liver, o, as in Man ; and after
traversing that organ it is discharged by numerous fine channels,
which converge towards the great abdominal trunk, or vena cava, n,
as it passes in close proximity to the liver, onwards to the sinus
venosus, q, or rudimentary auricle of the heart. This also receives
the jugular vein, r, from the head, which first, however, passes
downwards in front of the gill close to its inner edge, and meets a
vein, t, coming up from the abdomen, after which it turns abruptly
in the direction of the heart. Two other abdominal veins, u, meet
and pour their blood direct into the sinus venosus ; and into this
cavity also is poured the aerated blood returned from the gill by the
branchial vein, v, of which only the one on the right side can be
distinguished. — The lungs may be detected in a rudimentary state,
even in the very young tadpole ; being in that stage a pair of
minute tubular sacs, united at their upper extremities, and lying
behind the intestine and close to the spine. They may be best
brought into view by immersing the tadpole for a few days in
a weak solution of chromic acid, which renders the tissues friable,
so that the parts that conceal them may be more readily peeled
away. Their gradual enlargement may be traced during the period
of the tadpole's transparence ; but they can only be brought into
view by dissection, when the metamorphosis has been completed.
The following are Mr. Whitney's directions for displaying the
Circulation iu these organs :-—" Put the young Frog into a wine-
glass, and drop on him a single drop of chloroform. This suffices
to extinguish sensibility. Then lay him on the back on a piece of
cork, and fix him with small pins passed through the web of each
foot. Ilemove the skin of the abdomen with a fine pair of sharp
scissors and forceps. Turn aside the intestines from the left side,
and thus expose the left lung, which may now be seen as a glisten-
ing transparent sac, containing air-bubbles. With a fine camel-
hair pencil the lung may now be turned-out, so as to enable the
operator to see a large part of it by transmitted light. Unpin the
frog, and place him on a slip of glass, and then transmit the light
through the everted portion of lung. Remember that the lung is
very elastic, and is emptied and collapsed by very slight pressure.
Therefore, to succeed with this experiment, the lung should be
touched as little as possible, and in the lightest manner, with the
780 VEETEBEATED ANIMALS.
brush. If the heart is acting feebly, you will see simply a trans-
parent sac, shaped according to the quantity of air-bubbles it may
happen to contain, but void of red vascularity and circulation.
But should the operator succeed in getting the lung well placed,
full of air, and have the heart still beating vigorously, he will see
before him a brilliant picture of crimson network, alive with the
dance and dazzle of blood-globules, in rapid chase of one another
through the delicate and living lace-work which lines the chamber
of the lung." The position of the lungs in relation to the
heart and the great vascular trunks, is shown in Plate XXIV.,
fig. 6.
647. Injected Preparations. — Next to the Circulation of the
Blood in the living body, the varied distribution of the Capillaries
in its several organs, as shown by means of ' injections' of colouring
matter thrown into their principal vessels, is one of the most interest-
ing subjects of Microscopic examination. The art of making
successful preparations of this kind is one in which perfection can
usually be attained only by long practice, and by attention to a
great number of minute particulars ; and better specimens may be
obtained, therefore, from those who have made it a business to pre-
pare them, than are likely to be prepared by amateurs for them-
selves. For this reason, no more than a general account of the
process will be here offered ; the minute details which need to be
attended-to, in order to attain successful results, being readily
accessible elsewhere to such as desire to put it in practice.*
Injections may be either opaque or transparent, each method
having its special advantages. The former is most suitable where
solid form and inequalities of surface are specially to be displayed,
as in Figs. 424 and 430 ; the latter is preferable where the injected
tissue is so thin as to be transparent (as in the case of the retina
and other membranes of the eye), or where the distribution of its
blood-vessels and their relations to other parts may be displayed
by sections thin enough to be made transparent by mounting either
in Canada balsam or in Glycerine medium (Plate XXY.). — The
injection is usually thrown into the vessels by means of a brass
syringe expressly constructed for the purpose, which has several
jet-pipes of different sizes, adapted to the different dimensions of
the vessels to be injected ; and these should either be furnished
with a stopcock to prevent the return of the injection when the
syringe is withdrawn, or a set of small corks of different sizes
should be kept in readiness, with which they may be plugged.
The pipe should be inserted into the cut end of the trunk which is
to be injected, and should be tied therein by a silk thread. In
injecting the vessels of Fish, Mollusks, &c, the softness of the
* See especially the article 'Injection,' in the "Micrographic Dictionary;"
M. Eobin's work, "Du Microscope et des Injections ;" Prof. H. Frey's Treatise
"Das Mikroskop nnd die Mikroskopische Technik;" Dr. Beale's "How to
Work with the Microscope;" and the "Handbook to the Physiological Labora-
tory."
INJECTION OF CAPILLAEY BLOOD-VESSELS. 781
vessels renders them liable to break in the attempt to tie them ;
and it is therefore better for the operator to satisfy himself with
introducing a pipe as large as he can insert, and with passing it
into the vessel as far as he can without violence. All the vessels
from which the injection might escape should be tied, and some-
times it is better to put a ligature round a part of the organ or
tissue itself ; thus, for example, when a portion of the Intestinal
tube is to be injected through its branch of the Mesenteric artery,
not only should ligatures be put round any divided vessels of the
mesentery, but the cut ends of the intestinal tube should be firmly
tied. — For making those minute injections, however, which are
needed for the purposes of anatomical investigation, rather than to
furnish ' preparations' to be looked-at, the Author has found the
glass-syringe (Fig. 96), so frequently alluded-to, the most efficient
instrument ; since the Microscopist cpn himself draw its point to the
utmost fineness that will admit of the passage of the injection, and
can push this point without ligature, under the Simple Microscope,
into the narrowest orifice, or into the substance of the part into
which the injection is to be thrown. — Save in the cases in which the
operation has to be practised on living animals, it should either be
performed when the body or organ is as fresh as possible, or after
the expiry of sufficient time to allow the rigor mortis to pass-off,
the presence of this being very inimical to the success of the injec-
tion. The part should be thoroughly warmed, by soaking in warm
water for a time proportionate to its bulk ; and the injection, the
syringe, and the pipes should also have been subjected to a tem-
perature sufficiently high to ensure the free flow of the liquid. The
force used in pressing-down the piston should be very moderate at
first ; but should be gradually increased as the vessels become filled,
and it is better to keep-up a steady pressure for some time, than to
attempt to distend them by a more powerful pressure, which will
be certain to cause extravasation. This pressure should be main-
tained* until the injection begins to flow from the large veins, and
the tissue is thoroughly reddened ; and if one syringeful of injection
after another be required for this purpose, the return of the injec-
tion should be prevented by stopping the nozzle of the jet-pipe when
the syringe is removed for re-filling. When the injection has been
completed, any openings by which it can escape should be secured,
and the preparation should then be placed for some hours in cold
water, for the sake of causing the size to ' set.'f
* Simple mechanical arrangements for this purpose, by which the fatigtie
of maintaining this pressure with his hand is saved to the operator, are described
in the " Micrographic Dictionary."
f The Kidney of a Sheep or Pig is a very advantageous organ for the learner
to practise on ; and he should first master the filling of the vessels from the
arterial trunk alone, and then, when he has succeeded in this, he should fill the
tubuli urinifeii with white injection, before sending coloured injection into the
renal artery. The entire systemic circulation of small animals, as Mice, Eats,
Frogs, &c, may be injected from the aorta; and the pulmonary vessels from
the pulmonary artery.
782 VEETEBEATED ANIMALS.
648. For opaque injections, the best colouring-matter, when only
one set of vessels is to be injected, is Chinese vermilion. This,
however, as commonly sold, contains numerous particles of far too
large a size ; and it is necessary first to reduce it to a greater fine-
ness by continued trituration in a mortar (an agate or a steel
mortar is the best) with a small quantity of water, and then to get
rid of the larger particles by a process of ' levigation,' exactly cor-
responding to that by which the particles of coarse sand, &c, are
separated from the Diatomaceas (§ 261). The fine powder thus
obtained, ought not, when examined under a magnifying power of
200 diameters, to exhibit particles of any appreciable dimensions.
The size or gelatine should be of a fine and pure quality, and
should be of sufficient strength to form a tolerably firm jelly when
cold, whilst quite limpid when warm. It should be strained whilst
hot, through a piece of new flannel ; and great care should be
taken to preserve it free from dust, which may best be done by
putting it into clean jars, and covering its surface with a thin
layer of alcohol. The proportion of levigated vermilion to be
mixed with it for injection, is about 2 oz. to a pint ; and this is to
be stirred in the melted size, until the two are thoroughly incor-
porated, after which the mixture should be strained through muslin.
— Although no injections look so well by reflected light as those
which are made with vermilion, yet other colouring substances may
be advantageously employed for particular purposes. Thus a
bright yellow is given by the yellow chromate of lead, which is
precipitated when a solution of acetate of lead is mixed with a
solution of chromate of potass ; this is an extremely fine powder,
which 'runs' with great facility in an injection, and has the
advantage of being very cheaply prepared. The best method of
obtaining it is to dissolve 200 grains of acetate of lead and 105
grains of chromate of potass in separate quantities of water, to mix
these, and then, after the subsidence of the precipitate, to pour-ofF
the supernatant fluid so as to get-rid of the acetate of potash
which it contains, since this is apt to corrode the walls of the
vessels if the preparation be kept moist. The solutions should be
mixed cold, and the precipitate should not be allowed to dry before
being incorporated with the size, four ounces of which will be the
proportion appropriate to the quantity of the colouring-substance
produced by the above process. The same materials may be used
in such a manner that the decomposition takes-place within the
vessels themselves, one of the solutions being thrown-in first, and
then the other; and this process involves so little trouble or
expense, that it may be considered the best for those who are
novices in the operation, and who are desirous of perfecting them-
selves in the practice of the easier methods, before attempting the
more costly. By M. Doyere, who first devised this method, it was
simjxfy recommended to throw-in saturated solutions of the two
salts, one after the other ; but Dr. Goadby, who had much ex-
perience in the use of it, advised that gelatine should be employed,
INJECTION OF CAPILLAEY BLOOD-VESSELS.
•83
in the proportion of 2 oz. dissolved in 8 oz. of water, to 8 oz. of the
saturated solutions of each salt. This method answers very well
for the preparations that are to be mounted dry ; "but for such as
are to be preserved in fluid, it is subject to the disadvantage of
retaining in the vessels the solution of acetate of potash, which
exerts a gradual corrosive action upon them. Dr. G-oadby has
met this objection, however, by suggesting the substitution of
nitrate for acetate of lead ; the resulting nitrate of potash having
rather a preservative than a corrosive action on the vessels. — When
it is desired to inject two or more sets of vessels (as the arteries,
veins, and gland ducts) of the same preparation, different colouring
substances should be employed. For a ivhite injection, the carbo-
nate of lead (prepared by mixing solutions of acetate of lead and
carbonate of soda, and pouring-off the supernatant liquid when the
precipitate has fallen) is the best material. ISTo blue injections can
be much recommended, as they do not reflect light well, so that the
vessels filled with them seem almost black; the best is freshly
precipitated prussian blue (formed by mixing solutions of persul-
phate of iron and ferrocyanide of potassium), which, to avoid the
alteration of its colour by the free alkali of the blood, should be
triturated with its own weight of oxalic acid and a little water, and
the mixture should then be combined with size, in the proportion of
146 grains of the former to 4 oz. of the latter.
649. Opaque injections may be preserved either dry or in fluid.
The former method is well suited to sections of many solid organs,
in which the disposition of the vessels does not sustain much altera-
tion by drying ; for the co-
Fig. 424.
lours of the vessels are dis-
played with greater brilliancy
than by any other method,
when such slices, after being
well dried, are moistened with
turpentine and mounted in
Canada balsam. But for such
an injection as that shown in
Fig. 424, in which the form and
disposition of the intestinal
villi would be completely al-
tered by drying, it is indispen-
sable that the preparation
should be mounted in fluid, in
a cell deep enough to prevent
any pressure on its surface.
Either Goadby's solution or
weak Spirit answers the pur-
pose very well ; or by careful
management even such maybe mounted in Canada balsam or Gum
Damar (§ 176, 179).
660. Within the last few years, the art of making transparent
Villi of Small Intestine of Monkey.
784 VEETEBKATED ANIMALS.
Injections has "been much cultivated, especially in Germany ; and
beautiful preparations of this description have been sent over from
that country in large numbers. The colouring-matter chiefly
employed is carmine, which is dissolved in liquid ammonia; the
solution (after careful filtration) being added in the requisite amount
to liquid gelatine. The following is given by Dr. Carter as a
formula for a carmine injection which will run freely through the
most minute capillaries, and which will not tint the tissues beyond
the vessels themselves, a point of much importance : — Dissolve 60
grains of pure carmine in 120 grains of strong liquor ammonias
(Pharm. Brit.), and filter if necessary; with this mix thoroughly
I-! oz. of a hot solution of gelatine (1 to 6 of water) ; mix another
\ oz. of the gelatine solution with 86 minims of glacial acetic acid;
and drop this, little by little, into the solution of carmine, stirring
briskly the whole time. After the part has beeu injected, and has
been hardened either by partial drying or by immersion in the
Chromic acid solution or in Alcohol, thin sections are cut with a
sharp razor ; and these are usually dried and mounted in Canada
balsam. Many of these transparent injections (Plate XXY.) are
peculiarly well seen under the Binocular Microscope, which shows
the capillary network not only in two dimensions (length and
breadth), but also in its third dimension, that of its thickness;
this is especially interesting in such injections as that (Pig. 1)
of the villi of the Intestine (seen in situ in a transverse section of
its tube), a thin section of the Mouse's toe (Fig. 2), or the convo-
luted layer of the Brain (Fig. 3). The Stereoscopic effect is best
seen, if the light reflected through the object be moderated by a
ground-glass or even by a piece of tissue-paper placed behind it. —
This method, however, does not serve to display anything well, save
the distribution of the Capillary vessels ; the structures they traverse
being imperfectly shown. For the purpose of scientific research,
therefore, the method followed by Dr. Beale (for fall details of which
the reader is referred to his Treatise) is much to be preferred. The
material recommended by him for the finest injections is prepared as
follows : — Mix 10 drops of the tincture of perchloride of iron (Pharm.
Brit.) with 1 oz. of glycerine ; and mix 3 grains of ferrocyanide of
potassium, previously dissolved in a little water, with another 1 oz.
of glycerine. Add the first solution very gradually to the second,
shaking them well together; and lastly, add loz. of water, and 3 drops
of strong hydrochloric acid. This ' prussian blue fluid' though not a
solution, deposits very little sediment by keeping ; and it appears
like a solution even when examined under high magnifying powers,
in consequence of the minuteness of the particles of the colouring
matter. Where a second colour is required, a carmine injection
may be used, which is to be prepared as follows : — Mix 5 grains of
carmine with a few drops of water, and, when they are well in-
corporated, add about 5 drops of strong liquor ammonias. To this
dark red solution add about \ oz. of glycerine, shaking the bottle so
as to mix the two fluids thoroughly ; and then very gradually pour
PLATE XXV.
Distribution of Capillabies.
{To face p. 784.
INJECTION OF CAPILLARY BLOOD-VESSELS. 785
in another \ oz. of glycerine acidulated with 8 or 10 drops of acetic
or hydrochloric acid, frequently shaking the bottle. Test the
mixture with blue litmus paper; and mix with it another \ oz. of
glycerine, to which a few drops more acid should be added, if the
acid reaction of the liquid should not have previously been decided.
Finally, add gradually 2 drachms of alcohol previously well mixed
with 6 drachms of water, and incorporate the whole by thorough
shaking after the addition of each successive portion. — The staining
process (§ 161) may be combined with the injecting ; but Dr. Beale
has now come to prefer the following method, when such a com-
bination is desired. An alkaline carmine fluid rather stronger
than that ordinarily employed (carmine, 15 grs., strong liq. amnion.,
§ drachm, glycerine, 2 oz., alcohol, 6 drachms) is first to be injected
carefully with very slight pressure ; the ammonia having a ten-
dency to soften the walls of the vessels. When they are fully
distended, the preparation is to be left for from ] 2 to 24 hours, in
order that time may be allowed for the carmine liquid which has
permeated the capillaries, to soak through the different tissues and
stain the germinal matters fully. !Next a little pure glycerine is to
be injected, to get rid of the carmine liquid ; and the prussian blue
fluid is then to be injected with the utmost care. When the vessels
have been fully distended, the injected preparation is to be divided
into very small pieces ; and these are to be soaked for an hour or
two in a mixture of 2 parts of glycerine and 1 of water, and then
for three or four days in strong glycerine acidulated with acetic
acid (5 drops to 1 oz.). Preparations thus made are best mounted
in Glycerine jelly ; and may then be examined with the highest
powers of the Microscope. A well-injected preparation should
have its vessels completely filled through every part ; the particles
of the colouring matter should be so closely compacted together,
that they should not be distinguishable unless carefully looked-for ;
and there should be no patches of pale uninjected tissue. Still,
although the beauty of a specimen as a Microscopic object is much
impaired by a deficiency in the filling of its vessels, yet to the
Anatomist the disposition of the vessels will be as apparent when
they are only filled in part, as it is when they are fully distended ;
and imperfectly injected capillaries may often be better seen in thin
sections mounted as transparent objects, than such as have been
completely filled.
651. A relation may generally be traced between the disposition
of the Capillary vessels, and the functions they are destined to sub-
serve ; but that relation is obviously (so to speak) of a mechanical
kind ; the arrangement of the vessels not in any way determining
the function, but merely administering to it, like the arrange-
ment of water or gas-pipes in a manufactory. Thus in Fig. 425
we see that the capillaries of adipose substance are disposed in a
network with rounded meshes, so as to distribute the blood among
the Fat- cells (§ 634) ; whilst in Fig. 426 we see the meshes enor-
mously elongated, so as to permit the Muscular fibres (§ 637) to lie
3e
786
VEETEBEATED ANIMALS.
in them. Again, in Fig. 427 we observe the disposition of the
Capillaries aronnd the orifices of the follicles of a Mncons mem-
Fig. 426.
Capillary network around Fat-cells.
Capillary network of Muscle.
brane; whilst in Fig. 428 we see the looped arrangement which
exists in the papillary snrface of the Skin, and which is subser-
Fig. 427.
Fig. 428.
Distribution of Capillaries in
Mucous Membrane.
Distribution of Capillaries in
Skin of Finger.
vient to the nutrition of the epidermis and to the activity of the
sensory nerves (§ 642).
652. In no part of the Circulating apparatus, however, does
the disposition of the capillaries present more points of interest,
than it does in the Respiratory organs. In Fishes the respiratory
surface is formed by an outward extension into fringes of gills,
each of which consists of an arch with straight laminse hanging
down from it ; and every one of these laminas (Fig. 429) is fur-
nished with a double row of leaflets, which is most minutely sup-
plied with blood-vessels, their network (as seen at a) being so close
that its meshes (indicated by the dots in the figure) cover less space
than the vessels themselves. The gills of Fish are not ciliated on
their surface, like those of Mollusks and of the larva of the Water-
Newt ; the necessity for such a mode of renewing the fluid in
contact with them being superseded by the muscular apparatus
CAPILLAEIES OF EESPIEATORY OEGANS.
787
with which, their gill-chamber is furnished. — But in Eeptiles the
respiratory surface is formed by the walls of an internal cavity,
that of the lungs : these organs, however, are constructed on a
plan very different from that
Fig. 429.
which they present in higher
Yertebrata, the great extension
of surface which is effected in
the latter by the minute sub-
division of the cavity not being
here necessary. In the Frog
(for example) the cavity of each
lung is undivided; its walls,
which are thin and membra-
nous at the lower part, there
present a simple smooth ex-
panse; and it is only at the
upper part, where the exten-
sions of the tracheal cartilage
form a network over the inte-
rior, that its surface is de-
pressed into sacculi, whose
lining is crowded with blood-
vessels (Fig. 430). In this
manner a set of air-cells is
formed in the thickness of the
upper wall of the lung, which
communicate with the general
cavity, and very much increase
the surface over which the
blood comes into relation with Tw0 branehial pr0CeSses of the Gill of
the air ; but each air-cell has the Ee^ showing the branchial lamella :—
a capillary network of its own, a, portion of one of these processes en-
which lies on one side against larged, showing the capillary network of
its wall, so as _ only _ to be the lamellae,
exposed to the air on its free
surface. In the elongated lung of the Snake the same general
arrangement prevails ; but the cartilaginous reticulation of its upper
part projects much further into the cavity, and encloses in its
meshes (which are usually square, or nearly so) several layers of
air-cells, which communicate, one through another, with the general
cavity. — The structure of the lungs of Birds presents us with an
arrangement of a very different kind, the purpose of which is to
expose a very large amount of capillary surface to the influence of
the air. _ The entire mass of each lung may be considered as sub-
divided into an immense number of ' lobules' or ' lunglets' (Fig.
431, b), each of which has its own bronchial tube (or subdivision of
the windpipe), and its own system of blood-vessels, which have
very little communication with those of other lobules. Each lobule
3e2
VERTEBRATED ANIMALS.
Fig. 430.
has a central cavity, which closely resembles that of a Frog's lung
in miniature, having its walls strengthened by a network of cartilage
derived from the bronchial tube, in the interstices of which are
openings leading to sacculi in their substance. But each of these
cavities is surrounded by a
solid plexus of blood-vessels,
which does not seem to be
covered by any limiting mem-
brane, but which admits air
from the central cavity freely
between its meshes; and thus
its capillaries are in imme-
diate relation with air on all
sides, a provision that is
obviously very favourable to
the complete and rapid aera-
tion of the blood they con-
tain.— In the lung of Man
and Mammals, again, the
plan of structure differs from
;erior of upper part of Lung of Frog. the foregoing, though the
general effect of it is the
same. For its whole interior is divided up into minute air-cells,
Fig. 431.
Interior structure of Lung of Foiol, as displayed by a section,
A, passing in the direction of a bronchial tube, and by another
section, b, cutting it across.
which freely communicate with each other, and with the ulti-
mate ramifications of the air- tubes into which the trachea sub-
divides ; and the network of blood-vessels (Fig. 432) is so dis-
posed in the partitions between these cavities, that the blood is
CAPILLAEIES OF LUNG.
It has been calculated that
Fig. 43:
exposed to the air on both sides.
the number of these air-
cells grouped around the
termination of each air-
tube in Man is not less
than 18,000 ; and that the
total number in the entire
lungs is six hundred mil-
lions.
653. The following list
of the parts o£ the bodies
of Vertebrata, of which
injected preparations are
most interesting as Mi-
croscopic objects, may be
of service to those who
may be inclined to apply
themselves to their pro-
duction.— Alimentary Ca-
nal; stomach, showing
the orifices of the gastric follicles, and the rudimentary vill
near the pylorus; small intestine, showing the villi and the
orifices of the follicles, of Lieberkuhn, and at its lower part the
Peyerian glands ; large intestine, showing the various glandular
follicles: — Eespiratory Organs; lungs of Mammals, Birds, and
Keptiles ; gills and swimming -bladder of Fish : — Glandular Organs;
liver, gall-bladder, kidney, parotid: — Generative. Organs ; ovary of
Toad ; oviduct of Bird and Frog ; Mammalian placenta ; uterine
aud fcetal cotyledons of Ruminants : — Organs of Sense ; retina, iris,
choroid, and ciliary processes of eye, pupillary membrane of foetus ;
papilla? of tongue ; mucous membrane of nose, papilla? of skin of
finger: — Tegumentary Organs; skin of different parts, hairy and
smooth, with vertical sections showing the vessels of the hair-fol-
licles, sebaceous glands, and papillae ; matrix of nails, hoofs, &c. : —
Tissues ; fibrous, muscular, adipose, sheath of tendon : — Nervous
Centres ; sections of brain and spinal cord.
