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                           Transcriber’s Note


When italics were used in the original book, the corresponding text has
been surrounded by _underscores_ Mixed fractions have been displayed
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                                   ON

                   MOLECULAR AND MICROSCOPIC SCIENCE

                           VOLUME THE SECOND




                           LONDON: PRINTED BY
                SPOTTISWOODE AND CO., NEW-STREET SQUARE
                         AND PARLIAMENT STREET

_Fig. 118, p. 107._

[Illustration: GALEOLARIA LUTEA.]

                                             [_Frontispiece to Vol. II._




                                   ON

                               MOLECULAR

                                  AND

                          MICROSCOPIC SCIENCE




                           BY MARY SOMERVILLE

     AUTHOR OF ‘THE MECHANISM OF THE HEAVENS’ ‘PHYSICAL GEOGRAPHY’
               ‘CONNECTION OF THE PHYSICAL SCIENCES’ ETC.


       _Deus magnus in magnis, maximus in minimis_—St. Augustine


                        IN TWO VOLUMES—VOL. II.




                           WITH ILLUSTRATIONS




                                 LONDON
                     JOHN MURRAY, ALBEMARLE STREET
                                  1869

                 _The right of translation is reserved_




                                CONTENTS

                                   OF

                           THE SECOND VOLUME.


                               PART III.

                           ANIMAL ORGANISMS.

   SECT.                                                         PAGE

   I. FUNCTIONS OF THE ANIMAL FRAME                                 1

   II. PROTOZOA                                                    13

   III. HYDROZOA ZOOPHYTES                                         81

   IV. ANTHOZOA ZOOPHYTES                                         119

   V. ANNULOSA, OR WORMS                                          144

   VI. ECHINODERMATA                                              169

   VII. THE CRUSTACEA                                             188

   VIII. CIRRIPEDIA                                               213

   IX. BRYOZOA, OR POLYZOA                                        218

   X. TUNICATA, OR ASCIDIANS                                      222

   XI. MOLLUSCA                                                   229

   INDEX                                                          253




                             ILLUSTRATIONS

                                   TO

                           THE SECOND VOLUME.