Arrangement of the Capillaries on the walls
of the Air-cells of the Human Lung.
The study of the Embryonic Development of Yertebrated animals
has been pursued of late years with great zeal and success by
the assistance of the Microscope ; but as this is a department of
inquiry which needs for its successful pursuit a thorough scientific
culture, and is only likely to be taken-up by a professed Physiologist,
no good purpose seems likely to be served by here giving such an
imperfect outline of the process as could alone be introduced into
a work like the present ; and the reader who may desire informa-
tion upon it will find no difficulty in obtaining this elsewhere.*
* The Student cannot do better than master, in the first instance, the " Ele-
ments of Embryology," by Dr. Michael Foster and Mr. F. M. Balfour.
CHAPTEE XIX.
APPLICATIONS OP THE MICROSCOPE TO GEOLOGICAL INVESTIGATION.
654. The utility of the Microscope is by no means limited to the
determination of the structure and actions of the Organized beings
at present living on the surface of the Earth ; for a vast amount
of information is afforded by its means to the Geological inquirer,
not only with regard to the minute characters of the many
Vegetable and Animal remains that are entombed in the succes-
sive strata of which its crust is composed, but also with regard to
the essential nature and composition of many of those strata them-
selves.— We cannot have a better example of its value in both
these respects, than that which is afforded by the results of Micro-
scopic examination of lignite or fossilized wood, and of ordinary
coal, which there is every reason to regard as a product of the
decay of wood.
655. Specimens of fossilized wood, in a state of more or less
complete preservation, are found in numerous strata of very dif-
ferent ages, — more frequently, of course, in those whose materials
were directly furnished by the dry land, and were deposited in its
immediate proximity, than in those which were formed by the
deposition of sediments at the bottom of a deep ocean. Generally
speaking, it is only when the wood is found to have been pene-
trated by silex, that its organic structure is well preserved ; but
instances occur every now and then, in which penetration by
carbonate of lime has proved equally favourable. In either case,
transparent sections are needed for the full display of the organi-
zation ; but such sections, though made with great facility when
lime is the fossilizing material, require much labour and skill
when silex has to be dealt- with. Occasionally, however, it has
happened that the infiltration has filled the cavities of the cells and
vessels, without consolidating their walls ; and as the latter have
undergone decay without being replaced by any cementing mate-
rial, the lignite, thus composed of the internal ' casts' of the woody
tissues, is very friable, its fibres separating from each other like
those of asbestos ; and laminae split-asunder with a knife, or isolated
fibres separated by rubbing-down between the fingers, exhibit the
MICROSCOPIC EXAMINATION OF COAL. 791
characters of the woody structure extremely well, wheu mounted in
Cauada balsam. — Generally speaking, the lignites of the Tertiary
strata preseut a tolerably close resemblance to the woods of the
existing period: thus the ordinary structure of dicotyledonous and
monocotyledon-oils stems may be discovered in such lignites in the
ntmost perfection; and the peculiar modification presented by
coniferous wood is also most distinctly exhibited (Fig. 223). As we
descend, however, through the strata of the Secondary period, we
more and more rarely meet with the ordinary dicotyledonous struc-
ture ; and the lignites of the earliest deposits of these series are,
almost universally, either Gymnosperms* or Palms.
656. Descending into the Palaezoic series, we are presented in the
vast coal formations of our own and other countries with an extra-
ordinary proof of the prevalence of a most lnxuriant vegetation in
a comparatively-early period of the world's history ; and the Micro-
scope lends the Geologist essential assistance, not only in deter-
mining the nature of much of that vegetation, but also in demon-
strating (what had been suspected on other grounds) that Coal
itself is nothing else than a mass of decomposed vegetable matter,
derived from the decay of an ancient vegetation. The determina-
tion of the characters of the Ferns, Sigillarice, Lepidodendra, Gala-
mites, and other kinds of vegetation whose forms are preserved in
the shales or sandstones that are interposed between the strata of
Coal, has been hitherto chiefly based on their external characters ;
since it is very seldom that these specimens present any such traces
of minute internal structure as can be subjected to Microscopic
elucidation. But persevering search has recently brought to light
numerous examples of Coal-plants, whose internal structure is
sufficiently well preserved to allow of its being studied micro-
scopically : and the careful researches of Prof. W. C. Williamson
have shown that they formed a series of connecting links between
Cryptogamia and Flowering plants ; being obviously allied to
Eqiiisetacece, Lycopodiacecs, &c., in the character of their fructi-
fication, whilst their stem-structure foreshadowed both the ' endo-
genous' and 'exogenous' types of the latter .f Notwithstanding
the general absence of any definite form in the masses of decom-
posed wood of which Coal itself consists (these having apparently
been reduced to a pulpy state by decay, before the process of con-
solidation by pressure, aided perhaps by heat, commenced), the
traces of structure revealed by the Microscope are often sufficient —
especially in the ordinary ' bituminous' coal — not only to determine
its vegetable origin, but in some cases to justify the Botanist in
assigning the character of the vegetation from which it must have
been derived ; and even where the stems and leaves are represented
by nothing else than a structureless mass of black carbonaceous
* Under this head are included the Cycadece, along with the ordinary Coni-
fer<E or pine and fir tribe.
t See his succession of Memoirs on the Coal-Plants, in the recent volumes
of the "Philosophical Transactions."
792 APPLICATION TO GEOLOGICAL INVESTIGATION.
matter, there are found diffused through this a multitude of minute
resinoid yellowish-brown granules, which are sometimes aggregated
in clusters and enclosed in sacculi ; and these may now be pretty
certainly affirmed to represent the spores, while the sacculi repre-
sent the sporangia, of gigantic Lycopodiacece (club-mosses) of the
Carboniferous Flora. The larger the proportion of these granules,
the brighter and stronger is the flame with which the coal burns ;
thus in some blazing cannel-co&ls they abound to such a degree as
to make up the greater proportion of their substance ; whilst in
anthracite or ' stone-coal,' the want of them is shown by its dull
and slow combustion. It is curious that the dispersion of these
resinoid granules through the black carbonaceous matter is some-
times so regular as to give to transparent sections very much the
aspect of a section of vegetable cellular tissue, for which they have
been mistaken even by experienced microscopists ; but this resem-
blance disappears under a more extended scrutiny, which shows it
to be altogether accidental.
657. In examining the structure of Coal, various methods may
be followed. Of those kinds which have sufficient tenacity, thin
sections may be made ; but the opacity of the substance requires
that such sections should be ground extremely thin before they
become transparent ; and its friability renders this process one of
great difficulty. It may, however, be facilitated by using Marine
Glue, instead of Canada balsam, as the cement for attaching the
smoothed surface of the coal to the slip of glass on which it is
rubbed-down. Another method is recommended by the authors of
the " Micrographic Dictionary," (2nd Edit., p. 178) : — " The coal is
macerated for about a week in a solution of carbonate of potass ; at
the end of that time, it is possible to cut tolerably-thin slices with
a razor. These slices are then placed in a watch-glass with strong
nitric acid, covered, and gently heated ; they soon turn brownish,
then yellow, when the process must be arrested by dropping the
whole into a saucer of cold water, or else the coal would be dis-
solved. The slices thus treated appear of a darkish amber-colour,
very transparent, and exhibit the structure, when existing, most
clearly. "We have obtained longitudinal and transverse sections of
coniferous wood from various coals in this way. The specimens are
best preserved in glycerine, in cells ; we find that spirit renders
them opaque, and even Canada balsam has the same defect." —
When the coal is so friable that no sections can be made of it by
either of these methods, it may be ground to fine powder, and the
particles may then, after being mounted in Canada balsam, be
subjected to Microscopic examination : the results which this method
affords are by no means satisfactory in themselves, but they will
often enable the organic structure to be sufficiently determined, by
the comparison of the appearances presented by such fragments
with those which are more distinctly exhibited elsewhere. Valuable
information may often be obtained, too, by treating the ash of an
(Ordinary coal-fire in the same manner, or (still better) by burning
MICEOZOIC DEPOSITS ON SEA-BOTTOM.— LEVANT MUD. 793
to a white ash a specimen of coal that has been previously boiled
in nitric acid, and then carefully mounting the ash in Canada
balsam ; for mineral ' casts ' of vegetable cells and fibres may often
be distinctly recognized in such ash ; and such casts are not unfre-
quently best afforded by samples of coal in which the method of
section is least successful in bringing to light the traces of organic
structure, as is the case, for example, with the anthracite of Wales.
658. Passing on now to the Animal kingdom, we shall first cite
some parallel cases in which the essential nature of deposits that
from a very important part of the Earth's crust, has been deter-
mined by the assistance of the Microscope ; and shall then select a
few examples of the most important contributions which it has
afforded to our acquaintance with types of Animal life long since
extinct. — It is an admitted rule in Geological science, that the past
history of the Earth is to be interpreted, so far as may be found
possible, by the study of the changes which are still going on.
Thus, when we meet with an extensive stratum of fossilized Diato-
macece (§ 260) in what is now dry land, we can entertain no doubt
that this siliceous deposit originally accumulated either at the
bottom of a fresh-water lake or beneath the waters of the ocean ;
just as such deposits are formed at the present time by the produc-
tion and death of successive generations of these bodies, whose in-
destructible casings accumulate in the lapse of ages, so as to form
layers whose thickness is only limited by the time during which
this process has been in action (§ 259). In like manner, when we
meet with a Limestone-rock entirely composed of the calcareous
shells of Foraminifera, some of them entire, others broken up into
minute particles (as in the case of the Fusidina-limeatoYiQ of the
Carboniferous period (§ 448), and the Nummulitic limestone of the
Eocene (§ 452), we interpret the phenomenon by the fact that the
dredgings obtained from certain parts of the ocean-bottom consist
almost entirely of remains of existing Foraminifera, in which entire
shells, the animals of which may be yet alive, are mingled with the
debris of others that have been reduced by the action of the waves
to a fragmentary state. Such a deposit, consisting chiefly of Orbi-
tolites (§ 427), is at present in the act of formation on certain parts
of the shores of Australia, as the Author was informed by Mr. J.
Beete Jukes ; thus affording the exact parallel to the stratum of
Orbitolites (belonging, as his own investigations have led him to
believe, to the very same species) that forms part of the ' calcaire
grossier ' of the Paris basin. So in the fine white mud which is
brought up from almost every part of the sea-bottom of the Levant,
where it forms a stratum that is continually undergoing a slow but
steady increase in thickness, the Microscopic researches of Prof.
"Williamson* have shown, not only that it contains multitudes of
minute remains of living organisms, both Animal and Vegetable,
but that it is entirely or almost wholly composed of such remains.
* " Memoirs of the Manchester Literary and Philosophical Society," Vol. viii.
794
APPLICATION TO GEOLOGICAL INVESTIGATION.
Amongst these were about 26 species of Diatomacese (siliceous), 8
species of Forauiinifera (calcareous), and a miscellaneous group of
objects (Fig. 433), consisting of calcareous and siliceous spicules of
Sponges and Gorgonias, and fragments of the calcareous skeletons
Fig. 433.
Microscopic Or
in Levant Mud: — A, D, siliceous
spicules of Tethya; B, h, spicules of Geodia ; c, sponge-spicule
(unknown); E, calcareous spicule of Grantia; r, G, M, o, por-
tions of calcareous skeleton of Echinodermata ; H, I, calcareous
spicule of Gorgonia ; K, L, N, siliceous spicules of Halicliondria ;
p, portion of prismatic layer of shell of Pinna.
of Echinoderms and Mollusks. A collection of forms strongly re-
sembling that of the Levant mud, with the exception of the siliceous
FOEAMINIFEEAL ORIGIN OF CHALK. 795
Diatomaceas, is found in many parts of the 'calcaire grossier' of
the Paris basin, as well as in other extensive deposits of the same
early Tertiary period.
659. It is, however, in regard to the great Chalk Formation that
the information afforded by the Microscope has been most valuable.
Mention has already been made (§ 443) of the fact that a large
proportion of the North Atlantic sea-bed has been found to
be covered with an ' ooze ' chiefly formed of the shells of Globi-
germoB ; and this fact, first determined by the examination of the
small quantities brought up by the ' sounding ' apparatus, has
been fully confirmed by the results of the recent exploration of the
Deep-sea with the ' dredge;' which, bringing up half a ton of this
deposit at once, has shown that it is not a mere surface-film, hut
an enormous mass whoso thickness cannot be even guessed at.
" Under the Microscope," says Prof. Wyville Thomson,* of a sample
of 1| cwt. obtained by the dredge from a depth of nearly three
miles, " the surface-layer was found to consist chiefly of entire
shells of Globigerina bulloides, large and small, and of fragments
of such shells mixed with a quantity of amorphous calcareous
matter in fine particles, a little fine sand, and many spicules,
portions of spicules, and shells of Radiolaria, a few spicules of
Sponges, and a few frustules of Diatoms. Below the surface-layer
the sediment becomes gradually more compact, and a slight grey
colour, due, probably, to the decomposing organic matter, becomes
more pronounced, while perfect shells of Globigerina almost dis-
appear, fragments become smaller, and calcareous mud, structure-
less, and in a fine state of division, is in greatly preponderating
proportion. One can have no doubt, on examining this sediment,
that it is formed in the main by the accumulation and disintegra-
tion of the shells of Globigerina ; the shells fresh, whole, and living,
in the surface-layer of the deposit ; and in the lower layers dead,
and gradually crumbling down by the decomposition of their organic
cement, and by the pressure of the layers above." This white cal-
careous mud also contains in large amount the ' coccoliths ' and
' coccospheres ' formerly described (§ 367), these in its surface-layer
being imbedded in the viscous protoplasmic network, to which the
name Bathybius has been given (§ 366). It may be doubted, how-
ever, whether this is to be regarded as a distinct ' moneric ' organism,
or is formed by the fusion of the pseudopodial extensions of the
sarcode-bodies of the Globigerinae. — Now the resemblance which this
Globigerina-mud, when dried, bears to Chalk, is so close as at once to
suggest the similar origin of the latter ; and this is fully confirmed
by Microscopic examination. For many samples of it consist in
great part of the minuter kinds of Foraminifera, especially Globi-
gerince (Figs. 434, 435), whose shells are imbedded in a mass of
apparently amorphous iDarticles, many of which, nevertheless,
present indications of being the worn fragments of similar shells, or
of larger calcareous organisms. In the Chalk of some localities, the
* " The Depths of the Sea," p. 410.
796 APPLICATION TO GEOLOGICAL INVESTIGATION.
disintegrated prisms of Pinna (§ 522) or of other large shells of the
like structure (as Inoceramus) form the great bulk of the recogniza-
ble components ; whilst in other cases, again, the chief part is made
Fig. 434.
Microscopic Organisms in Chalk from Gravesend : — a, &, c, tZ,
Textularia globulosa ; e, e, e, Botalia aspera; /, Textularia
aculeata ; g, Planularia kexas ; h, Navicula.
up of the shells of Gytlierina, a marine form of Entomostracous
Crustacean (§ 564). Different specimens of Chalk vary greatly in
the proportion which the distinctly organic remains bear to the
amorphous residuum, and which the different kinds of the former
bear to each other ; and this is quite what might be anticipated,
when we bear in mind the predominance of one or another tribe
of Animals in the several parts of a large area; but it maybe fairly
concluded from what has been already stated of the amorphous
component of the Globigerina-mud, that the amorphous constituent
of Chalk likewise is the disintegrated residuum of Foraminiferal
shells. — But further, the Globigerina-mud now in process of forma-
tion is in some places literally crowded with Sponges having a
complete siliceous skeleton (§ 467) ; and some of these bear such an
extraordinarily close resemblance, alike in structure and in external
form, to the Ventriculites which are well known as Chalk-fossils, as
SPONGEOUS ORIGIN OF FLINTS. 797
to leave no reasonable doubt that these also lived as siliceous
sponges on the bottom of the Cretaceous sea. Other sponges, also,
are found in the Globigerina-mud, the structure of whose horny
skeleton corresponds so closely with the sponge-tissues which can
be recognized in sections of nodular Flints,* as to make it clear — ■
when taken in connection with correspondence of external form —
Fig. 135.
Microscopic Organisms in Clialh from Meudon ; seen partly as opaque, and
partly as transparent objects.
that such flints are really fossilized sponges, the silicifying material
having been furnished by the solution of the skeletons of the
siliceous sponges, or of deposits of Diatoms or Eadiolaria. Further,
in many sections of Flints there are found minute bodies termed
XantMdia, which bear a strong resemblance to the sporangia of
certain Desmicliacece (Fig. 126, d) ; and the Author has found
similar bodies in the midst of what appears to be sponge-tissue
imbedded in the Globigerina-mud. — All these correspondences show
that the formation of Chalk took place under conditions essentially
similar to those under which the deposit of Globigerina-mud is
* See Dr. Bowerhank's Memoirs in the "Transact, of the Geolog. Society,"
1840, and in the "Ann. of Nat. Hist.," 1st Ser., Vols, vii., x.
798 APPLICATION TO GEOLOGICAL INVESTIGATION.
being formed over the Atlantic sea-bed at the present time. And
there is strong evidence that this deposit is not merely a repetition
of the old Chalk-formation, but that it is an actual continuation of it ;
the bed of the Atlantic having probably been continuous in the Creta-
ceous epoch with that of the Sea which must have then covered the
large area now occupied by the Chalk of Europe, Asia, and America;
while the changes of elevation which this has undergone since it
became dry land, seem never to have been such as to bring up the
bottom of the Atlantic basin within many hundred fathoms of its
surface, so that the deposit of Globigerina-mud over its area has pro-
bably been going on over a large part of the Atlantic area through
the whole of the Tertiary and Quaternary epoch.*
660. In examining Chalk or other similar mixed aggregation,
whose component particles are easily separable from each other, it
is desirable to separate, with as little trouble as possible, the larger
and more definitely organized bodies from the minute amorphous
particles ; and the mode of doing this will depend upon whether we
are operating upon the large or upon the small scale. If the former,
a quantity of soft Chalk should be rubbed to powder with water,
by means of a soft brush ; and this water should then be proceeded
with according to the method of levigation already directed for
separating the Diatomaceas (§ 261). It will usually be found that
the first deposits contain the larger Foraminifera, fragments of
Shell, &c, and that the smaller Foraminifera and Sponge-spicules
fall next ; the fine amorphous particles remaining diffused through
the water after it has been standing for some time, so that they
may be poured-away. The organisms thus separated should be
dried and mounted in Canada balsam. — If the smaller scale of pre-
paration be preferred, as much Chalk scraped fine as will lie on the
point of a knife is to be laid on a drop of water on the glass slide,
and allowed to remain there for a few seconds ; the water, with any
particles still floating on it, should then be removed ; and the
sediment left on the glass should be dried and mounted in Bal-
sam.— For examining the structure of Flints, such chips as may
be obtained with a hammer will commonly serve very well : a clear
translucent flint being first selected, and the chips that are obtained
being soaked for a short time in turpentine (which increases their
transparence), those which show organic structure, whether Sponge-
tissue or Xanthidia, are to be selected and mounted in Canada
balsam. The most perfect specimens of Sponge-structure, how-
ever, are only to be obtained by slicing and polishing, — a process
which is best performed by the lapidary.
661. There are various other deposits, of less extent and im-
portance than the great Chalk-formation, which are, like it, com-
posed in great part of Microscopic organisms, chiefly minute
* The evidence in favour of this doctrine, which is now coming to be gene-
rally received among Geologists, will be found fully set forth by Prof. Wyville
Thomson, its originator, in his " Depths of the Sea."
MICEOZOIC COMPOSITION OF ROCKS. 799
Foraminifera ; and the presence of animals of this gronp may be
recognized, by the assistance of this instrument, in sections of cal-
careous rocks of various dates, whose chief materials seems to have
been derived from Corals, Encrinite-stems, or the shells of Mollusks.
Thus in the ' Crag' formation (Tertiary) of the eastern coast of
England, the greater portion of which is perceived by the unassisted
eye to be composed of fragments of Shells, Corals (or rather Poly-
zoaries, § 507), and Echinoderms, the Microscope enables us to
discover Foraminifera, minute fragments of Shells and Corals, and
spicules of Sponges; the aggregate being such as is at present in
process of formation on many parts of our shores, and having been,
therefore, in all probability, a 'littoral' formation; whilst the
Chalk (with other formations chiefly consisting of Foraminifera)
was deposited at the bottom of deeper waters. Many parts of the
Oolitic formation (Secondary) have an almost identical character,
save that the forms of organic life give evidence of a different age ;
and in those portions which exhibit the ' roe-stone' arrangement
from which the rock derives its name (such as is beautifully dis-
played in many specimens of Bath-stone and Portland-stone), it is
found by Microscopic examination of transparent sections, that
each rounded concretion is composed of a series of concentric
spheres enclosing a central nucleus, which nucleus is often a Fora-
miniferal shell. In the Carboniferous (palasozoic) limestone, again,
well-preserved specimens of Foraminifera present themselves ; and
there are certain bands of Limestone of this epoch in Eussia, vary-
ing in thickness from fifteen inches to five feet, and frequently
repeated through a vertical depth of two hundred feet, over very
wide areas, which are almost entirely composed of the extinct genus
Fusulina (§ 448) : thus prefiguring, as it were, the vast deposit of
Nummulitic limestone (§ 452) which marks the commencement of
the Tertiary epoch. — Mention has already been made (§ 450 note)
of Prof. Ehrenberg's very remarkable discovery that a large pro-
portion (to say the least) of the green sands which present them-
selves in various stratified deposits, from the Silurian epoch to the
Tertiary period, and which in certain localities constitute what is
known as tlie Greensand formation (beneath the Chalk), is com-
posed of the casts of the interior of minute shells of Foraminifera
and Mollusca, the shells themselves having entirely disappeared.
The material of these casts, which is chiefly Silex coloured by
Silicate of Iron, has not merely filled the chambers and their com-
municating passages (Fig. 277, a, b), but has also penetrated, even
to its minutest ramifications, the canal-system of the intermediate
skeleton (Figs. 280, 284). — Even this discovery pales in interest
before that more recent one to which it has led, and which may be
regarded as the most remarkable achievement of Microscopic
inquiry as applied to Geology : namely, the determination of the
organic nature of those Serpentine-limestones in the Laurentian
formations of Canada and elsewhere, which are products of the
growth of the gigantic Foraminiferal Eozoon over immense areas of
800 APPLICATION TO GEOLOGICAL INVESTIGATION.
the ancient sea-bottom (§§ 456-460). This discovery is alike inte-
resting to the Physiologist and Zoologist, on the one hand, and to
the Geologist on the other. For it presents to the former the
Hhizopod type of Animal life, than which nothing simpler can well
be conceived (§ 369), in an aspect of most unexpected magnitude :
whilst to the latter it affords evidence not merely of the prevalence
of Animal life, but of its important share in the production of rock
formations, in strata so far below those in which organic remains
had previously been detected, that, to use the words of Sir William
Logan, the appearance of the so-called ' Primordial Fauna' is a
comparatively modern event.
662. The foregoing general summary, taken in connection with
the more detailed statements that have been made in previous parts
of this work, will suffice to indicate the essential importance of
Microscopic examination, in determining, on the one hand, the real
character of various stratified deposits, and on the other, the nature
of the organic remains which these may include. The former
of these lines of inquiry has not yet attracted the attention
it deserves ; since, as is very natural, the greater number of Micro-
scopists are more attracted by those definite forms which they can
distinctly recognize, than by amorphous sediments which present
no definite structural characters. Yet it is a matter of extreme
interest to the Geologist, to determine how far these last also may
have had their origin in the disintegration of Organic structures ;
and much light may often be thrown upon this question by careful
Microscopic analysis. There is strong reason to believe, moreover,
that the deep-sea beds, of the Carboniferous limestone were really
formed by the agency of Foraminiferal life, very much in the con-
dition of Chalk ; and that they have been brought to their present
sub-crystalline form by a subsequent process of ' metamorphism,'
analogous to that which has converted the chalk of the Antrim coast
into a sort of white marble. It is interesting to remark, in this
connection, that whilst Fusulina does not show itself (so far as is
at present known) in any later epoch,, the arenaceous Saccamina,
which abounds in certain localities at the present epoch, has clearly
come down to us from the Carboniferous period (§ 435). Such a
line of inquiry was some time since systematically pursued by
Mr. Sorby; who applied himself to the Microscopic study of the
composition of freshwater Marls and Limestones, by ascertaining
the characters and appearances of the minute particles into which
shells resolve themselves by decay, and by estimating the relative
proportions of the organic and the inorganic ingredients of a
deposit, by delineating on paper (by means of the Camera Lucida)
the outlines of the particles visible in thin sections, then cutting
them out, and weighing the figures of each kind.*
663. It is obvious that, under ordinary circumstances, only the
* See his successive Memoirs in " Quart. Journ. of Geolog. Science," 1853,
p. 844, and subsequently.
MICEOSCOPIC PALEONTOLOGY. 801
hard parts of the bodies of Animals that have been entombed in
the depths of the earth are likely to be preserved ; but from these
a vast amount of information may be drawn ; and the inspection
of a microscopic fragment will often reveal, with the utmost cer-
tainty, the entire nature of the organism of which it formed part.
In the examination of the minuter "fossil Corals, and of those Poly-
zoaries (§ 507) which are commonly ranked with them, the
assistance of the Microscope is indispensable. Minute fragments
of the tests or spines of Echinodermata, and of all such Molluscous
shells as present distinct appearances of structure (this being espe-
cially the case with the Brachiopoda, and with certain families of
Lamellibranchiate bivalves), may be unerringly identified by its
means, when the external
form of these fragments Fig. 436.
would give no assistance
whatever. In the study
of the important ancient
group of Trilobites, not
only does a Microscopic
examination of the 'casts '
which have been preserved
of the surface of their Eye of jvitoftfte.
Eyes (Fig. 436), serve to
show the entire conformity in the structure of these organs to
the ' composite' type which is so remarkable a characteristic of
the higher Articulata (§ 586), but it also brings to light certain
peculiarities which help to determine the division of the great
Crustacean series with which this group has most alliance.*
Q64i. It is in the case of the Teeth, the Bones, and the Dermal
skeleton of Vertebrated animals, however, that the value of Micro-
scopic inquiry becomes most apparent; since the structure of these
presents so many characteristics which are subject to well-marked
variations in their several Classes, Orders, and Families, that a
knowledge of these characters frequently enables the Microscopist
to determine the nature of even the most fragmentary specimens,
with a positiveness which must appear altogether misplaced to
such as have not studied the evidence. It was in regard to teeth,
that the possibility of such determinations was first made clear b}r
the laborious researches of Prof. Owen,f and the following may be
given as examples of their value : — A rock -formation extends over
many parts of Russia, whose mineral characters might justify its
being likened either to the Old or to the New Red sandstone of this
country, and whose position relatively to other strata is such that
there is great difficulty in obtaining evidence from the usual sources
as to its place in the series. Hence the only hope of settling this
question (which was one of great practical importance, — since, if
* See Prof. Burraeister " On the Organization of the Trilobites," published,
by the Ray Society, p. 19.
f See his magnificent " Odontography."
3f
802 APPLICATION TO GEOLOGICAL INVESTIGATION.
the formation were Neiv Eed, Coal might he expected to underlie
it, whilst if Old Eed, no reasoaable hope of Coal could be entertained)
lay in the determination of the Organic remains which this stratum
might yield ; but unfortunately these were few and fragmentary,
consisting chiefly of teeth which are seldom perfectly preserved.
From the gigantic size of these teeth, together with their form, it
was at first inferred that they belonged to Saurian Eeptiles, in
which case the Sandstone must have been considered as New Eed;
but Microscopic examination of their intimate structure unmis-
takably proved them to belong to a genus of Fishes (Dendrodus)
which is exclusively Palaeozoic, and thus decided that the forma-
tion must be Old Eed. — So again, the Microscopic examination of
certain fragments of teeth found in a sandstone of Warwickshire,
disclosed a most remarkable type of tooth- structure (shown in
Fig. 437), which was also ascertained to exist in certain teeth that
had been discovered in the * Keupersandstein' of Wirtemberg ; and
Fig. 437.
Section of Tooth of Ldbyrinlhodon.
the identity or close resemblance of the animals to which these
teeth belonged having been thus established, it became almost
certain that the Warwickshire and Wirtemberg sandstones were
equivalent formations, a point of much Geological importance.
The next question arising out of this discovery, was the nature of
the animal (provisionally termed Labyrinthodon, a name expressive
of the most peculiar feature in its dental structure) to which these
teeth belonged. They had been referred, from external characters
merely, to the order of Saurian Eeptiles : but these characters were
by no means conclusive ; and as the nearest approaches to their
DETERMINATION OF FOSSIL TEETH AND BONES. 803
peculiar internal structure are presented by Fish-lizards and Lizard
like fish, it might be reasonably expected that the Labyrinthodon
would combine with its Reptilian characters an affinity to Fish.
This has been clearly proved to be the case, by the subsequent
discovery of parts of its skeleton in which such characters are very
obvious ; and by a very beautiful chain of reasoning, Prof. Owen
succeeded in establishing a strong probability, that the Labyrin-
thodon was a gigantic Frog-like animal five or six feet long, with
some peculiar affinities to Fishes, and a certain mixture also of
Crocodilian characters ; and that it made the well-known foot-
prints which have been brought to light, after an entombment
whose duration can scarcely be conceived (much less estimated), in
the Stourton quarries of Cheshire. This conclusion has been fully
confirmed by the subsequent discovery of a large number of remains
of Reptiles, some of them of yet earlier date, presenting similar
peculiarities of structure.
665. The researches of Prof. Quekett on the minute structure of
bone* have shown that from the average size and form of the
lacunae, their disposition in regard to each other and to the
Haversian canals, and the number and course of the canaliculi
(§ 012), the nature of even a minute fragment of Bone may often
be determined with a considerable approach to certainty ; as in the
following examples, among many which might be cited : — Dr.
Falconer, the distinguished investigator of the fossil remains of the
Himalayan region, and the discoverer of the gigantic fossil Tortoise
of the Sivalik hills, having met with certain small bones about which
he was doubtful, placed them in the hands of Prof. Quekett for
minute examination ; and was informed, on Microscopic evidence,
that they might certainly be pronounced Reptilian, and probably
belonged to an animal of the Tortoise tribe ; and this determination
was fully borne-out by other evidence, which led Dr. Falconer to
conclude that they were toe-bones of his great Tortoise. — Some
fragments of Bone were found, some years since, in a Chalk-pit,
which were considered by Prof. Owen to have formed part of the
wing-bones of a long- winged sea-bird allied to the Albatross. This
determination, founded solely on considerations derived from the
very imperfectly-preserved external forms of these fragments, was
called in question by some other Palaeontologists ; who thought it
more probable that these bones belonged to a large species of the
extinct genus Pterodactylus, a flying lizard whose wing was ex-
tended upon a single immensely -prolonged digit. No species of
Pterodactyle, however, at all comparable to this in dimensions, was
at that time known ; and the characters furnished by the configura-
tion of the bones not being in any degree decisive, the question
would have long remained unsettled, had not an appeal been made
to the Microscopic test. This appeal was so decisive, by showing
* See his Memoir on the ' Comparative Structure of Bone,' in the " Transact,
of the Microsc. Society," Ser. 1, Vol. ii. ; and the " Catalogue, of the Histologi-
cal Museum of the Roy. Coll. of Surgeons," Vol. ii.
3 f 2
804 APPLICATION TO GEOLOGICAL INVESTIGATION.
that the minute structure of the bone in question corresponded
exactly with that of Pterodactyle bone, and differed essentially from
that of every known Bird, that no one who placed much reliance
upon that evidence could entertain the slightest doubt on the matter.
By Prof. Owen, however, the validity of that evidence was questioned,
and the bone was still maintained to be that of a Bird ; until the
question was finally set at rest, and the value of the Microscopic
test triumphantly confirmed, by the discovery of undoubted Ptero-
dactyle bones of corresponding and even of greater dimensions, in
the same and other Chalk quarries .*
666. The application of the Microscope to Geology is not, how-
ever, limited to the determination or discovery of Organic structure ;
for, as has been now satisfactorily demonstrated, very important
information may be acquired by its means respecting the Mineral
composition of Rocks, and the mode of their formation. " As long,"
says Mr. David Forbes,f " as the Geologist encounters in the field
only rocks of so coarse and simple a structure as to admit of being
resolved by the naked eye into their constituent mineral species,
or of distinguishing the fragments of previously existing rocks of
which they have been built up, he may speculate with a fair chance
of success as to their probable origin or mode of formation. When,
however, as is more often the rule than the exception, rocks are
everywhere met with presenting so fine-grained and apparently
homogeneous a texture as to defy such attempts at ocular analysis,
all speculations as to their nature and formation based merely upon
observation in the field, can but be compared to groping in the
dark, with the faint hope of stumbling upon the truth. In these
cases the Geologist must call in the aid of Chemistry and the
Microscope ; by Chemical analysis he learns the per-centage com-
position of the rock in question ; whilst the Microscopic examina-
tion informs him how the Chemical components are Mineralogically
combined, and at the same time affords valuable information as to
the physical structure and arrangement of the components of the
rock-mass, tending to elucidate its formation and origin." The
mode recommened by Mr. D, Forbes of making transparent
sections of Bocks for Microscopic examination, is essentially the
same with that already described (§§ 154-156). A fragment from
one quarter to three quarters of an inch square, and of convenient
thickness, is chipped off the rock-specimen in the direction of the
required section, and ground down upon an iron or pewter plate in
a lapidary's lathe with emery, until a perfectly flat surface is
obtained. This surface is then worked down still finer upon a slab
of black marble, with less coarse emery, then upon a "Water of Ayr
stone with water alone, and lastly polished with water on a slab of
black marble. The polished surface being then cemented to a slip
* See Prof. Owen's Monograph on "The British Fossil Eeptilesof the Chalk
Formation " (published by the Palaeontographical Society), p. 80, et seq.
f ' The Microscope in Geology,' in the "Popular Science Review," October,
1867.
MICROSCOPIC PETEOLOGY. 805
of plate-glass, the other surface is to be worked down in the same
manner, until the section is reduced to a sufficient thinness ; when
it is to be transferred to a slide, and mounted in Canada balsam in
the usual mode. The examination of such a rock-section enables
a mineralogical analysis to be made even of the most compact and
apparently homogeneous rock ; for even when the glassy appearance
of a vitrified rock would discourage any hopes of structure being
discovered, some portion may generally be found in which the vitri-
fication is so far from being complete, as to enable the component
minerals to be distinctly recognized by Microscopic examination.
Thus in a specimen of glassy Pitchstone examined by Mr. Forbes,
the pyroxenic and feldspathic constituents of the rock were beauti-
fully apparent, notwithstanding that the rock itself looks like so
much dirty green bottle-glass. And in many cases in which the
specimens have been so perfectly vitrified as to show no trace of
structure in the first instance, this may be developed by carefully
acting upon the surface by gaseous or liquid hydrochloric acid.
Frequently, again, Mineral constituents are thus discovered, whose
existence had been previously unsuspected, from their being too
minute to be recognized by the eye ; and the presence of these may
have a most important bearing upon the question of the mode in
which the rock-masses have originated. Thus it has been shown
by Mr. Sorby that the quartz of granites contains water in
numerous minute cavities excavated in its solid crystals ; which
shows that granites have solidified at a heat far below the fusing
points of their constituent minerals, and at such a pressure as to
enable them to entangle and retain a small amount of aqueous
vapour. Similar cavities have been detected by Mr. Sorby not
merely in the quartz of volcanic rocks, but also in the felspar
and nepheline ejected from the crater of Vesuvius ; and this fact
renders it probable that the two classes of rocks were formed by
identical agencies, as might be concluded from the general arrange-
ment of their Mineral components. For it is affirmed by Mr. D.
Forbes that " the Microscopic examination already made of many
hundred sections of eruptive rocks, differing widely in Geological
age and Geographical distribution, shows that in all rocks of this
class, whether of the most compact, hard, and homogeneous ap-
pearance, or occurring in the softest and finest powder, like the
ashes and dust frequently thrown out by volcanoes, a similar
crystallized arrangement and structure is present and common to
them all. Lavas, Trachytes, Dolerites, Diorites, Porphyrites,
Syenites, Granites, &c, all possess the same general structural
features, serving to distinguish the eruptive rocks as a class from
all others." Again, it has been shown by Mr. Sorby that Micro-
scopic examination often allows the minerals formed at the time of
the solidification of the rock, to be distinguished from such as are
the products of subsequent alteration by the action of water, or by
atmospheric or other agencies. In the case of sedimentary rocks,
it frequently happens that Microscopic examination affords the only
806 APPLICATION TO GEOLOGICAL INVESTIGATION.
means by which the problem of their origin can be resolved ; the
most compact and apparently homogeneous specimens being thns
shown to be aggregations of more or less ronnded and water-worn
grains (often less than 1 -1000th of an inch in diameter) of Quartz,
weathered Felspar, Mica, soft and hard Clays, Clay-slate, Oxide
of Iron, Iron-pyrites, Carbonate of Lime, fragments of Fossil
Organisms, &c, arranged withont any trace of decided structure
or crystallization. And in rocks exhibiting Slaty Cleavage, this
may offcen be clearly demonstrated to be the result of pressure
applied at right angles to the structure itself, thereby causing an
elongation or flattening-out of some, along with a sliding move-
ment of other of the particles. — The foregoing examples are suffi-
cient to indicate the value of Microscopic inquiry in that depart-
ment of Geology which includes the study of the composition and
origin of Bocks, and which is now known as Petrology. It is a study,
however, which can only be profitably pursued by such as are pre-
pared for it by a large amount of Geological and Mineralogical
knowledge ; and to follow it out systematically. will require a large
expenditure of time and patience. As the limited scope of this
Treatise forbids any more extended notice of it, the Reader who
desires further information as to what has been already done, is re-
ferred to the sources mentioned below.*
* See the various Memoirs of Mr. Sorby in the Journal of the Geological
Society, the Proceedings of the Yorkshire Geological Society and elsewhere,
especially the following : — ' On some peculiarities in the Microscopic Struc-
ture of Crystals,' in "Journ. of Geolog. Society," Vol. xiv. p. 242; 'On the
Microscopical Structure of Crystals, indicating the Origin of Minerals and
Bocks,' Op. cit., p. 453 ; ' On the original nature and subsequent alteration of
Mica-Schist,' Op. cit., Vol. xix. p. 401 ; ' Sur l'Application du Microscope a
l'dtude de la Gdologie Physique,' in " Bull. Soc. Geol. de Paris," 1859-60,
p. 568 ; the Memoir by Mr. David Forbes, ' The Microscope in Geology,' in the
"Popular Science Beview," Oct. 1867; the Treatise of Vogelsang, "Philoso-
phic der Geologie und Mikroskopische Gesteinsstudien," Bonn, 1867 ; various
subsequent Memoirs by the same; the Treatise of Zirkel, " Mikroskopische Bes-
chaffenheit der Mineralien u. Gesteine," 1873 ; that of Bosenbusch, "Microsko-
pische Physiographie der petrographische wichtegen Mineralien," 1873, and
that of Jenzsch, " Mikroskopische Flora u. Fauna Krystallinische Mossen-
gesteine," 1868.
CHAPTEE XX.
CRYSTALLIZATION. — POLARIZATION. — MOLECULAR COALESCENCE.
667. Although by far the most numerous and most important
applications of the Microscope are those by which the structure
and actions of Organized beings are made known to us, yet there
are many Mineral substances which constitute both interesting
and beautiful objects ; being remarkable either for the elegance of
their forms or for the beauty of their colours, or for both combined.
The natural forms of Inorganic substances, when in any way sym-
metrical, are so in virtue of that peculiar arrangement of their
particles which is termed crystallization ; and each substance
which crystallizes at all, does so after a certain type or plan,— the
identity or difference of these types furnishing characters of primary
value to the Mineralogist. It does not follow, however, that the
form of the crystal shall be constantly the same for each substance ;
on the contrary, the same plan of crystallization may exhibit itself
under a great variety of forms ; and the study of these in such
minute crystals as are appropriate subjects for observation by the
Microscope, is not only a very interesting application of its powers,
but is capable of affording some valuable hints to the designer.
This is particularly the case with crystals of Snow, which belong
to the ' hexagonal system,' the basis of every figure being a hexagon
of six rays ; for these rays " become encrusted with an endless
variety of secondary formations of the same kind, some consisting
of thin lamina? alone, others of solid but translucent prisms heaped
one upon another, and others gorgeously combining lamina? and
prisms in the richest profusion ;"* the angles by which these figures
are bounded, being invariably 66° or 120°. Beautiful arborescent
forms are not unfrequently produced by the peculiar mode of aggre-
gation of individual crystals : of this we have often an example on
a large scale on a frosted window ; but microscopic crystallizations
sometimes present the same curious phenomenon (Fig. 438). — In
the following list are enumerated some of the most interesting
natural specimens which the Mineral kingdom affords as Micro-
scopic objects ; these should be viewed by reflected light, under a
very low power : —
* See Mr. Glaisher's Memoir on ' Snow-Crystals in 1855,' with numerous
beautiful figures, in " Quart. Joum. of Microsc. Science," VoL iii. (1855), p. 179.
308
CRYSTALLIZATION. — POLAEIZATION.
Antimony, sidphuret
Asbestos
Aventurine
Ditto, artificial
Copper, native
arseniate
malachite-ore
peacock-ore
pyrites (sulphuret)
■ ruby-ore
Iron, ilvaite or Elba-ore
pyrites (sulphuret)
Lapis lazuli
Lead, oxide (minium)
sulphuret (galena)
Silver, crystallized
Tin, crystallized
oxide
sulphuret
Zinc, crystallized.
Fig. 438.
Thin sections of Granite and other rocks of the more or less
regularly-crystalline structure adverted to in the preceding para-
graph, also of Agate, Arragonite, Tremolite, Zeolite, and other
Minerals, are very beautiful objects for the Polariscope.
668. The actual process of the Formation of Crystals maybe
watched under the Microscope
with the greatest facility ; all
that is necessary being to lay
on a slip of glass, previously
warmed, a saturated solution of
the Salt, and to incline the stage
in a slight degree, so that the
drop shall be thicker at its
lower than at its upper edge.
The crystallization will speedily
begin at the upper edge, where
the proportion of liquid to solid
is most quickly reduced by eva-
poration, and will gradually
extend downwards. If it should
go on too slowly, or should
cease altogether, whilst yet a
large proportion of the liquid
remains, the slide may be again
warmed, and the part already
solidified may be re-dissolved,
after which the process will recommence with increased rapidity. —
This interesting spectacle may be watched under any Microscope ;
and the works of Adams and others among the older observers tes-
tify to the great interest which it had for them. It becomes far more
striking, however, when the crystals, as they come into being, are
made to stand out bright upon a dark ground, by the use of the
Spot lens, the Paraboloid, or any other form of Black-ground
illumination ; still more beautiful is the spectacle when the Polar-
izing apparatus is employed, so as to invest the crystals with the
most gorgeous variety of hues. Yery interesting results may often
be obtained from a mixture of two or more Salts ; and some of the
Double Salts give forms of peculiar beauty.* A further variety
* The following directions have been given by Mr. Davies ("Quart. Journ.
of Microsc. Science," N.S., Vol. ii., 1862, p. 128, and Vol. v. p. 205) for obtain-
Crystallized Silver.
RADIATING CRYSTALLIZATION. 809
may be produced by fusing the film of the substance which has
crystallized from its solution ; since on the temperature of the glass
slide during the solidification will depend the size and arrange-
Fig. 439.
Radiating Crystallization of Santonine.
ment of the crystals. Thus Santonine, when crystallizing rapidly
on a very hot plate, forms large crystals radiating from centres
ing these. " He makes a nearly saturated solution, say of the double Sulphate
of Copper and Magnesia ; he dries rapidly a portion on a glass slide, allowing
it to become hot so as to fuse the salt in its water of crystallization ; there then
remains an amorphous film on the hot glass. On allowing the slide to cool
slowly, the particles of the salt will absorb moisture from the atmosphere, and
be<nn to arrange themselves on the glass, commencing from points. If then
placed under the Microscope, the points will be seen starting up here and
there; and from those centres the crystals may be watched as they burst into
blossom and spread their petals on the plate. Starting-points may be made at
pleasure, by touching the film with a fine needle, to enable the moisture to get
under it'; but this treatment renders the centres imperfect. If allowed to go
on, the crystals would slowly cover the plate, or if breathed-on they form im-
mediately ; whereas if it is desired to preserve the flower-like forms on a plain
ground, as soon as they are large enough development is suspended by again
applying gentle heat; the crystals are then covered with pure Canada balsam
and thin glass, to be finished off as usual. The balsam must cover the edges
of the film, or moisture will probably get under it, and crystallization go
creeping on."
810 CRYSTALLIZATION. — POLAKIZ ATION.
without any. undulations ; when the heat is less considerable, the
crystals are smaller, and show concentric waves of very decided
form (Fig. 439), but when the slip of glass is cool, the crystals are
exceedingly minute. It would seem as if these last results were
due to interruptions in the formative process at certain points,
consequent upon the hardening influence of cold, and the starting
of a fresh formation at those points.* A curious example of the
like kind in the crystallization of Sulphate of Copper to which a
small quantity of Sulphate of Magnesia has been added, is shown
in Fig. 440. The same principle has been carried out to a still
Fig. 440.
Radiating Crystallization of Sulphate of Copper and Magnesia.
greater extent in the case of Sulphate of Copper alone, by Mr. R.
Thomas,f who has succeeded, by keeping the slide at a temperature
of from 80° to 90°, in obtaining most singular and beautiful forms
* See Davies on ' Crystallization and the Microscope,' in " Quart. Journ. of
Microsc. Science," N.S., Vol. iv. p. 251.
t See his paper ' On the Crystallization at various Temperatures of the
Double Salt, Sulphate of Magnesia and Sulphate of Zinc,' in " Quart. Journ. of
Microsc. Science," KS., Vol. vi. pp. 137, 177. See also H. N. Draper on
' Crystals for the Micro-Polariscope,' in " Intellectual Observer," Vol. vi.
(1865), p. 437.
RADIATING CRYSTALLIZATION. 811
of spiral crystallization, such as that represented in Fig. 441. Mr.
Slack has shown that a great variety of spiral and curved forms
can be obtained by dissolving metallic salts, or Salicine, Santonine,
&c, in water containing 3 or 4 per cent, of colloid Silica. The
Fig. 441.
Spiral Crystallization of Sulphate of Copper.
nature of the action that takes place may be understood by allow-
ing a drop of the Silica-solution to dry upon a slide ; the result of
which will be the production of a complicated series of cracks,
many of them curvilinear. When a group of crystals in formation
tend to radiate from a centre, the contractions of the Silica will
often give them a tangential pull. Another action of the Silica is
to introduce a very slight curling with just enough elevation above
the slide to exhibit fragments of Newton's rings, when it is illu-
minated with Powell and Lealand's modification of Prof. Smith's
dark -ground illuminator for high powers, and viewed with a
l-8th Objective. With crystalline bodies, these actions add to the
variety of colours to be obtained with the Polariscope, the best
slides exhibiting a series of tertiary tints* — The following List
specifies the Salts and other substances whose crystalline forms
are most interesting. When these are viewed with Polarized light,
some of them exhibit a beautiful variety of colours of their own,
whilst others require the interposition of the Selenite plate for the
development of colour. The substances marked d are distinguished
* ' On the Employment of Colloid Silica in the preparation of Crystals for
the Polariscope,' in "Monthly Microscopical Journal," Vol. v. p. 50.
812 INORGANIC OR MINERAL KINGDOM. — POLARIZATION.
by the curious property termed dicliroism, which was first noticed
by Dr. Wollaston, but has been specially investigated by Sir D.
Brewster.* This property consists in the exhibition of different
colours by these crystals, according to the direction in which the
light is transmitted through them ; a crystal of Chloride of Pla-
tinum, for example, appearing of a deep red when the light passes
along its axis, and of a vivid green when the light is transmitted in
the opposite direction, with various intermediate shades. It is
only possessed by doubly-refracting substances ; and it depends on
the absorption of some of the coloured rays of the light which is
polarized during its passage through the crystal, so that the two
pencils formed by double refraction become differently coloured, —
the degree of difference being regulated by the inclination of the
incident ray to the axis of double refraction.
Acetate of Copper, a
of Manganese
■ of Soda
— — of Zinc
Alum
Arseniate of Potass
Asparagine
Aspartic Acid
Bicarbonate of Potass
Bichromate of Potass
Bichloride of Mercury
Binoxalate of Chromium and Potass
Bitartrate of Ammonia
of Lime
of Potass
Boracic Acid
Borate of Ammonia
of Soda (borax)
Carbonate of Lime (from urine of
horse)
Carbonate of Potass
of Soda
Chlorate of Potass
Chloride of Barium
. of Cobalt
of Copper and Ammonia
■ Palladium, d
of Sodium
Cholesterine
Chromate of Potass
Cinchonoidine
Citric Acid
Cyanide of Mercury
Hippuric Acid
Hypercnanganate of Potass
Iodide of Potassium
■ of Quinine
Mannite
Margarine
Murexide
Muriate of Ammonia
Nitrate of Ammonia
of Barytes
• of Bismuth
■ of Copper
of Potass
■ of Soda
• of Strontian
of Uranium
Oxalic Acid
Oxalate of Ammonia
of Chromium
of Chromium and Ammonia, d
■ ■ of Chromium and Potass, d
- of Lime
of Potass
of Soda
Oxalurate of Ammonia
Phosphate of Ammonia
— Ammoniaco-Magnesian
(triple of urine)
of Lead, d
• of Soda
Platino-chloride of Thallium
Platino-cyanide of Ammonia, d
Prussiate of Potass (red)
Ditto ditto (yellow)
Quinidine
Salicine
Saliginine
Santonine
Stearine
Sugar
Sulphate of Ammonia
of Cadmium
. of Copper
"Philosophical Transactions," 1819.
POLAEIZATION-OBJECTS.
813
Sulphate of Copper and Ammonia
■ of Copper and Magnesia
of Copper and Potass
■ of Iron
> of Iron and Cobalt
of Magnesia
of Mckel
■ of Potassa
Sulphate of Soda
— — — of Zinc
Tartaric Acid
Tartrate of Soda
Uric Acid
Urate of Ammonia
of Soda
It not unfrequently happens that a remarkably -beautiful spe-
cimen of Crystallization developes itself, which the observer desires
to keep for display. In order to do this successfully, it is necessary
to exclude the air ; and Mr. Warrington recommends Castor-oil as
the best preservative. A small quantity of this should be poured
on the crystallized surface, a gentle warmth applied, and a thin
glass cover then laid upon the drop and gradually pressed down ;
and after the superfluous oil has been removed from the margin, a
coat of Gold-size or other varnish is to be applied. — Although most
of the objects furnished by Yegetable and Animal structures, which
are advantageously shown by Polarized light, have been already
noticed in their appropriate places, it will be useful here to reca-
pitulate the principal, with some additions.
Vegetable.
Cuticles, Hairs, and Scales, from
Leaves (§§ 317, 350)
Fibres of Cotton and Flax
Eaphides (§ 328)
Spiral cells' and vessels (§§ 326, 331)
Starch-grains (§ 327)
Wood, longitudinal sections of,
mounted in balsam (§ 340)
Animal.
Fibres and Spicules of Sponges (§ 467)
Polypidoms of Hydrozoa (§ 479)
Spicules of Gorgonise (§ 487)
Polyzoaries (§ 507)
Tongues (Palates) of Gasteropoda
mounted in balsam (§ 538)
Cuttle-fish bone (§ 533)
Scales of Fishes (§§ 617, 618)
Sections of Egg-shells (§ 669)
of Hairs (§§ 621, 622)
, of Quills (§ 623)
, of Horns (§ 624)
of Shells (§§ 522-531)
of Skin (§630)
of Teeth (§§ 615, 616)
of Tendon, longitudinal
(§ 628)
669. Molecular Coalescence. — Eemarkable modifications are
shown in the ordinary forms of crystallizable substances, when the
aggregation of the inorganic particles takes place in the presence
of certain kinds of organic matter ; and a class of facts of great
interest in their bearing upon the mode of formation of various
calcified structures in the bodies of Animals, was brought to light
by the ingenious researches of Mr. Eainey,* whose method of ex-
perimenting essentially consisted in bringing- about a slow decom-
position of the salts of Lime contained in G-um-arabic, by the
agency of Subcarbonate of Potash. The result is the formation of
* See his Treatise " On the Mode of Formation of the Shells of Animals, of
Bone, and of several other structures, by a process of Molecular Coalescence,
demonstrable in certain artificially-formed products" (1858); and his 'Further
Experiments and Observations," in " Quart. Journ. of Microsc. Science," N.S.,
Vol. i. (1861), p. 23.
814 MOLECULAR COALESCENCE.
spheroidal concretions of Carbonate of Lime, which progressively
increase in diameter at the expense of an amorphous deposit
which at first intervenes between them ; two such spherules some-
times coalescing to produce ' dumb-bells,' whilst the coalescence
of a larger number gives rise to the mulberry-like body shown
in Fig. 442, 5. The particles of such composite spherules appear
subsequently to undergo re-arrangement according to a definite
plan, of which the stages are shown at c and d ; and it is upon
this plan that the further increase takes place, by which such larger
concretions as are shown at a, a, are gradually produced. The
structure of these, especially when examined by Polarized light, is
found to correspond very closely with that of the small calculous
concretions which are common in the urine of the Horse, and which
were at one time supposed to have a matrix of cellular structure.
The small calcareous concretions termed ' otoliths,' or ear-stones,
found in the auditory sacs of Fishes, present an arrangement of
their particles essentially the same. Similar concretionary spheroids
have already been mentioned (§ 573) as occurring in the skin of the
Shrimp and other imperfectly-calcified shells of Crustacea; they
occur also in certain imperfect layers of the shells of Mollusca ; and
we have a very good example of them in the outer layer of the
envelope of what is commonly known as a ' soft egg,9 or an * egg
without shell,' the calcareous deposit in the fibrous matting already
described (§ 628) being here insufficient to solidify it. In the ex-
Fig. 442.
Artificial Concretions of Carbonate of Lime.
ternal layer of an ordinary egg-shell, on the other hand, the concre-
tions have enlarged themselves by the progressive accretion of
calcareous particles, so as to form a continuous layer, which con-
sists of a series of polygonal plates resembling those of a tesselated
pavement. In the solid ' shells' of the eggs of the Ostrich and
ARTIFICIAL PRODUCTION OF ORGANIC FORMS. 815
Cassowary, this concretionary layer is of considerable thickness ;
and vertical as well as horizontal sections of it are very interesting
objects, showing also beautiful effects of colour under Polarized
light. And from the researches of Prof. W. 0. "Williamson on the
scales of Pishes (§ 617\ there can be no doubt that much of the
calcareous deposit which they contain is formed upon the same
plan.
670. This line of inquiry has been contemporaneously pursued
by Prof. Harting, of Utrecht, who, working on a plan fundamentally
the same as t.iat of Mr. Rainey (viz., the slow precipitation of
insoluble salts of Lime in the presence of an Organic ' colloid'),
has not only confirmed but greatly extended his results ; showing
that with animal colloids (such as egg-albumen, blood- serum, or a
solution of gelatine) a much greater variety of forms may be thus
produced, many of them having a strong resemblance to Calcareous
structures hitherto known only as occurring in the bodies of Animals
of various classes. The mode of experimenting usually followed by
Prof. Harting, was to cover the hollow of an ordinary porcelain
plate with a layer of the organic liquid, to the depth of from 0'4 to
0*6 of an inch ; and then to immerse in the border of the liquid,
but at diametrically opposite points, the solid salts intended to act
on one another by double decomposition, such as Muriate, Nitrate,
or Acetate of Lime, and Carbonate of Potass or Soda ; so that,
being very gradually dissolved, the two substances may come
slowly to act upon each other, and may throw down their precipitate
in the midst of the ' colloid.' The whole is then covered with a
plate of glass, and left for some days in a state of perfect tran-
quillity ; when there begin to appear at various spots on the
surface, minute points reflecting light, which gradually increase
and coalesce, so as to form a crust that comes to adhere to the
border of the plate ; whilst another portion of the precipitate sub-
sides, and covers the bottom of the plate. Round the two spots
where the salts are placed in the first instance, the calcareous
deposits have a different character ; so that in the same experi-
ment several very distinct products are generally obtained, each in
some particular spot. The length of time requisite is found to vary
with the temperature, being generally from two to eight weeks.
By the introduction of such a colouring matter as madder, log-
wood, or carmine, the concretions take the hue of the one employed.
"When these concretions are treated with dilute acid, so that their
calcareous particles are wholly dissolved- out, there is found to
remain a basis- substance which preserves the form of each ;
this, which consists of the ' colloid' somewhat modified, is termed
by Harting calco-globuline. — Besides the globular concretions with
the peculiar concentric and radiating arrangement obtained by
Mr. Rainey (Pig. 442), Prof. Harting obtained a great variety of
forms bearing a more or less close resemblance to the following : —
1. The ' discoliths' and < cyatholiths' of Prof. Huxley (§§ 367, 368) ;
the presence of which alike in the protoplasmic Bathybius and in
816 MOLECULAR COALESCENCE.— MICRO-CHEMISTRY.
the Radiolarian MyxobracMa is thus accounted for.* 2. The tuber-
culated ' spicules' of Alcyonaria (Figs. 308. 309), and the very
similar spicules in the mantle of some species of Boris (§ 532).
3. Lamellae of 'prismatic shell- substance' (§ 522), which are very
closely imitated by crusts formed of flattened polyhedra, found
on the surface of the ' colloid.' 4: The spheroidal concretions
which form a sort of rudimentary shell within the body of Limax
(§ 532). 5. The sinuous lamellee which intervene between the
parallel plates of the ' sepiostaire' of the Cuttle-fish (§ 533) ; the
imitation of this being singularly exact. 6. The calcareous con-
cretions that give solidity to the ' shell' of the Bird's egg ; the
semblance of which Prof. Harting was able to produce in situ, by
dissolving away the calcareous component of the egg-shell by dilute
acid, then immersing the entire egg in a concentrated solution of
chloride of calcium, and transferring it thence to a concentrated
solution of carbonate of potass, with which, in some cases, a little
phosphate of soda was mixed.f Other forms of remarkable regu-
larity and definiteness, differing entirely from anything that
ordinary crystallization would produce, but not known to have their
parallels in living bodies, have been obtained by Prof. Harting.
Looking to the relations between the calcareous deposits in the
scales of Fishes (§§ 617, 618) and those by which Bones and Teeth
are solidified, it can scarcely be doubted that the principle of 'mole-
cular coalescence' is applicable to the latter, as well as to the
former ; and that an extension and variation of this method of
experimenting would throw much light on the process of ossification
and tooth-formation.
671. Micro- Chemistry of Poisons. — By a judicious combination
of Microscopical with Chemical research, the application of re-agents
may be made effectual for the detection of Poisonous or other sub-
stances, in quantities far more minute than have been previously
supposed to be recognizable. Thus it is stated by Dr. WormleyJ
that Micro- Chemical analysis enables us by a very few minutes'
labour to recognize with unerring certainty the reaction of the
100,000th part of a grain of either Hydrocyanic Acid, Mercury, or
Arsenic ; and that in many other instances we can easily detect by
its means the presence of very minute quantities of substances, the
true nature of which could only be otherwise determined in com-
paratively large quantity, and by considerable labour. This
inquiry may be prosecuted, however, not only by the application of
* It is a fact of no little interest that Prof. Giinibel has been able to discover
'coccoliths' in Calcareous strata of various Geological periods, extending back
to the Silurian. See "Neues Jahrb. f. Mineral., GeoL.u. Paheont. ," 1870, p. 753 ;
and " Nature," Nov. 3rd, 1870, Vol. iii. p. 16.
t See Prof. Harting' s "Kecherches de Morphologie Synthe'tique sur la pro-
duction artificielle de quelques Formations Calcaires Inorganiques, publics
par l'Acadenrie Ecyale Neerlandaise des Sciences," Amsterdam, 1872; and
"Quart. Journ. of Microsc. Science," Vol. xii. p. 118; also a Memoir on "Mole-
cular Coalescence," by W. M. Ord, M.B., in the same volume, p. 219.
I " Micro-Chemistry of Poisons," New York, 1867.
MICKO-CHEMISTEY. 817
ordinary Chemical Tests under the Microscope, but also by the use
of other means of recognition which the use of the Microscope
affords. Thus it was originally shown by Dr. Guy* that by the
careful sublimation of Arsenic and Arsenious Acid, — the sublimates
being deposited upon small disks of thin -glass, — these are dis-
tinctly recognizable by the forms they present under the Microscope
(especially the Binocular) in extremely minute quantities ; and that
the same method of procedure may be applied to the volatile metals,
Mercury, Cadmium, Selenium, Tellurium, and some of their Salts,
and to some other volatile bodies, as Sal- Ammoniac, Camphor, and
Sulphur. The method of sublimation was afterwards extended by
Dr. Helwigf to the Vegetable Alkaloids, such as Morphine, Strych-
nine, Yeratrine, &c. And subsequently Dr. Guy, repeating and
confirming Dr. Helwig's observations, has shown that the same
method may be further extended to such Animal products as the
constituents of the Blood and of Urine, and to volatile and decom-
posable Organic substances generally. J It maybe anticipated that
by the careful prosecution of Micro-Chemical inquiry, especially
with the aid of the Spectroscope, the detection of Poisons and other
substances in very minute quantity will come to be accomplished
with such facility and certainty as have until lately been scarcely
conceivable.
* 'On the Microscopic Characters of the Crystals of Arsenious Acid,' in
" Trans, of Microsc. Society," Vol. ix. (1861), p. 50.
t " Das Mikroskop in der Toxikologie," 1865.
j ' On Microscopic Sublimates ; and especially on the Sublimates of the
Alkaloids,' in " Trans, of Eoyal Microsc. Soc," Vol. xvi. (1868), p. 1 ; also
' ; Pharmaceutical Journal," June to September, 1867.
3g
APPENDIX.
[The passage of the latter portion of this volume through the press
having been delayed for more than a twelvemonth by other de-
mands upon the Author's time, he has here to mention some of the
more important improvements in the Microscope and its appliances,
which have come under his notice since its earlier Chapters were
printed off.]
New Portable Compound Microscope.
Fig. 443.
-A portable Microscope
Swift's Portable Microscope, as set up for us?.
was long since devised by Messrs. Powell and Lealand, which can
be packed into a flat case of convenient size by unscrewing the
SWIFT'S PORTABLE MICROSCOPE. 819
body from the arm, folding together the legs of the tripod-stand,
and turning the stage on a joint, so as to lie parallel to the pillar.
By introducing a similar joint into the arm itself, Mr. Swift makes
the body fold back upon the pillars without any unscrewing ; and
whilst his Portable Microscope when set up for use (Fig. 443)
is a steady and convenient instrument, suitable for all ordinary
work, it packs, when folded together (Fig. 444), into a box only 9
inches long, 4 inches wide, and 2-| inches deep, which also holds a
Fig. Ui.
Swift's Portable Microscope, as folded for packing.
good deal of accessory apparatus. The rack-movement and fine
adjustment are both very good ; the stage is of full size, and has
an object-carrier working on glass bearings for smoothness of
action; and its aperture is surrounded by a rotating ring, into
which may be fitted either a slide-holder for rotating the object
in the axis of the body, or a film of mica or selenite for varying the
action of Polarized light, the ring being made to revolve by pressing
the finger against a milled-head at the front of the stage. To the
under side of the stage may be adapted a special form of Achromatic
Condenser (including Polarizing prism) devised by Mr. Swift, of
which a description will be presently given. And the small box
which holds the Microscope and two objectives, can also be made
to receive a double ISTose-piece, Camera Lucida, Stage-forceps,
Side-condenser, Live-box, Analyzing prism, and Zoophyte-trough.
Loiv-angled Objectives. — The Author has been very glad to learn
that the doctrine he has advocated throughout, as to the superior
value of Objectives of moderate aperture for most purposes of
scientific investigation, is now coming to be generally recognized ;
several Makers having recently devoted themselves specially to the
construction of such combinations, in which the most perfect correction
possible shall be attained, — instead of making objectives of small
aperture by stopping-down combinations which had been con-
structed for larger apertures, but were not good enough to bear
them. Besides the superiority in focal depth which such Objectives
possess, they further admit of being used much more conveniently
(in consequence of the greater distance that can be obtained between
the front lens and the object) for the examination of opaque objects
with side-illumination. This is especially the case with the excellent
small-angled 1-oth and l-6th made by Mr. Swift expressly with this
view.
3 g 2
820
APPENDIX. — BLANKLEY'S MICA-SELENITE STAGE.
Glass Revolving Stage. — The invention of this stage (Fig. 139),
attributed to MM. Nachet, is claimed by Mr. Zentmayer, of
Philadelphia ; who states that he first constructed it in 1862, and
that a Microscope which he made in 1864 for Dr. Keen, of Phila-
delphia, was shown by Dr. K. to MM. Nachet, who copied from it
the arrangement in question.
Combination of Mica-film with Selenite.- — -The variety of tints
given by a Selenite-film under Polarized light, is so greatly in-
creased by the interposition of a rotating film of Mica, that two
Selenites — red and blue — with a Mica-film, are found to give the
entire series of colours obtainable from any number of Selenite-
films, either separately or in combination with each other. The
Revolving Mica- Selenite Stage (Fig. 445) devised by Mr. Blankley,
and made by Mr. Swift, furnishes a very simple and effective
Fig. 445.
f wBnpPil
Blankley's Bevolving Mica-Selerdte Stage.
means of obtaining these beautiful effects ; the Mica-film being set
in a diaphragm which can be made to rotate by applying the
finger at the front edge of the stage ; whilst the Selenites are so
placed in a slide, that either of them can be brought under
the aperture as desired.
Swift's New Achromatic Condenser. — In this ingenious piece of
apparatus (Fig. 446) are combined the advantages of (1) an Achro-
matic Condenser, a, centred by two milled-headed screws, c, c, and
having an angle of 140°, which fits it for use with Objectives of
very wide angular aperture, whilst, by removing the upper com-
bination, it is made to suit lower powers ; (2) a contracting Dia-
phragm worked by the lever b ; (8) a revolving Diaphragm, e, with
four apertures, into which can be fitted either (a) a series of three
central stops, giving a Black-ground illumination scarcely inferior
to that of the paraboloid, and capable of being used with the small
angle l-5th, (b) tinted or ground-glass Moderators, or (c) two
Selenite-films for the Polarizing apparatus ; (4) a Polarizing
prism, f, mounted on an excentric arm, so as to be brought under
the axis of the condenser when not in use, and thrown out when
not wanted ; and (5) an upper arm carrying two revolving cells
geared together by fine teeth (one of them shown at d, while the
SWIFT'S NEW ACHROMATIC CONDENSER.
821
other is under the condenser), so that a revolving motion may be
given to either by acting on the other; one of these cells carries a
plate of mica, the revolution of which over the selenite-films gives a
Fig. 446.
Swift's New Achromatic Condenser.
great variety of colour-tints with Polarized light ; while the other
serves to receive oblique-light disks, to which rotation can be given
by the same means. — The special advantage of this Condenser lies
in its having the polarizing prism, the selenite- and mica-films, the
black-ground and oblique-light stops, and the moderator, all brought
close under the back lens of the Achromatic ; whilst it combines in
itself all the most important appliances which the ' sub-stage' of
822 APPENDIX.— SWIFT'S POETABLE MICROSCOPE LAMP.
Fig. 447.
Fig. 448.
Messrs. Ross's or of Messrs. Powell and Lealand's large Microscope,
or the ' secondary body ' of Messrs. Beck's, is adapted to receive,
either separately or in combination.
Siviftfs Portable Microscope Lamp. — Every Microscopist who
desires to exhibit his objects by artificial light elsewhere than at
his own home, has desired a lamp suitable for this purpose,
adjustable to any height, and ca-
pable of being packed in a small
compass and of being carried in any
position without spilling the liquid it
burns. This desideratumis now sup-
plied by Mr. Swift, who has devoted
much ingenuity to the construction
of such a lamp ; the special diffi-
culty being to prevent leakage
from the passage through which
the wick rises, with-
out interfering with
the ascent of the
fluid. The lamp
(Fig. 447) is mount-
ed on a telescope-
pillar, which sup-
ports it steadily at
any height from 4
to 12 inches ; and
this is screwed into
a tripod foot. By
pushing in the tele-
scope - pillar, un-
screwing the tripod,
and inverting it
over the chimney
(Fig. 448), the lamp
can be packed into
a tube 7-| inches
long and If inch in
diameter. It gives
a good flame, and burns for two hours. The size of the reser-
voir might of course be increased, so as to enable the lamp to
burn longer ; but this would add to the bulk of its case.
Section-Gutting Machines. — An entirely new apparatus for cut-
ting thin sections has been devised by Prof. Biscoe (U.S.), which
has the great advantage of being adaptable to the stage of a
Microscope, so that the section may be cut in view of the
magnified picture, instead of under the guidance of ordinary vision.
The principle of the apparatus is that the object is attached to
the platform, whilst the cutter is carried in a frame which slides
over it, supported by three micrometer- screws ; by turning which
SECTIOX-CUTTEES. — FREEZING MICBOTOME. 823
tlie height of the cutter above the platform, and consequently the
thickness of the section, are regulated.* — Another apparatus,
devised by Mr. George Hoggan, M.B., is adapted for cutting
sections either of hard or of soft substances. The peculiarity
of its arrangement for the former consists in the fixation of the
body to be cut (such as a piece of bone, a tooth, or an Echinus-
spine) on a horizontal carriage, progressively advanced by a micro-
meter-screw ; while the sections are cut with a fine saw work-
ing in a vertical plane between guides, so that, as the blade
cannot swerve in the least, the face of the section is perfectly
true, and slices may be cut both thin and smooth enough to admit
of being mounted for the purposes of the Microscopist, without
any further preparation than washing-off the sawdust. By a
modification in the arrangement of its parts, this apparatus can be
used also for cutting sections of soft substances with a knife or
razor.f
Freezing Microtome. — Notwithstanding the various methods
which have been devised for hardening soft tissues of which it is
desired to obtain very thin sections, and supporting them by enve-
lopes of paraffin, carrot, or elder-pith, there are some to which no
hardening process is so applicable as that of freezing ; and Prof.
Eutherford has devised a Microtome for this purpose, which has
been found extremely effective. It consists, in principle, of an
ordinary Section-instrument (Fig. 108), the tube of which is sur-
rounded by a box containing a freezing mixture ; and the requisite
hardening is thus secured during the whole process of section-
cutting. For success in the operation, however, several minute
precautions must be observed, which are fully detailed by the
inventor, whose directions should be implicitly followed.* This
Microtome may be equally well employed for cutting sections of
substances which do not require to be hardened by freezing.
Sunk Cells. — The ' sand-blast' process has been applied to the
excavation of small deep cavities in glass, which are very convenient
for mounting certain classes of objects either in Balsam or liquid.
Although the bottom of the cell is left by this process with a
roughened surface, yet when the cell is filled with balsam, the
granulation disappears ; and if the cell is to be filled with some
fluid whose refractive index differs much from that of glass, a little
balsam may be first run-in and hardened, whereby the surface will be
rendered clear. For dry or opaque objects no such preparation is
necessary, the ground-glass bottom making a soft and agreeable
back-ground ; but if a black back-ground should be desired, a little
Asphalte or Brunswick-black varnish may be first run-in. — These
cells may be obtained from Mr. C. Baker, Holborn.
Cement for Covering-glass. — It frequently happens that it is
desirable to remove the covering-glass from objects that have been
* See " Quart. Journ. of Microsc. Science," Vol. xiv. p. 182.
t " Journ. of the Quekett Microsc. Club,'' Vol. iii. p. 266.
% " See Monthly Microsc. Journal," Vol. x. p. 18o.
824 APPENDIX— NACHET'S OPTICAL ILLUSION.
dry-mounted; either in order to examine the objects without the
intervention of any medium, or because (as has frequently happened
in the Author's experience) the under side of the covering-glass has
become dimmed by the deposit of a fine dew. It is very desirable,
therefore, that the cement used for attaching the cover should be
one which, while sufficiently firm to hold it securely, should be so
easily liquefied as to allow of its ready removal. Mr. T. Charters
"White has found a mixture of four or five parts of ordinary yellow
bees-wax with one part of Canada balsam fulfil these require-
ments perfectly. If a little of this cement be melted in a spoon,
it may be painted-on with a warm smooth wire, so as to fill-in the
angle between the edge of the covering-glass and the slide; and
it has the great advantage over other cements of not ' running-in,'
as it is at once cooled on touching the slide ; while a very gentle
warmth is sufficient to loosen it, so as to allow of the cover being
readily removed when desired.*
M. Nachefs Optical Illusion. — It was discovered by M. ISTachet,
in the course of his Microscopic examination of the markings of
Diatoms, that the hexagonal form commonly attributed to them is
really due to a visual or (more probably) a mental illusion. For
he found that if a series of round black dots be made upon a white
or light-coloured ground, arranged as in Fig. 449, with narrow in-
terspaces between them, the dots will appear hexagonal. The illu-
Fig. 449.
sion is so strong that even when we know the dots to be circular,
it is difficult to accept them as such, when the paper is held at
about eighteen inches from the eye.
* " Journ. of Quekett Microsc. Club," Vol. iii. p. 232.
INDEX.
Aberration, Chromatic, 41, 42.
Spherical, 38 — 39.
means of reducing and
correcting, 39 — 43.
Absorption bands, 115 — 121.
Acalephs, see Medusa.
Acanthometrina, 566.
Acarida, 728, 729.
Acklya prolifera, 355, 356.
Achnanthes, 334, 335.
Achromatic Condenser, 134—137 ;
use of, 186, 187.
Achromatic Correction, 6 ; principle
of, 42, 43,
Achromatic Objectives, see Object-
Glasses.
-4cz'we£a- parasitism in Infusoria, 498.
Acrocladia, spines of, 589.
Actinocyclus, 328.
Actinophrys, 470 — 472; reproduction
of, 478—480.
Actinoptychus, 329.
Actinotrocha larva of Sipunculus, 667.
Actinozoa, 588—590.
Adipose Tissue, 763.
Adjustment of Focus, 95, 176 — 179.
Adjustment of Object-glass, 44, 45,
179—182.
JEthalium septicurn, 391.
Agamic eggs, of Botifera, 507, 508 ;
of Entomostraca, 680, 681 ; of
Insects, 726, 727.
Agarics, generation of, 394, 395.
Agassiz, Prof., on scales of Fish, 745.
Agrion, circulation in larva of, 714.
Air-bubbles, miscroscopic appearances
of, 198 ; in microscopic prepara-
tions, 248, 255, 264.
Air-pump, use of, in mounting objects,
248.
Albuminous substances, tests for, 229.
I Alburnum, 429, 438.
! Alcyonian Zoophytes, 588, 590.
Alcyonidium, 620.
ALG.E, higher, miscroscopic structure
of; 370 — 377 ; (see Protophyta).
Allman, Prof., on Tubuiarida, 595
note ; on Fresh-water Polyzoa, 622
note ; on Appendicularia, 631 note.
Alternation of Generations, 414, 5S8
—592.
Alveolina, 523, 524.
Amaranthus, seeds of, 459, 460.
Amaroucium, 625 — 627.
Ambulacral disks of Echinida, 597,
598.
Amici, Prof., his early construction
of Achromatic lenses, 43 ; his in-
vention of the Immersion system,
46 ; his Prism for oblique illumi-
nation, 138 ; his drawing Camera,
127.
Amoeba, 471 — 476 ; reproduction of,
477—479.
Amoeboid state of Volvox, 287, 288 ;
of protoplasm of Chara, 369 note ;
of protoplasm of roots of Mosses,
399 ; of Myxogastric Fungi, 391 ;
of colourless Blood-corpuscles, 754,
755.
AmpMpleura pellucida, resolution of,
213.
Amphistegina, 545.
Amphitetras, 332.
Anacharis alsinastrum, formation of
cells in, 419 : cyclosis in, 420, 421.
Anagallis, petal of, 454.
Androspores of CEdogonium, 361.
Anguillulce, 661.
Angular Apertui'e of Object-glasses,
43, 201 note: means of determining,
202 note; limitation of, for Bino-
826
INDEX.
cular, 69—72 ; real value of, 202—
208.
Anguliferece, 332, 333.
Animal Tissues, formation of, 732 —
736.
Animalcule- cage, 158, 159.
Animalcules, 482 ; (see Infusoria,
Bhizopoda, and Botifera).
Animals, distinction of, from Plants,
270—272, 462—464.
Annelida, 664 — 673 ; marine, circu-
lation in, 665, 666 ; metamor-
phoses of, 666 — 668 ; remarkable
forms of, 668 — 671; luminosity of,
671 ; fresh-water, 672, 673.
Annual Layers of Wood, 437, 438.
Annular Ducts, 431.
Annulosa, 659 ; see Entozoa, Tur-
bellaria, and Annelida.
Annulus of Ferns, 407.
Anodon, shell of, 637 ; parasitic embryo
of, 648, 649 ; ciliary action on gills
of, 656.
Anomia, fungi in shell of, 388.
Ant, red, integument of, 691.
Antedon, development of, 613 — 615.
Antennae of Insects, 707 — 709.
Antheridia, of Chara, 867 ; of Mar-
chantia, 398 ; of Mosses, 402 ; of
Ferns, 410 ; — see Antherozoids.
Antherozoids, of Volvox, 289 ; of
Vaucheria, 355 ; of Sphaeroplea,
360 ; of (Edogonium, 361 ; of
Characea?, 367 ; of Fuci, 372 ; of
Floridese, 375 ; of Marchantia, 398 ;
of Mosses, 402; of Ferns, 411.
Anthers, structure of, 454, 455.
Anthony, Dr., on scale of Lepisma,
697 ; on battledoor scales, 695 ;
on tongue of Fly, 711 note.
Antirrhinum, seeds of, 459, 460.
Aperture, Angular, see Angular
Aperture.
Aphides, agamic reproduction of, 726,
727.
Aphthae, fungus of, 388.
Aplanatic Searcher, 40.
Apothecia of Lichens, 378.
Appendicularia, 630, 631.
Apple, cuticle of, 446.
Aptinoptychus, 329.
Apus, 676, 679.
Aquarium Miser oscope, 108.
Aquatic Box, 158, 159.
Arachnida, microscopic forms of,
728, 729 ; eyes of, 729 ; respira-
tory organs of, 730 ; feet of, 730 ;
spinning apparatus of, 730, 731.
Arachnoidiscus, 330.
Aralia, cellular parenchyma of, 416.
Arcella, 476, 477.
Archegonia, of Marchantia, 398 ; of
Mosses, 402; of Ferns, 410, 411.
Archer, Mr., on zoospores of Des-
midiaceaB, 296 ; on production of
Amceboids, 369 ; on fresh-water
Badiolaria, 473.
Arenicola, 664.
Areolar tissue, 757, 758.
Argulus, 683.
Aristolochia, stem of, 443.
Artemia, 677, 680.
Ascaris, 60; fungous vegetation on,
387.
Asci, of Lichens. 378; of Fungi,
391.
Ascidia parallelogramma, 624, 625.
Ascidians, 624 ; Compound, 625 —
627 ; Social, 627—629 ; develop-
ment of, 629—630.
Asphalte-varnish, 237.
A spidis ca-form of Trichoda, 493.
Aspddium, fructification of, 406.
Asplanchna, 506, 507, 512.
Asteriada, skeleton of, 603 ; meta-
morphoses of, 609, 610.
Asterolampra, 829.
Asteromphalus, 329.
Astr omnia, 563.
Astrophyton, 602.
Astrorhiza, 477.
Auditory vesicles of Mollusks, 657 ;
development of, 651, 655.
Aulacodiscus, 331.
Avicida, nacre of, 636, 637.
Avicularia of Polyzoa, 622, 623.
Axile bodies of sensory papillae,
752.
Axis-cylinder of Nerve-fibres, 771 ;
ultimate ditribution of, 772.
Azure-blue butterfly, scales of, 695.
Bacillaria paradoxa, 321, 324 ; move-
ments of, 318.
Bacteria, 3S0, 381.
Bacteriastrum, 333.
INDEX.
827
Baer, Von, on development, 17.
Bailey, Prof., his Diatomaceous tests,
213 ; on siliceous cuticle, 413 ; on
internal siliceous casts of Forami-
nifera, 546 note.
Baker, Mr., his Travelling Micro-
scope, 107, 108 ; his Air-pump,
248 note : his Pond-stick, 267.
Balanus, metamorphoses of, 684, 685.
Balbiani, M., on generation of In-
fusoria, 496—498.
Balsam, Canada, see Canada Balsam.
Barbadoes, Polycystina of, 565.
Bark, structure of, 441, 442.
Barnacle, metamorphoses of, 684, 685.
Basidia of Fungi, 391.
Bastian, Dr., on production of Bac-
teria, 380, 381.
Bat, hair of, 747, 748; cartilage of
ear of, 764.
Batrachospermece, 364, 365.
Battledoor scale of Polyommatus, 695.
Bathybius, 465, 795.
Beading of Insect-scales, Dr. Royston
Pigott on, 693—701.
Beale, Prof., his Pocket Microscope,
106 ; his Demonstrating Micro-
scope, 106 ; his use of viscid media,
231, 232 ; his preservative liquid,
252 ; his blue injection, 784 ; his
method of making thin-glass cells,
258 ; of making deep cells, 262 ;
his staining-fluid, 228, 229, 785;
his views of Tissue-formation, 733
— 735 ; his observations on Blood-
corpuscles, 751.
Beck, Messrs., their Student's Micro-
scope, 91, 92 ; their Popular Mi-
croscope, 96, 97 ; their Large Com-
pound Microscope, 104, 105 ; their
Achromatic Condenser, 135 ; their
arrangement of Polarizing appa-
ratus, 146 ; their Compiessoriums,
163, 164; their Binocular Magnifier,
218 note.
Mr. Joseph, on scales of Thysa-
nurae, 696—700.
Mr. Eichd., his Dissecting Micro-
scope, 83 — 85 ; his Disk-holder,
155, 156 ; his Side-Keflector, 150 ;
his Vertical Illuminator, 153, 154 ;
on scales of Thysanurae, 700 ; on
Spider's threads, 731.
Bee, eyes of, 704—706 ; hairs of, 702;
proboscis, 711, 712 ; wings of, 720 ;
sting of, 724 ; reproduction of, 727.
Bei-g-mehl, 341.
Bermuda-earth, 329, 330.
Beroe, 593.
Biddulphia, 331 ; markings on, 308 ;
self-division of, 313, 314.
Biliary Follicles, 765.
Biloculina, 521.
Binary Subdivision, of Palmoglaea,
276 ; of Protococcus, 278, 279 ; of
Desmidiacece, 293—295 ; of Diato-
maceae, 313 — 315; of Confervaceae,
358 ; of cells of Phanerogamia, 418 ;
of Rhizopods, 478 ; of Infusoria,
489, 490 ; of Cartilage-cells, 764.
Binocular Eye-piece, 66.
Magnifier, Nachet's, 83 —
85 ; Beck's, 218 note.
Microscopes, Stereoscopic,
principles of construction of, 57-
60; advantages of, 72, 73; Ob-
jectives appropriate to, 69 — 72 ;
different forms of, Compound, 60 —
69 ; Simple, 83—85 ; Student's, 96
— 98; Non- Stereoscopic, 110, 111.
Vision, 57—60, 71
Bipinnaria-larvfi of Star-fish, 609.
Bird, Dr. Golding, on preparation of
Zoophytes, 583.
Birds, bone of, 738, 739 ; feathers of,
747 ; blood of, 758 ; lungs of, 787,
788.
Bird's-head processes of Polyzoa, 622.
Bisulphide of Carbon, mounting ob-
jects in, 252, 327, 328.
Bivalve Mollusks, shells of, 632—641.
Black-ground Illuminators, 140 — 144.
Black- Japan varnish, 237.
Blankley, Mr., his Selenite Stage,
820.
Blenny, viviparous, scales of, 743.
Blights, of Corn, 392, ?93.
Blood, Absorption-bands of, 120, 121.
Blood-disks of Vertebrata, 751 — 754;
mode of examining and preserving,
754, 755 ; circulation of, see Cir-
culation.
Blood-vessels, injection of, 780 — 785 ;
disposition of, in different parts,
785—789
Bockett Lamp, 171.
INDEX.
Bone, structure of, 736 — 739 ; mode
of making sections of, 739, 740.
Bones, fossil, examination of, 764.
Botryllians, 587, 588.
Botrytis, of silkworms, 383—385 ; of
potato, 393.
Bowerbank, Dr., his researches on
Sponges, 571 note; on structure
of Shells, 635, 642 ; on Agates,
758.
BowerbanMa, 618—620.
Brachionus, 502, 506, 512.
Brachiopoda, structure of Shell of,
639—641.
Brady, Mr. H. B., on Saccamina, 532 ;
on Loftusia, 538.
Braitbwaite, Dr., on cell formation,
274 ; on Sphagnacese, 404 — 406.
Bran ckiop oda, 6 7 7 — 6 8 0.
Branchipus, 680.
Braun, Prof., on development of
Pediastrese, 301—303.
Brewster, SirD., on single magnifiers,
50 ; on siliceous cuticles, 412 ; on
structure of Nacre, 635 ; on Di-
chroism, 767.
Brightwell, Mr., on Diatomaceae, 332
note; 333 note; on Asplanchna,
506, 507; on Noctiluca, 595 note.
Brooke, Mr., his nose-piece, 130.
Brownian Movement, 199.
Browning, Mr., his Rotating Micro-
scope, 95 ; his Spectroscope Eye-
piece, 115, 116 ; his Spectro- micro-
meter, 117—121.
Brunswick-black varnish, 237.
Bryozoa, see Poltzoa.
Buccinum, palate of, 645, 647 ; egg-
capsules of, 649 ; development of,
652.
Bugs, 690 ; wings of, 721.
Bugula avicularia, 622, 623.
Built-up Cells, 261, 262.
Bulbels of Chara, 367 ; of Marchantia,
397.
Bulimina, 541.
Bull's Eye Condenser, 148 — 150 ; use
of, 191—193.
Burdock, stem of, 443.
Busk, Mr. G., on Volvox, 284—289 ;
on structure of Starch-grain, 427 ;
on Polyzoa, 623.
Butterflies, see Lepidoptera.
Cabinets, Microscopic, 266.
Cactus, raphides of, 428.
Calcaire Gx-ossier, 793, 795.
Calcareous Deposits, Rainey and
Harting on, 813—816.
Calcareous Sponges, 567, 571 note.
Calcarina, 544.
Calycantlius, stem of, 442.
Calyptra of Mosses, 402.
Cambium-layer, 442.
Camera Lucida, 126 — 128 ; use of in
Micrometry, 129, 130.
Campanularidce, 581.
Campylodiscus, 324.
Canada Balsam, use of as Cement,
237, 238 ; mounting of objects in,
242—251.
Canaliculi of Bone, 738, 739.
Canal svstem of Foraminifera, 520,
543—560.
Capillaries, circulation in, 774 — 777 ;
injection of, 780 — 785 ; distribution
of, 785—789.
Capsule of Mosses, 402 ; of Ferns, 407.
Carmine Injections, 784, 785 ; Stain-
ing liquid, 230, 231, 785.
Carp, scales of, 744, 745.
Carpenteria, 541.
Carrot, seeds of, 460.
Carter, Mr. H. J., on Volvox, 290
note ; on production of Rhizopods
from Plants, 369 note ; on sexes in
Rhizopods, 479 ; on development of
Sponges, 567, 572.
Cartilage, structure of, 764, 765.
Caryophillia, 588.
Caryopltyllum, seeds of, 459, 460.
Caterpillars, feet of, 724.
Cedar, stem of, 439.
Cells for mounting ohjects, of Cement,
257, 258 ; of Thin-glass, 258, 259 ;
of Plate-glass, 259, 260 , of Tube,
261 ; of Metal, 261 ; built-up, 261 ;
262 ; sand-blast. 823 ; mounting
objects in, 262—264.
Animal, formation of, 732 —
735.
Vegetable, 272—275 ; in Pba-
nerogamia, 415 — 429 ; cyclosis in,
419 — 423 ; thickening deposits in,
423 — 425 ; spiral deposits in, 425,
426 ; starch-grains in, 426, 427 J
raphides in, 428.
INDEX.
829
Cellular Tissue, Vegetable, ordinary
form of, 415—417; stellate, 417,
418 ; formation of, 419.
Cellulose, 273.
Cements, Microscopic, 236—239, 824.
Cement-Cells, mode of making, 257.
Cementum of Teeth, 743.
Cephalopods, shell of, 643 ; chroma-
tophores of, 657, 658.
Ceramiacece, 375 — 377.
Ceramidium, 376.
Cercomonad, Messrs. Dallinger and
Drysdale on development of, 494 —
496.
Cestoid Entozoa, 659, 660.
Chcetocerece, 332, 333.
Chcetophoracece, 363, 364.
Chalk, Foraminifera, &c, of, 466 ;
formation of, 795 — 798.
Characece, 365 — 369 ; cyclosis of fluid
in, 366 ; multiplication of by
gonidia, 367 ; sexual apparatus of,
367—369.
Cheilostomata, 621.
Cherry-stone, cells of, 424. .
Chemical Re agents, use of, in Micro-
scopic research, 227 — 230.
Chemistry, microscopical, 816.
Chevalier, M., his early construction
of Achromatic objectives, 43 ; his
drawing Camera, 128.
Chilodon, teeth of, 486 , self-division
of, 489.
Chirodota, calcareous skeleton of, 607.
Chitine of Insects, 691.
Choroid, pigment of, 760.
Chromatic Aberration, 41, 42 ; means
of reducing and correcting, 42, 43.
Chromatophores of Cephalopods, 657,
658.
Chrysaora, development of, 585 — 588.
Chyle, corpuscles of, 753.
Cidaris, spines of, 600.
Ciliary action, nature of, 501, 502 ;
in Protophytes* 271, 279, 283; in
Infusoria, 486—488 ; on gills of
Mollusks, 656 ; on epithelium of
Yertebrata, 762.
Ciliobrachiata, 617.
Circulation of Blood, in Vertebrata,
771—780; in Insects, 713—715;
alternating, in Tunicata, 624, 629.
Circulation, Vegetable, see Cyclosis.
Cirrhipeds, metamorphoses of, 15,
684, 685.
Cladocera, 679.
Claparede, M., on development of
Neritina, 655 note ; on Tomopteris,
671 note.
Claparede and Lachmann, on Lie-
berkiihnia, 468 ; on Amoeba, 475 ;
on Infusoria, 513 note.
Clark, Prof. Jas., on Sponges, 568.
Clavellinidce, 627—629.
Cleanliness, importance of, to Micro-
scope, 173, 174; in mounting ob-
jects, 264, 265.
Clematis, stem of, 436.
Closterium, movement of fluid in, 291
— 293 ; binary subdivision of, 293,
294 ; multiplication of by gonidia,
296 ; conjugation of, 297, 298.
Clypeaster, spines of, 600.
Coal, nature of, 790—792.
Coalescence, molecular, 813 — 816.
Cobweb-Micrometer, 121, 122.
Coccoliths, 465, 816.
Coccospheres, 465, 466.
Cocconeidce, 333, 334.
Cockchafer, cellular integument of,
691 ; eyes of, 705 ; antenna of, 708,
709 ; spiracle of larva of, 717.
Cockle of Wheat, 661.
Coddington lens, 51.
Coenosarc of Hydrozoa, 579.
Ccenurus, 660.
Cohn, Dr., his account of various
states of Piotococcus, 278—282 ;
his researches on Volvox, 288 —
290 ; on Stephanosphsera, 290 ; on
Sphseroplea, 359, 360 ; on repro-
duction of Rotifera, 509.
Coleoptera, integument of, 691 ; an-
tennae of, 707, 708 ; mouth of, 709.
Collection of Objects, general direc-
tions for, 266—269.
Collema, 352.
Collins, Mr., his Harley Binocular,
97, 98 ; his Eye- piece caps, 97 ; his
Aquarium Microscope, 108 ; his
Graduating Diaphragm, 134, 137 ;
his Air-pump, 248 note ; his Book-
Cabinet, 266.
Collomia, spiral fibres of, 425, 426.
Colonial nervous system of Polyzoa,
619, 620.
830
INDEX.
Colourless corpuscles of Blood, 758 —
760.
Columella of Mosses, 404.
Comatula, metamorphosis of, 613 —
615 ; nervous system of, 751.
Compound Microscope, optical prin-
ciples of, 52 — 56 ; mechanical con-
struction of, 74—77, 85—87 ; Third
class, 87 — 90 ; Second class, 90 —
98 ; First class, 99 — 105 ; for spe-
cial purposes, 106 — 111, 818.
Compressorium, 161 — 164 ; use of,
182, 183.
Concave lenses, refraction by, 36.
Conceptacles of Marchantia, 397,398.
Concretions, calcareous, 813 — 816.
Condenser, Achromatic, use of, 134 —
136; Webster, 136; Swift's new,
820.
Hemispherical, 139.
for Opaque objects, ordi-
nary, 148 ; Bull's eye, 149 ; mode
of using, 191—193.
Confervacece, 358 ; self-division of,
358 ; zoospores of, 359 ; sexual re-
production of, 359—362.
Coniferce, peculiar woody fibre of,
430 ; absence of ducts in, 432 ;
structure of stem in, 439; fossil,
791.
Conjugates, 362, 363.
Conjugation, of Palmoglaea, 276 ; of
Desmidiacese, 296— 298 ; ofDiato-
rnacese, 315 — 317; of Conjuga-
tes, 362 — 363; (supposed) of Ac-
tinophrys, 478, 479 ; of Gregari-
nida, 481 ; of Infusoria, 496—498.
Connective Tissue, 757 ; corpuscles of,
735, 756, 758.
Conochilus, 507.
Contractile vesicle, of Volvox, 284 ;
of Actinophrys, 471 ; of Infusoria,
488, 489.
Conversion of Belief, 58—60, 67, 68.
Convex lenses, refraction by, 83 — 36,
formation of images by, 37.
Copepoda, 678.
Coquilla-nut, cells of, 424.
Corallines, true, 376 ; Zoophytic, 581.
Cork, 441.
Corn, blights of, 392, 393, 661.
Corn-grains, husk of, 461.
Corns, structure of, 761.
Cornnspira, 520.
Coi-puscles of Blood, 751 — 755.
Correction of Object-glasses, for
Spherical Aberration, 39, 40 ; for
Chromatic Aberration, 42, 43 ; for
thickness of covering glass, 44, 45,
179—182.
Corynactis, thread-cells of, 590.
Cotyledons, 458.
Coscinodiscece, 327, 328.
Cosmarium, swarming of granules in,
293 ; self-division of, 294 ; conju-
gation of, 297 ; development of,
297.
Crab, shell-structure of, 686 ; meta-
morphoses of, 687.
Crabro, integument of, 691.
Crag-Formation, 799.
Cricket, gastric teeth of, 713 ; sounds
produced by, 720.
Crinoidea, skeleton of, 604 ; meta-
morphosis of, 613 — 615.
Cristatella, 621.
Cristellaria, 540.
Crouch, Mr., his Educational Mi-
croscope, 87, 88 ; his Student's Bi-
nocular, 96 ; his adapter for Beck's
Side-reflector, 151.
Crusta Petrosa of Teeth, 743.
Crustacea, 674 — 688 ; lower forms
of, 674 — 676 ; Entomostracous, 676
—682 ; Suctorial, 683 ; Cirrh'iped,
684—685 ; Decapod, shell of, 686 ;
metamorphoses of, 687, 688.
Cryptogamia, general plan of struc-
ture of, 370, 414 ; see Protophyta,
Algse, Lichens, Fungi, Hepaticas,
Mosses, Ferns, &c.
Crystallization, Microscopic, 807 —
813.
Ctenoid scales of Fish, 744, 745.
Ctenophora, 592, 594.
Ctenosomata, 621.
Curcidionidce, scales of, 692 ; elytra
of, 703 ; foot of, 723.
Cuticle of Animals, 759.
of Equisetacese, 412 ; of
Flowering Plants, 445—452.
Cutis Vera, 758.
Cuttle-fish, shell of, 643 ; chromato-
phores, 658.
Cyanthus, seeds of, 460.
Cycloclypeus, 552, 553.
INDEX.
831
Cycloid scales of Fish, 744, 745.
Cyclops, 678 ; fertility of, 681.
Cyclosis, in Closterium, 291, 292 ; in
Diatornacea?, 305 ; in Chara, 365
— 367 ; in cells of Phanerogamia,
419—423 ; in Phizopods, 468.
Cyclosto?nata, 621.
Cydippe, 592, 593.
Cymbellece, 335.
Cynipidce, ovipositor of, 724.
Cypris, 677.
Cyprcea, structure of shell of, 642.
Cystic Entozoa, 660.
Cysticercus, 660.
Cytherina, 677, 758.
Dactylocalix, 570 note.
Dallinger, Mr., on development of
Infusoria, 494, 495.
Dalyell, Sir J. G. , on development of
Medusae, 585—587.
Damar- Varnish, 237, 251.
Dapknia, 679 ; ephippial eggs of, 681,
682 ; development of, 682.
Davies, Mr., on Microscopic. Crystal-
lization, 808 note.
Dawson, Dr., on Eozoon Canadense,
555.
Deane's Gelatine, 253.
De Barv, Dr., on Myxogastric Fungi,
391, 392.
Decapod Crustacea, shell of, 686, 687 ;
metamorphoses of, 687, 688.
Defining power of Object-glasses, 200,
201.
Demodex follicidorum, 729.
Demonstrating Microscope, Beale's,
106, 107.
Dendritina, 522.
Dendrodns, teeth of, 802.
Dentine of Teeth, 740—742.
Depressions, distinction of, from ele-
vations, 197.
Dermestes, hair of, 702.
Desiccation, tolerance of, by Infuso-
ria, 495, 496 ; by Potifera, 509,
510.
Desmidiacece, general structure of,
290, 291 ; movement of fluid in,
291 — 292 ; binary subdivision of,
293 — 295 ; formation of gonidia
by, 296 : origination and multipli-
cation of varieties in, 304 ; conju-
gation of, 296—298 ; development
of, 297 ; classification of, 298—299 ;
collection of, 300.
Deutzia, stellate hairs of, 448.
Development, of Annelida, 666—671 ;
of Anodon, 648, 649 ; of Asci-
dians, 629 ; ofCirrhipeds, 684—685;
of Crab, 687, 688; of Desmidiaceas,
297; of Diatomacese, 317 ; ofEchino-
dermata, 60S — 615 ; of Embryo
(Animal) 572, 573, 749 ; of Embryo
(Vegetable) 457, 458 ; of Ento-
mostraca, 678 — 682; of Ferns,
408—412 ; of Gasteropods, 649—
655 ; of Leaves, 418, 419 ; of
Medusae, 579 — 588 ; of Mosses,
404 ; of Nudibranchiata, 650 ; of
Palmoglaea, 276 ; of Pollen-grains,
454, 455; of Protococcus, 278 —
280; of Sponges, 572, 573; of
Stem, 442—444 ; of Volvox, 285—
287.
Diagonal Scales, 124, 130.
Diamond-beetle, scales of, 692 , elytra
of, 703 ; foot of, 723.
Diaphragm Eye-piece, Slack's, 126.
Diaphragm-Plate, 133—137.
Diatoma, 322, 323.
Diatornacea, Vegetable nature of, 304,
305 ; cohesion of frustules of, 306,
307 ; siliceous envelope of, 308,
309 ; markings of, 308—312 ; bi-
nary subdivision of, 313 — 315 ; go-
nidia of, 315 ; conjugation of, 315 —
317 ; limits of species of, 318—339 ;
movements of, 318, 319 ; classifica-
tion of, 319, 320, ; general habits
of, 339, 340 ; fossilized deposits of,
340—342, 755 ; collection of, 342,
344 ; mounting of, 344, 345 ; their
value as tests, 211 — 214 ; erroneous
appearances of, 196, 824.
Dichroism, 812.
Dicotyledonous Stems, structure of,
435—444.
Dictyocalyx, 570.
Dictyoloma, seeds of, 460.
Didemnians, 627.
Didymoprium, self-division of, 293 ;
conjugation of, 298, 299.
Differentiation, progressive, in Vege-
table Cell-formation, 272 ; in Ani-
mal Cell-formation, 734.
832
INDEX.
Difflugia, 476, 477.
Diffraction of Light, errors arising
from, 195, 196, 210.
Diphtheria, fungus of, 389.
Dipping-Tubes, 165.
Diptera, mouth of, 710 ; halteres of,
721 ; ovipositors of, 725.
Discorbina, 542.
Disk- holder, Beck's, 155 ; Morris's,
156.
Dispersion, chromatic, 41, 42.
Dissecting Microscope, Quekett's
simple, 80, 81; Field's, 81, 82;
Beck's, 83—85 ; Nachet's, 85.
Dissection, Microscopic, 217 — 226.
Distoma, 661.
Docidium, microgonidia of, 296.
Dog, epidermis of foot of, 761.
D'Orbigny, M., his Classification of
Foraminifera, 515, 517.
Doris, palate of, 646 ; spicules of,
643 ; development of, 650—652.
Dorsal Vessel of Insects, 714.
Dotted Ducts, 431, 432.
Double-bodied Miscroscope, 110.
Doublet, Wollaston's, 50.
Dragon-fly, eyes of, 705 ; larva of,
714, 718.
Drawing Apparatus, 126 — 129.
Draw-Tube, 112.
Dropping Bottle, 256.
Drosera, hairs of, 448.
Dry-mounting of objects, 239 — 242.
Drysdale, Dr., on development of
Infusoria, 494, 495.
Ducts, of Plants, 431, 432.
Dujardin, M., on Sarcode, 462 ; on
Bhizopods, 13, 466 ; on Foramini-
fera, 515 ; on Botifera, 510 — 513.
Duramen, 429, 438.
Dusideia, skeleton of, 571.
Dytiscus, foot of, 723 ; trachea and
spiracle of, 716, 717.
Eagle-Ray, teeth of, 741.
Earvng, wings of, 720.
Eccremocarpus, seeds of, 460.
Echinida, shell of, 596, 597 ; ambu-
lacra! disks of, 597, 598 ; spines of,
598 — 601 ; mode of making sections
of, 604—606; pedicellarias of, 601;
teeth of, 601 — 603 ; metamor-
phosis of, 610—613.
Echinodekmata, skeleton of, 596 —
608 ; metamorphoses of, 608 — 615.
Ecker, Prof., on eggs of Hydra,
578.
Ectosarc of Bhizopods, 467, 733.
Educational Microscopes, 87 — 89.
Edwards, Prof. (U.S.), on develop-
ment of spores of QEdogonium, 359 ;
on Amoeba, 476.
Eel, scales of, 744 ; gills of, 786, 787.
Eels, of paste and vinegar, 660.
Eggs of Insects, 725, 726 ; see
Winter-eggs.
Egg-shell, fibrous structure of, 756 ;
calcareovs deposit in, 814, 816.
Ehrenbei-g, Prof., his researches on In-
fusoria, 13, 482, 483 ; on Rotifera,
14, 482, 483, 507 ; on Polycystina,
565, 566 note; on composition of
Greensands, 546 note.
Elastic Ligaments, 757.
Elaters of Marchantia, 399.
Elementarv Parts of Animal body,
732—736 ; see Tissues.
Elevations, distinction of, from de-
pressionsj 197.
Elytra of Beetles, 720.
Embryo, Animal ; — see Development.
Vegetable, development of,
in Phanerogamia, 457 — 459; in
Ferns, 411, 412.
Empusa, 385.
Enamel of Teeth, 742.
Encrinites, 604, 613.
Encysting process of Infusoria, 490 —
496.
End-bulbs of Nerves, 773.
Endochrome of Vegetable cell, 273,
274 ; of Diatomacese, 305.
Endogenous Stems, structure of, 434,
435.
Endosarc of Bhizopods, 467, 733.
Enterobryus, 386—388.
Entomostracoits Crustacea, 676 — 682 ;
classification of, 677 — 680; repro-
duction of, 680—682.
Entozoa, 659—662 ; Cystic, 660 ;
Nematoid, 660 — 661 ; Trematode,
662.
Eozoic Limestones, 560 note, 799, 800.
Eozoon Canadense, 555 — 560.
Ephemera, larva of, 690, 714, 718.
Ephippium of Daphnia, 681, 682.
INDEX.
833
Epidermis, structure of, 759 — 761.
Epithelium, 761 ; ciliated, 762.
Epithemia, 320 ; conjugation of, 316.
Equisetacece, cuticle of, 412 ; spores of,
413.
Erecting Binocular, 65.
Erecting Prism, Nachet's, 114.
Erector, Lister's, 113.
Errors of Interpretation, 193 — 200.
Eunotiece, 320, 321.
Euplectella, 569.
Eupodiscece, 331.
Euryale, skeleton of,- 602.
Exogenous Stems, structure of, 435 —
444.
Eyes, care of, 172, 173.
Eyes of Mollusks, 656, 657; of
Insects, 704—707; of Trilobite,801.
Eye-piece, 54; Huyghenian, 54, 55
Bamsden's, 56 ; Kellner's, 56
Binocular, 66 ; Erecting, 114
Spectroscopic, 116 ; Micrometric,
121—125; Diaphragm, 126.
Collins's shades for, 97.
Ealconer, Dr., on bones of fossil
Tortoise, 803.
Fallacies of Microscopy, 193—200.
Earrants's Medium, 254.
Earre, Dr. A., his researches on
Bowerbankia, 15, 620.
Fat-cells, 763 ; capillaries of, 785.
Feathers, structure of, 746, 750.
Feet of Insects, 721 — 723 ; of Spiders,
730.
Fermentation, influence of vegeta-
tion on, 379—382.
Ferns, 402 — 406 ; scalariform ducts
of, 406; fructification of, 406—
408 ; spores of, 408 ; prothallium
of, 409; antheridia of, 410; arche-
gonia of, 410, 411 ; generation and
development of, 413.
Fertilization of ovule, in Flowering-
plants, 458, 459.
Fibre- cells of anthers, 455 ; of seeds,
425, 426.
Fibres, Muscular, 766 — 770.
Nervous, 770 — 774.
Spiral, of Plants, 425, 426,
430, 431.
Fibrillse of Muscle, structure of, 767,
768.
Fibro-Cartilage, 765.
Fibro- Vascular Tissue, 420.
Fibrous Tissues of Animals, 756 — 758 ;
formation of, 735.
Fiddian's Lamp, 171.
Field's Dissecting and Mounting
Microscope, 81, 82.
Educational Miscroscope, 87.
Filiferous capsules of Zoophytes, 589,
590.
Finders, 131—133.
Fine Adjustment, 75 ; uses of, 177 —
179.
Fin-feet of Branchiopoda, 677 — 680.
Fishes, bone of, 738, 739 ; teeth of,
740, 741 ; scales of, 743 - 746 ;
blood of, 751 — 753 ; circulation in,
777 ; gills of, 786, 787.
Fishing-tubes, 165.
Flatness of field of Object-glasses,
203, 204.
Flint, organic structure in, 797 ; ex-
amination of, 798.
Flint-Glass, dispersive power of, 42.
Floridece, 375—377.
Floscularians, 510, 511.
Flowers, small, as Microscopic ob-
jects, 453 ; structure of parts of,
453—461.
Fluid, mounting objects in, 255 — 257,
262—264.
Fluke, 661.
Flustra, 14, 15, 616—620.
Fly, fungous disease of, 385 ; number
of objects furnished by, 689 ; cir-
culation in, 715; tongue of, 710;
spiracle of, 717; wing of, 719;
foot of, 722.
Focal Adjustment, 176 ; precautions
in making, 177 ; errors arising from
imperfection of, 178, 179, 196,197.
Focal Depth of Objectives, 201, 202 ;
increase of with Binocular, 72.
Focke, on Closterium, 296 ; on Dia-
tomaceae, 315.
Follicles of Glands, 765, 766.
Foot of Fly, 722 ; of Dytiscus, 723 ;
of Spider, 730.
Foraminifera, 514 — 562 ; their re-
lation to Bhizopods, 470, 515 ;
their general structure, 515 — 520 ;
porcellanous, 520 — 529; arenaceous,
529—539 : vitreous, 539 — 560 ;
3H
834
INDEX.
collection and mounting of, 560 —
562 ; fossil deposits of, see Fossil
Foraminifera ; mode of making
sections of, 224 note.
Forbes, Mr. D., on structure of
Eocks, 804—806.
Forbes, Prof. Ed., on Hydroids and
Medusa?, 582, 5J38.
Forceps. 166 ; stage, 155 ; slider, 246.
Forflculidce, wings of, 720.
Formed Material, 733—736.
Fossil Bone, 803, 804.
Diatomacese, 340—342, 793,
794.
Foraminifera, 524, 532, 536—
538, 544. 555—560, 793—800.
Polycystina, 565, 566.
Sponges, 796, 797.
Teeth, 801—803.
Wood, 790—792.
Fowl, lung of, 7
Fragillarieoz, 322.
Freezing Microtome, 823.
Frog, blood of, 752 — 754 ; pigment-
cells of, 760, 761 ; circulation in
web of, 774 — 776; in tongue of,
776 ; in lung of, 776 : structure
of lung of, 787, 788.
Fructification, of Cbara, 367—369;
of Fuci, 372—375 ; of FlorideaB,
375—377 ; of Lichens, 377 ; of
Fungi, 391, 395 ; of Marcbantia,
395, 398 ; of Mosses, 402—404 ;
of Ferns, 406 — 410 ; of Equisetaceae,
413.
Fucacece, 372 — -375 ; sexual apparatus
of, 372—374; development of, 375.
Fungi, simplest forms of, 378 — 383 ;
in bodies of living Animals, 383 —
389; in substance, or on surface, of
Plants, 392, 393 ; amoeboid states
of, 391—392; higher forms of,
394, 395 ; universal diffusion of
sporules of, 390—393.
Furcularians, 512.
Furlong,Mr.,on Polycystina, 566 note.
Fusulina, 544, 545.
Gad-flies, ovipositor of, 725.
Gall-flies, ovipositor of, 724.
Gallionella, 326.
Galls of Plants, 724.
Ganglion-Cells, 770.
Ganoid scales of Fish, 745.
Gasteropoda, structure of shell of,
642, 643 ; palates of, 644—647 ;
development of, 649 — 655 ; organs
of sense of, 656, 657.
Gastric teeth of Insects, 713.
Gastrula, 573, 651, 727.
Gelatine, Deane's, 253 ; see Glyce-
rine-jelly.
Gelatinous Nerve-fibres, 771 — 773.
Generation, distinguished from
Growth, 276, 414.
Geology, applications of Microscope
to, 790—806.
Geranium-j)eta.\, peculiar cells of,
453.
Germinal Matter, 733—736.
Gillett, Mr., his White-cloud illumi-
nator, 144.
Gills, of Mollusks, ciliary motion on,
650 ; of Fishes, distribution of
vessels in, 786, 787 ; of Water-
newt, circulation in, 776.
Gizzard, of Insects, 713.
Glands, structure of, 765, 766.
Glandular woody fibre of Coniferse,
430.
Glass Slides, 233.
Thin, 234—236.
Glaucium, cyclosis in hairs of, 423.
Globigerina, 540.
Globigerina-mud, 272, 464, 540 ; its
relation to Chalk-formation, 795 —
798.
Globigerinida, 540 — 545.
Glochidium, 648. 649.
Glue, Liquid, uses of, 237, 242.
, Marine, uses of, 238, 239, 792.
Glycerine, use of, in mounting objects,
231, 232, 253—255.
Glycerine- Jelly, Lawrance's, 253 ;
Pimmington's, 254 note.
Glycerine-Medium, Farrants's, 254.
Gnats, transparent larvae of, 714.
Goadby's Solution, 255.
Gold-size, use of, 236, 237.
Goniometer, 100, 125.
Gomphonemeo3, 335, 336.
Gonidia, multiplication by, in Des-
midiaceas, 296; in Pediastreae, 302;
in Diatoniaceae, 314 — 317 ; in Hy-
drodictyon, 357; in Chara, 367;
in Lichens, 377.
INDEX.
' S35
Gonozooids of Hydrozoa, 5/9.
Gordius, 661.
Gorgonia, spicules of, 591.
Gosse, Mr., on masticatory appa-
ratus of Rotifera, 504 — 506 ; on
sexes of Rotifera, 507 ; on Meli-
certa, 511 ; on thread-cells of
Zoophytes, 589, 590.
Grammatophora, 325; its use as test,
213.
Grantia, structure of, 567, 571, 573.
Grasses, silicified cuticle of, 448.
Gray, Dr., on palates of Gastero-
pods, 646; on development of
Bucciuum, 652.
Green-sands, Prof. Ekrenberg on
composition of, 546 note, 799.
Gregarinida, 479 — 481.
Gromia, 469, 470.
Growing-Slide, 157, 158.
Growth, distinguished from Gene-
ration, 276, 414.
Guano, Diatomaceas of, 343.
Gumbel, Dr., on Eozoon, 560 note.
Guy, Dr., on sublimation .of Alka-
loids, 816.
Haeckel, Prof., on Monerozoa, 464,
465 ; on Myxobrachia, 466 : on
Thalassicolla, 482 ; on Polycystina,
564 ; on Calcareous Sponges, 571
note, 572 ; on Gastraaa theory, 572
note; on Coelenterata, 574 note.
Hcematococcus 347 ; its relations to
Protococcus, 278.
Haime, M. Jules, on metamorphosis
of Trichoda, 491—493.
Hairs, of Insects, 702 ; of Mam-
mals, 747—749.
, of Vegetable cuticles, 448,
rotation of fluid in, 422, 423.
Halichondria, spicules of, 568.
Halifax, Dr., on making Sections of
Insects, 691.
Haliomma, 564, 565.
Ealiotis, palate of, 646.
Balodactylus, 620.
Halteres of Diptera, 721.
Hand-Magnifiers, 51, 52, 77, 78.
Harley Binocular, 97, 9S.
Hartig, Prof., on production of
Pthizopods from Plants, 369
note.
Harting, Prof., on calcareous Concre,
tions, 815, 816.
Hartnack, M., his Immersion-lense,
46 ; his diagonal Micrometer, 124 ;
on Surirella, 214.
Hartwig, Dr., onRhizopods, 468 note,
47S.
Harvest-bug, 729.
Haversian Canals of bone, 737.
Haustellate Mouth, 712, 713.
Hazel, stem of, 438.
Hearing, supposed organs of in Insects,
709.
Heat, tolerance of, by Infusoria, 495-
496.
Heliopelta, 330, 331.
Helix, palate of, 644, 645.
Hemiptera, wings of, 721.
Hemispherical Condenser, Reade's,
139.
Hendry, Mr., on Diatom-tests, 212.
Hepaticce, 395 — 399 ; see Marchantia.
Hep worth, Mr., on feet of Insects,
722.
Heterostegina, 552.
Hexiradiate Sponges, 569.
Hicks, Dr., on Amoeboid state of
Volvox, 287, 288 ; on Unicellular
Algae, 347 ; on gonidia of Lichens,
352, 377 ; on Amoeboid production
in root-fibres of Mosses, 399 ; on
eyes of Insects, 705 ; on peculiar
organs of sense in Insects, 709, 713,
721.
Hincks, Mr. T., on Hydroid Zoo-
phytes, 579.
Himantidiurii, 320.
Hippocrepian Polyzoa, 621, 622.
Hofmeister, Prof., on Higher Crypt-
togamia, 411 note.
Hogg, Mr., on development of
L}"mn8eus, 652.
Hoggan, Mr. G., his Section-cutter,
823.
Hollyhock, pollen-grains of, 206, 457.
Holothurida, skeletons of, 606 — 608 ;
development of, 613 note.
Holtenia, 569.
Hoofs, structure of, 750, 751.
Hooker, Dr. J. D., on Antarctic Dia-
tomaceee, 340.
Hornet, wings of, 720.
Horns, structure of, 750, 751.
3h2
S$6
INDEX.
Houghton, Rev. W., on Glochidium,
649.
Hudson, Dr., on Fedalion, 507.
Huxley, Prof., on cell- formation in
Spbagnacese, 404 ; on Bathybius,
465 ; on Coecoliths, 465, 466 ; on
Botifera, 509, 513 ; on Thalassi-
eolla, 481 ; on Sponges, 572 ; on
Noctiluca, 594 ; on Shell of Mol-
lusca, 635 ; on Appendicularia, 631 ;
on Blood of Annelida, 665 ; on
Shell of Crustacea, 686 note ; on
Reproduction of Aphides, 726, 727.
Huyghenian eye-piece, 54, 55.
Hyalodiscus, 213, 327.
Hyalonema, 570.
Hydatinu, 512 ; reproduction of, 503.
Hydra, 3 ; structure of, 574 — 577 ;
multiplication of, 577, 578.
Hydra tuba, development of Acalephs
from, 585-588.
Hydrodlctyon, 356, 357.
Htdrozoa, 574 — 578 ; production of
Medusae from, 17, 579—588.
Hyla, preparation of nerves of, 773.
Hymenoptera, proboscis of, 711 ;
wings of, 719 ; stings and ovi-
positors of, 724, 725.
Ice-Plant, cuticle of, 448.
Jchneumonidos , ovipositor of, 724.
Illumination of Opaque objects, 191—
193 ; of Transparent objects, 185 —
190.
Illuminator, Black-ground, 140 — 142,
189, 190.
Oblique, 137—140, 187
-189.
Parabolic, 141, 142.
Reade's Hemispherical,
139, 140.
Side, 148, 151.
Vertical, 151, 154.
Wenham's Keflex,
143.
142,
White- Cloud, 144, 145.
Immersion-Lenses, 46.
Images, formation of, by convex
lenses, 37.
Indian Corn, cuticle of, 446, 449.
Indicator, Quekett's, 126.
Indusium of Ferns, 407.
Infusorial Earths, 341, 342.
Infusokia, 483 — 501 ; forms of, 4S4
—486 ; movements of, 486, 487 ;
internal structure of, 487 — 489 ;
binary subdivision of, 489, 490 ;
encysting process of, 490 — 495;
sexual generation of, 496 — 499 ;
peculiar forms of, 499, 500.
Injections of Bloodvessels, mode of
making, 780—785.
Inman, Dr., on mounting petals, 453.
Insects, great numbers of objects
furnished by, 689, 690 ; micro-
scopic forms of, 690 , antennas of,
707, 709 ; circulation of blood in,
713—715 ; eggs of, 725, 726 ; eyes
of, 704—707 ; feet of, 721—724 ;
gastric teeth of, 713 ; hairs of, 702 ;
integument of, 691 ; mouth of,
709 — 713 ; organs of hearing in,
709 ; of smell in, 721 ; of taste in,
713 ; ovipositors of, 724, 725 ;
scales of, 692 — 702; spiracles of,
717, 718; stings of, 724 ; traehese
of, 715—717 ; wings of, 721.
Intermediate Skeleton of Forami-
nifera, 520, 544, 547.
Internal Casts of Foraminifera, 542,
546,554, 558, 788.
Inverted Microscope, Dr. L. Smith's,
108, 109.
Iris, structure of leaf of, 449, 451.
Iris-diaphragm, 134.
Isthmia, 331, 332 ; markings on, 308,
309 ; self-division of, 314,
Itch-Acarus, 728.
lulus, fungous vegetation in, 386.
Jackson, Mr., his Eye-piece Micro-
meter, 122, 123.
Jackson -model for Compound Micro-
scope, 86.
Jelly-fish, development of, 584 — 588.
Jewel-lenses, 50.
Jukes, Prof., on Foraminiferal reef,
793.
Kellner's Eye-piece, 56.
Kidneys, structure of, 766.
Kingsley, Rev. C, 14, 24, 28.
Kleinenberg, Dr., on Hydra, 578 note,
733 note.
Kolliker, Prof., on Fungi in Shells,&c,
388 note.
IXDEX.
S37
Labelling of Objects, 265, 266.
Labyrinthodon, tooth of, 763, 764.
Lachmann, see Claparede and Lach-
mann.'
Lacinularia, Huxley on,- -507 note.
Lacunae of Bone, 695, 696.
Ladd's Student's Microscope, 91, 92.
Lagena, 515, 539, 540.
Laguncula, 617 — 619.
LameUicoraes, antennae of, 708.
Lamps, microscope, 169 — 171, 822.
Lankester, Mr. E. Ray, on cell-
layers of Embryo, 572 note ; on
development of Limnseus, 651.
Larvae of Echinoderms, 608 — 615.
Laticiferous vessels, 441.
Laurentian Formation of Canada, 555,
799, 800 ; of Europe, 560 note.
Leaves, structure of, 445 — 452 ; mode
of examining, 452.
Leech, teeth of, 672.
Leeson, Dr., his double-refracting Go-
niometer,l25 ; his Selenite-plate, 147.
Leeuenhoek, his early researches, 2.
Legg, Mr., on collection of Eorarui-
nifera, 560, 561.
Leidy, Dr., on parasitic Eungi, 386 —
388.
Lenses, refraction by, 33 — 46.
Lepidocyrtus, scales of, 698 — 701.
Lepidoptera, scales of, 692 — 696 ;
proboscis of, 712, 713 ; wings of,
703, 720 ; eggs of, 725, 726.
Lepidosteus, bony scales of, 739, 745.
Lepisma, scales of, 696, 697.
Lepralia, 617, 621.
Lemcea, 683.
Levant-Mud, microscopic organisms
of, 793, 794.
Lever of Contact, 235.
Libellula, eyes of, 705 ; respiration of
larva of, 718.
Liber, 441, 442.
Lichens, 377, 378.
Lichmophorece, 321, 322.
Lieberkuhn, on Gregarina, 481 ; on
Spongilla, 572.
Lieberkiihu(speculum),151, 152; mode
of using, 193.
Lieberkiihnia, 468.
Ligaments, structure of, 756, 757.
Light, suitable for Microscope, 169 —
172; position of, 171, 172; arrange-
ment of, for Transparent objects,
182—190 ; for Opaque objects, 195
—198.
Ligneous Tissue, 429, 430.
Ligula of Insects, 710, 711.
Limax, shell of, 612 ; palate of, 645.
Limpet, palate of, 645.
Liquid Glue, use of, 237, 242.
Lister, Mr., his improvements in
Achromatic lenses, 44; his Erector,
113; his Zoophyte-trough, 160,
161 ; his observations oh Zoophyteg,
581 ; on Social Ascidians, 6^8,
630 note.
Lituolida, 529—539.
Live-Box, 158, 159.
Liver, structure of, 724, 725.
Liverwort, see Marchautia.
Lobb, Mr., on binary subdivision in.
Micrasterias, 294, 295.
Lobosa, 467, 473 — 477.
Loftusia, 538.
Logan, Sir W., on Laurentian For-
mation, 555 note, 800.
Lophophore of Polyzoa, 617*
Lophyropoda, 676, 677.
Lowne, Mr., on feet of Insects, 723;
on development of Insects, 727,
728.
Lubbock, Sir J., on Daphnia, 6S1 ;
on Thysanura, 696 — 700.
Luders, Mad., her observations on
yeast, 379—381.
Luminosity of Noctiluca, 594, 595.
Lunss of Reptiles, 787; of Birds,
787, 788 ; of Mammals, 789.
Lyccenidce, scales of, 692, 695.
Lymnceus, development of, 651, 652.
Lymph, corpuscles of, 753.
Machilis, 697.
Macro-gonidia, of Volvox, 286, of
Pediastrea?, 301 ; of Hydrodictyon,
357.
Maddox, Dr., his Growing-Slide, 158.
Magnifying power, mode of deter-
mining, 214 — 216; augmentation
of, 175, 176 ; of different Objec-
tives, 214—216.
Mahogany, section of, 441.
Mallow, pollen-grains of, 456, 457;
their use as tests, 71 — 206.
Malpighian bodies of Kidney, 766.
INDEX.
Malpighian layer of Skin, 760.
Maltwood's Finder, 132.
Mammals, bone of, 736 — 739 ; teeth
of, 742, 743; hairs, &c, of, 747,
748 ; blood of, 751 — 755 ; lungs of,
78S, 789.
Man, teeth of, 742, 743 ; hair of, 748,
749; blood of, 751—755.
Mandibulate mouth of Insects, 709.
Marchantia, general structure of, 395 ;
stomata of, 396 ; conceptacles of,
397, 398 ; sexual apparatus of, 398,
399.
Margaritacece, shells of, 635 — 637.
Marine Glue, uses of, 238, 239, 792.
Masticating apparatus of Rotifera,
' 504, 505.
Mastogloia, 338, 339.
Media, Preservative, 252 — 255.
Medullary Rays, 417, 439—441.
Sheath, 430, 436.
Medusa, development of, from Zoo-
phytes, 584—588.
Medusoids of Hydroida, 579—582.
Megalopa-l&rva, of Crab, 687, 688.
Megatherium, teeth of, 743.
Melanospevmeos, 372 — 375.
M cliceriwns, 510, 511.
Melolontha, see Cockchafer.
Melosira, 326 ; self-division of, 314 ;
conjugation of, 317.
Menelaus, scale of, 694.
Meniscus Lenses, refraction by, 37.
Meridion, 320, 321.
Mesembryanthemum, cuticle of, 448.
Mesocarpus, 363.
Mesogloia, 370, 371.
Metamorphosis, 15 : of Annelids, 666
—671 ; of Cirrhipeds, 684, 685 ; of
Ascidians, 629, 630; of higher
Crustacea, 6S7, 688 ; of Entomo-
straca, 682; of Echinoderms, 608—
615; of Infusoria, 491—493; of
Mollusks, 648—655.
Mica-Selenite Stage, S20.
Micrasterias, binary sub -division of,
294, 295 ; gonidia of, 296.
Micro- Chemistry, 816, 817.
Micro-gonidia, of Protococcus, 280 ;
of Desmidiaceas, 296 ; of Pedias-
treae, 302 ; of Hydrodictyon, 357.
Micrometer, Cobweb, 121; Eye-piece,
122—125.
Micrometry, by Micrometer, 121 —
125 ; by Camera Lucida, 129.
Micropyle of Vegetable Ovule, 457.
Microscope, support required for,
168, 169; care of 173, 174; general
arrangement of, 174 — 182; for
Transparent objects, 182 — 190 ;
for Opaque objects, 190—193.
Binocular, see Binocular
Microscope.
Compound, see Com-
pound Microscope.
Simple, see Simple
Microscope.
, Aquarium, 108.
Demonstrating, 106.
Dissecting, 80 — 85.
Double-bodied, 110.
Educational, 87—89,
— Inverted. 108, 109.
Pocket, 106.
Popular, 96, 97.
Portable, new, 819.
Students, 90—98.
Travelling, 107, 108.
Microscopic Dissection, 217 — 220.
Micro-Spectroscope, 115, 116; appli-
cations of, 115 — 121.
Microtome, 219.
Microzymes, 3S2.
Mildew, fungous vegetation of, 390 —
392.
Miliolida, 520—529.
Millon's test for Albuminous sub-
stances, 229
Milne-Edwards, M., his researches
on Compound Ascidians, 15, 630
note ; on Development of Annelida,
666 note.
Mineral Objects, 807—813.
Minnoio, circulation in, 776.
Mites, 728.
Moderator, Rainey's, 171.
Molecular Coalescence, 813 — 816.
Movement, 199, 200.
Mollusca, shells of, 632 — 644; pa-
lates of, 644 — 647; development of,
648 — 655 ; ciliary motion on gills
of, 656 ; organs of sense of, 656,
657.
Monerozoa, 464, 466, 530.
Monocotyledonous Stems, structure
of, 434, 435.
INDEX.
Monothalamous Foraniinifera, 515.
Morula, 572.
Morehouse, Mr., on Lepisma-scale,
698.
Morris, Mr., his Object-holder, 156;
his method of mounting Zoophytes,
583.
Mosses, structure of, 399, 400 ;
sexual apparatus of, 401 — 404 ;
urns of, 402 ; peristome of, 402,
403 : development of spores of, 404.
Mother-of- Pearl, structure of, 636.
Moths, see Lepidoptera.
Moulds, fungous, 379, 390.
Mounting of objects, see Objects.
Mounting-Instrument, 245, 247.
Mounting- Microscope, Field's, 81, 82.
Mounting-Plate, 238, 239.
Mouse, hair of, 748 ; cartilage of ear
of, 764 ; vessels of toe of, 784.
Mouth of Insects, 709, 713.
Mucous Membranes, structure of,
758 ; capillaries of, 786.
Miiller, Dr. Fritz, on colonial ner-
vous system of Polyzoa, 619, 620.
Miiller, Prof. J., his researches on
Polycystina, 564 ; on Echinoderm
larvae, 608—615.
Muscardine, or Silk-worm disease,
384, 385.
Muscular Fibre, structure of, 766 —
770 ; mo le of examining and pre-
paring, 768 : capillaries of, 785, 786.
Mush-deer, hair of, 747 ; minute
blood- corpuscles of, 758.
Mussel, ciliary action on gills of, 656 ;
development of, 649.
Mya, structure of hinge-tooth of, 638.
Mycelium of Fungi, 389—394.
Mycetozoa, 391.
Myliobates, teeth of, 740, 741.
Myriapods, hairs of, 702.
Myxobrachia, 466, 816.
Myxogastric Fungi, 391, 392.
Nachet, M. M., their Stereoscopic
Binocular, 60, 61 ; Stereo-Pseudo-
scopic Binocular, 67 — 69 ; Bino-
cular Magnifier, 85 ; Student's
Microscope, 93 — 95 ; Double-
bodied Microscope, 110 ; Erecting
Prism, 114; Cameras, 123, 129;
130.
Nacre, structure of, 635 — 637.
Nais, 67^, 673.
Nassula, teeth of, 486.
Navicellse of Gregarinida, 480.
Nautilus, shell of, 643.
Navicular, 386, 387; movements of,
318.
Needles for Dissection, mode of
mounting, 219.
Nematoid Entozoa, 660, 661.
Nemertes, larva of, 668.
Nepa, tracheal system of, 715.
Nepenthes, spiral vessels of, 430.
Nervous Tissue, structure of, 770 —
773 ; mode of examining, 773, 774.
Net, Collector's, 267, 268.
Nettle, sting of, 448.
Neuroptera, circulation in, 714, 718 ;
wings of, 719.
Neutral-tint Eeflector, 129.
Newt, circulation in larva of, 776.
Nicol-Prism, 145.
Nitella, 365.
Nitzschiece, 323.
Nobert's Test, 209, 210.
Noctiluca, 594, 595.
Nodosaria, 540.
Nonionina, 547.
Non-striated Muscular fibre, 769, 770.
Nose-piece, Brooke's, 130.
Nostochacece, 354.
Nucleus of Vegetable cells, 274, 275,
423 ; of Bhizopoda, 471, 474, 479 ;
of Infusoria, 496 ; of Gregarinida,
479, 480 ; of Animal cells, 734.
Nudibranchs, development of, 650 —
652.
Nummulinida, 519, 545 — 560.
Nummulite, structure of, 519, 549 —
552.
Nummulitic Limestone, 549, 793.
Nupkar lutea, parenchyma of, 417,
418.
Object-Finders, 131—133.
Object-Glasses, Achromatic principle
of, 40 — 42 ; construction of, 43 —
47 ; adjustment of, for covering
glass, 44, 45; 179—182; adapta-
tion of to Binocular, 69 — 72 ; de-
fining power of, 200 ; penetrating
power of, 201, 202 ; increase of focal
depth with Binocular, 72 ; resolving
840
INDEX.
power of, 202, 203 ; flatness of field
of, 203, 204 ; comparative value of,
200—205 ; different powers of, 205
—209 ; tests for, 205—214 ; deter-
mination of magnifying power of,
214—216.
Object-Marker, 130, 131.
Objects, mode of mounting, dry, 239
—242; in Canada balsam, 242—
251 ; in preservative Media, 252 —
255 ; in cells, 262—264; see Opaque
and Transparent Objects.
Objects, labelling and preserving of,
265, 266.
collection of, 266—269.
Oblique Illuminators, 137—140, 187—
189.
Ocelli of Insects, 704—706.
Octospores of Fuci, 373.
(Edogonium, zoospore of, 359 ; sexual
reproduction of, 361, 362.
Oersted, Prof., on sexuality of Agarics,
394, 395.
Oidium, 393.
Oil-globules, microscopic appearances
of, 198.
Oleander, cuticle of, 447 ; stomata of,
450.
Oncidium, spiral cells of, 425.
Onion, raphides of, 428.
Oolite, structure of, 799.
Oo-spores, of Yolvox, 289 ; of Yau-
cheria, 355 ; of Spbaeroplea, 359,
360; of (Edogonium, 361.
Opaque Objects, arrangement of Mi-
scroscope for, 190 — 192 ; various
modes of illuminating, 192, 193 ;
modes of mounting, 240 — 242.
Opercula of Mosses, 402.
Operculina, 548.
Ophiocoma, teeth and spines of, 603."
Ophiurida, skeleton of, 603 ; develop-
ment of, 610.
OpJirydince, 499.
Orbiculina, plan of growth of, 522,
523.
Orbitoides, structure of, 553, 554.
Orbitolina, 543.
Orbitolites, structure and development
of, 524—529 ; fossil, 793.
Orbidina, 540.
Orchideous Plants, 425, 458.
Ornithorhynchus, hair of, 718.
Orthoptera, wings of, 720.
Osmunda, pro thallium of, 412 note.
Oscillatoriacece, 350 — 352.
Ostracece, shells of, 637—639.
Ostracoda, 677.
Otoliths of Gasteropods, 657 ; of
Fishes, 814.
Ovipositors of Insects, 724, 725.
Ovules of Pbanerogamia, 457 ; fer-
tilization of, 458 ; mode of study-
ing, 458, 459.
Owen, Prof., on structure of Teeth,
19, 20 ; on fossil Teeth, 800—803 ;
on fossil Bone, 803, 804.
Oxytricha-iovm of Ti ichoda, 491 — 493.
Oyster, shell of, 637, 639.
Pachymatisma, spicules of, 571.
Pceony, starch-cells of, 427.
Pacinian corpuscles, 773.
Palates of Gasteropods, 644 — 647.
Palm, stem of, 434, 435.
Palmella, 346.
Palmellacece, 346, 347.
Palmodictyon, 347.
Palmoglcea macrococca, life- his tor}- of,
275—277.
Papilke of Skin, structure of, 759,
772 ; capillaries of, 786 ; of Tongue,
772.
Parabolic Speculum, 150, 151.
Paraboloid, 140, 141.
Paramecium, superficial pellicle of,
484 ; contractile vesicles of, 489 ;
binary subdivision of, 490 ; sexual
generation of, 496.
Paraphyses of Lichens, 378 ; of
Mosses, 402.
Parasitic Fungi, 383—389.
Parkeria, 536— 5c 8.
Passulus, fungous vegetation in, 387.
Paste, Eels of, 661.
Pasteur, M., his researches on fer-
ments, 381 ; on pebrine, 382.
Patella, palatal tube of, 645.
Pearls, structure of, 637.
Pebrine, 382.
Pecari, hair of, 748.
Pecten, eyes of, 656 ; tentacles of, 657.
Pedalion, 507.
Pediastrece, structure of, 300, 301 ;
multiplication and development of,
302, 303 ; varieties of, 304.
INDEX.
841
Pedicellarice of Echinoderms, 601.
Pedicellina, 621.
Peneroplis, 516, 521, 522.
Penetrating power of Object-glasses,
201, 202 ; increase of, with Bi-
riocular, 72.
Pelargonium, cells of petal of, 453.
Pentacrinoid larva of Comatula, 613 —
615.
Pentacrinus, skeleton of, 604.
Perennibrancliiata, bone of, 639 ;
blood-corpuscles of, 753.
Peristome of Mosses, 40^, 403.
Perophora. 628, 629.
Petals of Flowers, structure of, 453,
454.
Petrology, Microscopic, 804—809.
Pettenkofer's test, 229.
Phanerogamia, elementary tissues of,
415, 433 ; (see Tissues of Plants);
Stems and Roots of, 434—445 ;
Cuticles and Leaves of, 445 — 452 ;
Flowers of, 452 — 459 ; Seeds of,
459—461.
Phyllopoda, 679.
Pieridce, scales of, 692, 694.
Pigott, Dr. Poyston, his Aplanatic
Searcher, 40 ; his Micrometers,
125 ; on Nobert's Test, 211 ; on
scales of Insects, 673 — 702.
Pigment-cells, 760, 761; of Cuttle-
fish, 658.
Pigmentum nigrum, 760.
Pilidmm-\&rv& of Nemertes, 668.
Pillischer, Mr., his Student's Micro-
scope, 89, 90 ; his Lamp, 170.
Pilulina, 532.
Pinna, structure of shell of, 633 — 635 ;
fossil, in Cnalk, 796.
Pinnularia, 336 ; multiplication of,
315.
Pistillidia, see Archegonia.
Pith, structure of, 416, 436.
Placoid scales of Fish, 745, 746.
Planaria, 662—664.
Planorbulina, 542.
Plantago, cyclosis in hairs of, 423.
Plants, distinction of from Animals,
270—272, 462—464.
Plate-glass Cells, 259. 260.
Pleurosigma, 386 ; nature of markings
on, 196, 310—312; value of as
Test, 212.
Pluteus-hrva, of Echinus, 610, 611.
Plumules of Butterflies, 692.
Pocket Microscope, Beale's, 106.
Podura, scale of, 693 — 702 ; use of,
as Test-object, 208.
Poisons, detection of, 816, 817.
Polarization, Objects suitable for,
812, 813.
Polarizing Apparatus, 145 — 147.
Polistes, fungous vegetation in, 385.
Pollen-grains, development of, 454,
455 ; structure and markings of,
455 — 457.
Pollen-tubes, fertilizing action of, 458.
Polycelis, 663.
Polyclinians, 625.
Polycystina, nature of, 473, 562 ; dis-
tribution of, 56^—566.
Polygastrica, see Infusoria.
Polymorphina, 540.
Polyommatus argus, scale of, 695.
Polypes, see Hydra and Zoophytes.
Polypide of Polyzoa, 617.
Polypodium, fructification of, 406, 407.
Polysiomella, 545 — 547.
Polythalamous Foraminifera, 514 —
520.
Polytrema, 533.
Polyzoa, 616 — 62.3 ; general struc-
ture of, 616 — 621 ; classification
" of, 621—623.
Pond-Stick, Baker's, 267.
Poppy, seeds of, 459.
Popular Microscope, Beck's, 96.
Porcellaaous Foraminifera, 518, 520
—529.
Porcellanous shells of Gasteropods,
642.
Porcupine, quill of, 748.
Porifera, see Sponges.
Portable Microscope, Swift's, 817.
Potato-disease, 393.
Powell and Lea-land's Microscopes,
102 — 104 ; their Binocular for high
powers, 110, 111 ; their Achromatic
Condenser, 135 ; their White-cloud
Illuminator, 144 ; their Vertical
Illuminator, 153.
Prawn, shell of, 687.
Preservative Media, 252—255.
Primordial Cell, 273, 274, 411.
Utricle, 273, 274, 419.
Pringsheim, Dr., his observations on
842
INDEX.
Vaucheria, 355 ; on Hydrodictyon,
357 ; on Q3dogonium, 361 ; on
Sphacelaria, 372.
Prismatic Shell-substance, 632 — 635.
Prism, Amici's, 138, 139 ; Nachet's
Erecting, 114 ; Wenham's, 62 ;
Camera Lucida, 126—129; Spec-
troscope, 116; Polarizing, 145, 146.
Proboscis of Bee, 711, 712 ; of But-
terfly^ 712, 713; of Fly, 710, 711.
Proteonina, 533.
Proteus, blood-corpuscles of, 753.
Pro thallium of Ferns, 409 — 411.
Protista, 464.
Protococcus, life-history of, 277 — 282 ;
conditions influencing changes of,
281, 282 ; its relation to Ulvaceje,
348.
Protoplasm, of Vegetable cell, 273 —
275, 419—421; of Animals, 733—
735.
Protophyta, general characters of,
270—275.
Peotozoa, their relations to Proto-
phyta, 271, 462—464.
Pseudembryo of Echinoderms, 609 —
615.
Pseudo-navicellse of Gregarinida, 480.
Pseudopodia of Rhizopods, 466 — 477.
Pseudoscope, 59.
Pseudoscopic Microscope of MM.
Nachet, 67, 68.
Vision, 58.
Pteris, fructification of, 406 ; pro-
thallium of, 409—411.
Pterodactyle, bone of, 803, 804.
Puccinia, 392.
Purpura, egg-capsules of, 651 ; de-
velopment of, 652 — 655.
Pycnogonidce, 674 — 676.
Quekett, Prof. J., his Dissecting
Microscope, 80, 81 ; his Indi-
cator, 126 ; on Raphides, 428, 429 ;
on structure of Bone, 19, 738, 739,
803, 804.
Quinqueloculina, 521.
Radiating Crystallization, 809, 810.
Badiolaria, 467, 470—473, 562—
567.
Rainey, Mr., his Moderator, 169 ; on
Molecular coalescence, 813 — 815.
Ralfs,Mr., onDesmidiace£e,290— 304 ;
on DiatomaceEe, 305 note.
Ramsden's Eye-piece, 5Q.
Raphides, 428.
Reade, Rev. J. B., his Hemispheri-
cal Condenser, 139, 140.
Re-agents, Chemical, use of in Micro-
scopic research, 227—230, 816, 817.
Red Corpuscles of blood, 751 — 753.
Red Snow, 346.
Reflection by Prisms, 32, 33.
Reflex Illuminator, Wenham's, 142,
143.
Refraction, laws of, 30 — 32 ; by con-
vex lenses, 32 — 36 ; by concave
and meniscus lenses, 36, 37.
Rein-deer, hair of, 748.
Reptiles, bone of, 738, 739, 803;
teeth of, 742 ; scales of, 743—746 ;
blood of, 751—756 ; lungs of, 787,
788.
Resolving power of Object-glasses,
202, 203.
Reticularia, 467 — 470.
Reticulated Ducts, 431.
Rhabdammina, 531, 534.
Rhinoceros, horn of, 751,
Rhizopoda, 466, 467 ; their subdivi-
sions, 468 — 477 ; their reproduc-
tion, 477 — 479 ; their relation to
higher Animals, 733 — 735, 754 —
772.
Rhizosolenia, 333.
Rhizostoma, 586.
RhodospermecE, 375—377.
Rhubarb, raphides of, 428.
Rhynchonellidce, structure of Shell of,
641.
Rice-Paper, 416, 417.
Ricinice, 729.
Ring-Cells, Metallic, 261.
Ring-Net, 267—269.
Rochea, cuticle of, 447, 448.
Rocks, structure of, 798—800, 804—
806.
Roots, structure of, 444 ; mode of
making sections of, 445.
Ross, Mr., on correction of Object-
glass, 44, 45 ; his Compound Mi-
croscope, 99 — 102 ; his Achromatic
Condenser, 135, 136 ; his Simple
Microscope, 78, 79 ; his Lever of
contact, 235 ; his Compressorium,
INDEX.
843
163 ; bis eye-piece Micrometer,
122.
Ross-Model for Compound Micro-
scope, 86.
Rotalia, 516, 543, 544.
Eotaline Foraminifera, 517, 542 — 546.
Rotating Microscope, Browning's, 95.
Rotifer, anatomy of, 503 — 507 ; re-
production of, 507 — 509 ; tenacity
of life of, 509 ; occurrence of in
leaves of Sphagnum, 404, 501.
Rotifera, general structure of, 501
— 507 ; reproduction of, 507 — 509 ;
desiccation of, 509; classification
of, 510—513.
Rush, stellate parenchyma of, 417.
Rust, of Corn, 392.
Sable, hair of, 705.
Saccamina, 531, 532.
Safety-Stage, Stephenson's, 154.
Salter, Mr. Jas., on teeth of Echinida,
601—603.
Salts, crystallization of, 809 — 812.
Salvia, spiral fibres of seed of, 426.
Sand-blast cells, 823.
Sand-wasp, integument of, 691.
Sarcina ventriculi, 383.
Sarcode, of Protozoa, 462.
Sctrcoptes scabiei, 728.
Sarsia, 580.
Savi-flies, ovipositor of, 724.
Scalariform ducts of Ferns, 406, 431.
Scales, of cuticle of Plants, 448.
of Fish, 743—745, 815.
of Insects, 692 — 704 ; their
use as Test-objects, 207, 208.
of Reptiles and Mammals, 746.
Schafer, Mr., on Muscular Fibre,
769 note.
Schizonemece, 337, 338.
Schleiden, doctrines of, 7.
Schultz's test, 229.
Schultze, Prof. Max., on movement of
fluid in Diatoms, 305 ; on surface-
markings of Diatoms, 313 note ; on
Sarcode in higher Animals, 734
note; on Foraminifera, 515.
Schwann, doctrines of, 21, 732.
Scissors for microscopic dissection,
219 ; for cutting thin sections, 220.
Sclerogen, deposit of, on walls of Cells,
424, 425.
Scolopendrum, sori of, 406.
Sea Anemone, 588 — 590.
Section- Instruments, 221, 222, 822,
823.
Sections, thin, mode of making, of
soft substances, 220, 221 ; of sub-
stances of medium hardness, 221,
222 ; of hard substances, 222—227 ;
of Foraminifera, 224 note ; of
Leaves, 452 ; of Wood, 444, 445; of
Echinus-spines, 604, 605 ; of Insects,
691 ; of Bones and Teeth, 739, 740 ;
of Hairs, 749, 750.
Seeds, microscopic characters of, 459,
461.
Segmentation of Yolk-mass, 651,
653.
Selenite-Plate, 146, 147.
Stages, 147, 820.
Selligues, M., his early construction
of Achromatic lenses, 43.
Sepiola, eggs of, 658.
Sepiostaire of Cuttle-fish, 643.
Serialaria, colonial nervous system
of, 619.
Serous Membranes, structure of, 758.
Serpentine-limestone, 555 — 560, 799.
Sertularidce, 581 — 584.
Shadbolt, Mr., on Arachnoidiscus,
330 ; his annular Condenser, 140
note ; his Turn-table, 257, 258.
Shark, teeth of, 740, 741 ; scales,
&c, of, 745, 746.
Shell of Crustacea, 686, 687; of
Echinida, 596, 597 ; of Foramini-
fera, 515- 520 ; of Mollusca, 632—
644 ; Fungi in, 389.
Shrimp, shell of, 687.
Side-Illuminator, 147 — 150.
Side-Reflector, Beck's, 150, 151.
Siebold, Prof., on reproduction of
Bee, 727.
Silica crack-slide, 197.
Siliceous Cuticles, 412, 448.
Siliceous Sponges, 569, 570.
Silk-worm disease, 382 — 385.
Silver, crystallized, 808.
Simple Microscope, optical principles
of, 48 — 51 ; various forms of, 77 —
85.
Siphonacece, 353 — 357.
Sipunculus, larva of, 667, 668.
Siricidai, ovipositors of, 724.
844
INDEX.
Skin, structure of, 759 ; papillae of,
772, 786.
Slack, Mr., his Diaphragm-Eyepiece,
126 ; his White-cloud illumination,
145 ; his Stage-vice, 155 ; his Com-
pressoriums, 163, 164; his Silica
crack-slide, 197; his crystalliza-
tions from silica solutions, 811.
Slider-Forceps, 246.
Slides, Glass, 233.
Wooden, 241.
Slug, rudimentary shell of, 642 ;
palate of, 644, 645 ; eyes of, 657.
Smith, Mr. Jas., his Mounting In-
strument, 245, 247 : his Selenite
Stage, HI note; his Object Cabi-
net, 235.
Smith, Dr. Lawrence, his Inverted
Microscope, 108, 109.
Smith, Prof., (U.S.) his Binocular
Eyepiece, 66 ; his vertical Illumi-
nator, 153 ; his Growing-slide, 158;
his views on Diatoms, 315.
Smith, Prof. W., on Diatomaceae,
212,305 note; 308, 330.
Smith and Beck, see Beck, Messrs.
Smut, of Wheat, 393.
Snail, palate of, 644, 645 ; eyes of,
657.
Snake, lung of, 747.
Snow- crystals, 806.
Social Ascidiaus, 627—629.
Soemmering, his speculum, 127.
Sole, skin and scales of, 744, 745.
Sollitt, Mr., on Diatom-tests, 211,
212.
Sorby, Mr., his Spectroscope Eye-
piece, 115; his Microscopic exami-
nation of Pocks, 804, 806.
Soredia of Lichens, 378.
Sori of Ferns, 406. 407.
Spatangidium, 329.
Spatangus, spines of, 601.
Spencer, Mr., his method of clean-
ing thin glass, 236.
Spectacles, for Dissection, 2 '8.
Spectro-Micrometer, Browning's, 117.
Spectroscope Eye-piece, 115 — 120.
Spectroscopic Analysis, principles of,
115, 116.
Speculum, Parabolic, 150, 151.
Spermatia, and Spermogonia of lichens,
378.
Sphacelaria, 371, 372.
Spliairia, development of within
Animals, 386.
Sphceroplea, sexual reproduction of.
359, 360.
Sphcerosira volvox, 286.
Splicer ozoum, 481.
Spliagnaceoz, peculiarities of, 404,
405 ; occurrence of Potifer in leaf-
cells of, 404.
Spherical Aberration, 38, 39 ; means
of reducing and correcting, 39,
40.
Spicules, of Sponges, 568 — 571 ; pre-
paration of, 573 ; of Alcyonian
Zoophytes, 591 ; of Doris, 643.
Spiders, eyes of, 720 ; respiratory
organs of, 721 ; feet of, 721 ; spin-
ning apparatus of, 721, 722.
Spines of Echimda, 598, 601 ; mode of
making sections of, 604 — 606.
Spinning-apparatus of Spiders, 730,
731.
Spiracles of Insects, 717, 718.
Spiral Cells of Sphagnum, 404 ; of
Orchideae, 425 ; of anthers, 455.
■ — Crystallization, 811.
—Fibres, 426.
Vessels, in petals, 454.
Spiriferido?, shell-structure of, 641.
Spirillina, 539.
Spirogyra, 363.
Spirolina, 522.
Spiroloculina, 521.
Sponges, their structure, 567 ; ciliary
action in, 568 ; skeleton of, 568 —
571; reproduction of, 572, 573;
examination of, 573 , fossil, 797.
Spongilla, 568, 572.
Spongiole of Root, 444.
Sporangia, of Desmidiacess, 297,
298 ; of Diatomacese, 316, 317; of
Fuci, 373; of Hepaticse, 398.
Spores of Paluioglaea, 276, of Conju-
gates, 368 ; of Fuci, 374 ; of
Hepaticse, 399 ; of Mosses, 402
—404; of Ferns, 409, 410; of
Equisetaceaa, 413.
Spot-Lens, 140.
Spring-Clip, 240.
Press, 246.
-Scissors, 219.
Squirrel, hair of, 747, 748.
INDEX.
845
Stage, Glass, 93, 96, 820 ; Rotating,
90 ; Safety, 154.
Stage-Forceps, 155.
Stage-Plate, glass, 157, 158.
Stage- Vice, 155.
Staining Processes, 230, 231, 785.
Stanhope Lens, 51.
Stanhoscope, 52.
Star-Anise, cells of seed-coat of,
424.
Starch -granules, in Cells, 426 — 428 ;
appearance of, by Polarized light,
427.
Star-fish, Bipinnarian larva of, 609,
610.
Stato-spores, of Yolvox, 287, 289 ; of
Hydrodictyon, 357.
Staurastruiii, prominences of, 291 ;
self-division of, 294 ; varieties of,
304.
Stauroneis, 337.
Steenstrup, Prof., on Alternation of
generations, 5S7.
Stein, Dr., his doctrine of Acineta
forms, 498 ; his researches on In-
fusoria, 513 note.
Stellaria, spiral vessels in petal of,
454.
Stellate cells of Push, 417 ; of Water-
lily, 418.
Stemmata of Insects, 706.
Stems, Endogenous, structure of,
434, 435 ; Exogenous, structure and
development of, 434, 435 ; mode
of making sections of, 444, 445.
Stentor, 487, 499 ; its conjugation.
497.
Stephanoceros Eichornii, 510. 511.
Stepkanosphcera pluvialis, 290 note.
Stereoscope, 57.
Stereoscopic Spectacles, 218.
Vision, principles of, 57
— 60 : application of, to Compound
Microscope, 60 — 73 ; to Simple
Microscope, 83 — 85.
Stephenson, Mr., his Binocular Mi-
croscope, 64, 66 ; his safety- stage,
154 ; on mounting in bisulphide of
carbon, 252 ; on Coscinodiscus,
327, 328.
Stewart, Mr., on internal skeleton of
Echinodermata, 606.
Stick-net, 268, 269.
Stigmata of Insects, 717, 718.
Stings of Plants, structure of, 448 ;
of Insects, 724, 725.
Stokes, Prof., on Absorption-bands of
blood, 120, 121.
Stomata of Marchantia, 396 ; of
Flowering Plants, 449, 450.
Striatellece, 325.
Student's Microscopes, Pillischer's,
89, 90 ; Beck's, 91, 96; Ladd's, 92 ;
Nachet's, 93 — 95 ; Crouch's Bi-
nocular, 96 ; Harley Binocular,
97, 98.
Storied Cmstacea, 683, 684.
Sulphate of Copper and Magnesia,
radiating crystallization of, 810.
Sulphate of Copper, spiral crystalli-
zation of, 811.
Suminski, Count, on Ferns, 11.
Sundew, hairs of, 448.
Sunk Cells, 259.
Surirella, 324 ; conjugation of, 315 ;
316 ; use of as test, 214.
Swarming of granules in Desmidia-
cese, 293.
Swift's Portable Microscope, 818 ; his
Achromatic Condenser, 820 ; his
Portable Lamp, 822.
Synapta, calcareous skeleton of, 607 ;
development of, 613 note.
Syncoryne, 580.
Syncrypta, 286.
Synedrece, 323.
Syringe, small glass, 165, 166 ; uses
of, 183, 228, 244, 247, 256, 655 note.
Syringes for Injection, 780, 781.
Tabanus, ovipositor of, 725.
Table for Microscope, 168,
Tadpole, pigment-cells of, 761 ; circu-
lation in, 776 — 780.
Tcenia, 659, 660.
Tardigrada, 512, 513 ; desiccation
of, 509—510.
Teeth, of Echinida, 601—603 ; of
Mollusks, 644—647 ; of Leech,
635 ; of Vertebrata, structure of,
740—743 ; fossil, 801—803 ; mode
of making sections of, 740.
Tendon, structure of, 757.
Tenthredinidce, ovipositors of, 724.
Terebella, circulation and respiration
in, 664—666.
846
INDEX.
Terebralida, structure of shell of,
639 — 641 ; muscular fibre of,
768. ^
Terpsinoe, 325.
Test-Bottles, 228;
Test-Liquids, 229, 230.
Test-Objects, 205—214.
Tethya, sexual generation of, 572.
Tetraspores of Florideae, 375, 376.
Textularia, 541, 542.
Tlialassicolla, 481, 482.
Thallus of lower Cryptogamia, 370,
377.
Thaumantias, 584.
Thecas of Fungi, 391 ; of Ferns, 407 ;
of Eo^uisetacese, 413.
Thin Glass, 234—236.
Tom- Glass Cells, 258, 259.
Thomas, Mrs. H., on Cosmarium,
294, 297.
Thomas, Mr. P., on microscopic Crys-
tallization, 810.
Thompson, Mr, J. V., on Polyzoa,
616 ; on development of Comatula,
61 4 ; on metamorphosis of Cirrhi-
peds, 684 ; on metamorphosis of
Crustacea, 687.
Thomson, Prof. Wyville, on nutrition
of Marine animals, 272 ; on Sili-
ceous Sponges, 569, ^570 ; on de-
velopment of Echinodermata, 610
note, 613 note, 615 ; on Chalk-forma-
tion, 795, 798.
Thread-cells of Zoophytes, 589, 590.
Thrush, fungous vegetation of, 388.
Thwaites, Mr., his fluid for Algse,
252 ; on conjugation of Diatoms,
316 ; on filamentous extensions of
Paltnelleaa, 317 note, 377.
Ticks, 729.
Tinea favosa, fungus of, 388.
Tinoporus, 542.
Tipida, larva of, 718,
Tissues, Elementary, of Animals,
microscopic study of, 732 ; forma-
tion of, 733 — 736 ; see Blood, Bone,
Capillaries, Cartilage, Epidermis,
Epithelium, Fat, Feathers, Fibrous
Tissues, Glands, Hair, Horn, Mu-
cous Membranes, Muscle, Nervous
Tissue, Pigment-cells, Scales, Se-
rous Membranes, Teeth.
Tissues, Elementary, of Plants, 414;
Cellular, 415-428; Woody, 429-
430 : Fibro-vascular, 429 ; Vascu,
lar, 430 ; Vasiform, 431, 432 ; dis-
section of, 432, 433.
Tomes, Mr., his Object-marker, 131.
Tomopterls, 668, 671.
Tongues of Gasteropods, 644 — 647;
of Insects, 710 — 712.
Torida ccrevisice, 378, 379.
Tow-Net, 268, 269.
Trachea of Insects, 715 — 717 ; mode
of preparing, 718, 719.
Tradescantia, cyclosisin hairs of, 422,
423.
Transparent Objects, arrangement of
Microscope for, 182 — 186 ; various
modes of illuminating, 186 — 190.
Trematode Entozoa, 661.
Triceratium, 332 ; markings on, 310,
312.
Trichoda, bristles of, 486 ; metamor-
phosis of, 491—493.
Trdobite, eye of, 801.
Trttocidina, 521.
Trochammina, 531, 534.
Trochus, palate of, 646, 647.
Trout, circulation in young, 777.
Tube-cells, 261.
Tubular Nerve-substance, 770, 771.
Tubularia, 580.
Tulasne, M., on Lichens, 378 ; on
Fungi, 395 note.
Tulley, Mr., his early production of
Achromatic objectives, 43.
Tunicata, general organization of,
623—625 ; composite types of, 625,
628 ; alternating circulation in,
624, 629; development of, 629,
630.
Turbellaria, 662—664.
Turn-table, Shadbolt's, 257, 258.
Vlvacece, 348—350.
Unicellular Plants, 275.
Unionidce, shells of, 637—639.
Uredo, 392.
Urns of Mosses, 402.
Uvella, 286.
Vacuoles, 274, 284, 471, 488; mi-
croscopic appearances of, 199.
Valentin's Knife, 221.
Vallisneria, cyclosis in, 420.
INDEX.
847
Vanessa, haustellium of, 712.
Variation, tendency to, in Desmidia-
cese, 302 ; in Diatoinaceae, 339 ; in
Polycystina, 562.
Varnishes useful to Microscopists,
236, 237.
Vasiform Tissue, 431, 432.
Vaucher, M., on Confervse, 4.
Vaucheria, zoospores of, 353, 354 ;
sexual reproduction of, 354 — 356.
Vegetable Ivory, 425.
Ventriculites, 796.
Vermilion Injections, 782.
Vertebrata, elementary structure of,
732, (see Tissues) ; blood of, 751 —
755 ; circulation in, 774—780.
Vertical Illuminators, 153, 154.
Vegetable Kingdom, differentiated
from Animal, 270 — 272.
"Vesicular Nerve-substance, 770.
Vibracula of Polyzoa, 622.
Vibriones, 380, 381 .
Villi of intestine, injections of, 783,
784.
Vine-disease, 393.
Vinegar, Eels of, 661.
Viscid Media, Prof, Beale's use of,
231, 232.
Vitreous Foraminifera, 518, 539 —
560.
Volvox, structure of, 282 — 285 ; de-
velopment and multiplication of,
285—287 ; amoeboid state of, 287,
288 ; generation of, 288, 289.
Vorticella, 485, 486, 499 ; encysting
process in, 491.
Wallicb, Dr., on making sections of
Foraminifera, 224 note ; on surface-
markings of Diatoms, 312 note;
on Coccospheres, 465 ; on Rhizo-
pods, 468 note; on Amceba, 476,
477 note; on Polycystina, 563 note.
Warts, structure of, 761.
Water-Lily, stellate cells of, 417, 418 ;
leaf of, 452.
Water-Newt, circulation in larva of,
776.
Water- Vascular system, of Eotifera,
506, 507;. of Entozoa, 659.
Webster-Condenser, 136, 137.
Welcker, Prof., on distinction between
elevations and depressions, 197.
Wenbam, Mr., his new Achromatic
combination, 47 ; his Binocular
Microscope, 62, 63 ; his Illuminator
for the Binocular, 140 note; his
Parabolic Illuminator, 140 note;
his Keflex Illuminator, 142, 143 ;
on adjustmentof Object-glasses, 180,
181 ; his observations on Pleuro-
sigma, 312 -note; on Cyclosis, 421,
423 ; on Podura-scale, 701.
Whalebone, structure of, 751.
Wheat, blights of. 392, 393, 661.
Wheats'tone, Sir C, his invention of
the Stereoscope, 57, 58 ; of the
Pseudoscope, 59, 60.
Wheel- Animalcules, see Potifera.
White-cloud Illuminator, 144, 145.
White Corpuscles of blood, 753 — 755.
White Fibrous tissue, 756, 757.
Whitney, Mr., on circulation in
Tadpole, 777 — 780.
Williamson, Prof. W. C, on Volvox,
290; on shells of Crustacea, 649 ; on
scales of Fishes, 744 — 746 ; on
Coal-plants, 791 ; on Levant-mud,
793—795.
Wings of Insects, 719 — 721 ; scales
of, 692—702.
Winter- eggs, of Rotifera, 509 ; of
Hydra, 578 ; of Enlomostraca, 682.
Wollaston, Dr., his Doublet, 50 ;
his Camera Lucida, 126.
i Wood, of Exogenous stems, 436 — 441.
Woodward, Col. Dr., his resolution
of Nobert's Test, 210, 211 ; of
Araphipleura pellucida, 213 ; of
Surirella gemma, 214 ; on structure
of Diatom-valves, 312 ; on Podura-
scale, 701.
| Woody Fibre, 429 ; glandular, of
Coniferse, 430.
Wormley, Dr., on Micro-Chemistry,
816, 817.
Xanthidia of Flints, 297 note, 797.
Yeast-plant, 378, 379.
Yellow Fibrous tissue, 757.
Yucca, cuticle of, 445, 446 ; stomata
of, 449.
Zenker, Dr., on contractile vesicle
of Infusoria, 472.
843
INDEX.
Zoea-larva of Crab, 680.
Zoophyte-Trough, 160, 161.
Zoophttes, 574 — 575 ; Hydroid,
574 — 581 ; preparation of for Mi-
croscope, 582, 583 ; development
of Acalephte from. 584 — 588 ;
Alcyonian, 590—592 ; Actiniform,
588—590.
Zoospores, formation of, by Pro-
tococcus, 279, 280 ; by Des-
midiaceEe, 296 ; by Pediastreae,
301 ;■ by Ulvacese, 349 ; by
Vaucheria, 353, 354 ; by Achlya,
355, 356 ; by Confervacese, 359 ;
by Chsetophoracese, 364 ; by
Fucacese, 375.
Zygnema, 363.
Zygosis of Khizopods, 478 ; of Gre-
garinida, 481.
THE END.
LONDON:
SAVILL, EDWAEDS AND CO., PBINTEBS, CHANDOS STBEET,
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