 FIG.      PAGE

 118. Galeolaria lutea (_Voght_)                         _frontispiece_
  86. Amœba princeps                                                 14
  87. Actinophrys sol                                                17
  88. Acanthometra bulbosa  }                              _to face_ 19
  89. Eucyrtidium cranoides } (_Haeckel_)[A]  _frontispiece to vol. i._
  90. Dictyopodium trilobum }                              _to face_ 20
  91. Podocyrtis Schomburgi                                          20
  92. Aulocantha scolymantha }                             _to face_ 21
  93. Actinomma drymodes     } (_Haeckel_)                 _to face_ 21
  94. Haliomma echinaster    }                             _to face_ 21
  95. Simple Rhizopods                                               22
  96. Gromia oviformis                                               26
  97. Various forms of Foraminifera                                  28
  98. Simple disk of Orbitolites complanatus                         34
  99. Animal of Orbitolites complanatus                              34
 100. Rosalina ornata (_Voght_)                            _to face_ 41
 101. Section of Faujasina                                           45
 102. Interior of the Operculina                                     46
 103. Section of Sponge                                              59
 104. Paramœcium caudatum                                            69
 105. Kerona silurus                                                 69
 106. Noctiluca                                                      73
 107. Vorticellæ                                                     76
 108. Acineta                                                        77
 109. Thread-cells and darts                                         82
 110. Hydra fusca                                                    84
 111. Syncoryna Sarsii with Medusa-buds                              90
 112. Thaumantia pilosella                                           92
 113. Otolites of magnified Thaumantias                              93
 114. Development of Medusa-buds                                     95
 115. Rhizostoma                                                     98
 116. Cydippe pileus and Beroë Forskalia                            102
 117. Praya diphys            }                           _to face_ 103
 118. Galeolaria lutea        } (_Voght_)[B]             _frontispiece_
 119. Apolemia contorta       }                           _to face_ 108
 120. Physophora hydrostatica }                                     109
 121. The Physalia                                                  112
 122. Velella spirans (_Voght_)                                     115
 123. Alcyonian polypes, highly magnified                           120
 124. Polype of Alcyonidium elegans                                 120
 125. Spicula of Alcyonium digitatum                                121
 126. Red coral branch                                              126
 127. Red coral greatly magnified                                   127
 128. Tubipora musica                                               130
 129. Actinian polype                                               131
 130. Lobophylla angulosa                                           135
 131. Nervous system of Leech                                       151
 132. Foot of Naïs                                                  152
 133. Terebella conchilega                                          154
 134. Pushing poles of Serpula                                      155
 135. Foot of a Polynoë                                             160
 136. Brachionus pala                                               163
 137. Common Rotifer                                                167
 138. Section of shell of Echinus                                   177
 139. Sucker-plate of Sea-Egg                                       179
 140. Section of a sucker-plate                                     179
 141. Spine of Echinus miliaris                                     181
 142. Pluteus of the Echinus                                        181
 143. Larvæ of Echinus in various stages of development             182
 144. Skeleton of Synapta                                           185
 145. Wheel-like plates of Chirodota violacea                       186
 146. Ear of Crab                                                   191
 147. Section of a Crab                                             193
 148. Young of Carcinus mœnas in various stages of development      195
 149. Lucifer, a stomapod crustacean                                200
 150. Female Cyclops                                                205
 151. Cypris                                                        207
 152. Section of Daphnia pulex                                      208
 153. Balanus culcatus                                              213
 154. Tentacles or feet of the Balanus                              214
 155. Section of Lepas anatifera                                    215
 156. Development of Balanus balanoïdes                             216
 157. Lepas                                                         217
 158. Cells of Lepraliæ                                             219
 159. Cellularia ciliata and Bugula avicularia                      220
 160. Magnified group of Perophora                                  222
 161. Highly magnified Perophora                                    223
 162. Ascidia virginea                                              225
 163. Salpa maxima                                                  227
 164. Young of Salpa zonaria                                        227
 165. Cardium or Cockle                                             230
 166. Foot of Cockle                                                231
 167. Section of shell of Pinna transversely to the direction of
      its prisms                                                    233
 168. Membranous basis of the shell of the Pinna                    233
 169. Section of nacreous lining of the shell of
      Avicula margaritacea (pearl oyster)                           234
 170. Tongue of Helix aspersa                                       237
 171. Palate of Trochus zizyphinus                                  237
 172. Granulated Trochus                                            238
 173. Tongue of Limpet                                              238
 174. Whelk                                                         240
 175. The Crowned Eolis                                             240
 176. Tongue-teeth of Eolis coronata                                241
 177. Hyalæa and Clio                                               243
 178. Clione borealis                                               243
 179. Cuttle Fish                                                   245
 180. Arm of Octopus                                                247

Footnote A:

  From Dr. Ernst Haeckel’s ‘Radiolarien.’

Footnote B:

  From Voght’s ‘Syphonophores de la Mer de Nice’.




                   MOLECULAR AND MICROSCOPIC SCIENCE.




                               PART III.

                           ANIMAL ORGANISMS.




                               SECTION I.

                     FUNCTIONS OF THE ANIMAL FRAME.


ALTHOUGH animal life is only known to us as a manifestation of divine
power not to be explained, yet the various phases of life, growth, and
structure in animals, from the microscopic Monad to Man, are legitimate
subjects of physical inquiry, being totally independent of those high
moral and religious sentiments which are peculiar to Man alone.

The same simple elements chemically combined in definite but different
proportions form the base of animal as well as of vegetable life. But
besides the elementary gases and carbon, many substances, simple and
compound, are found in the animal frame; the phosphate and carbonate of
lime, iron which colours the blood, and common salt which, with the
exception of water, is the only article of food we use in a mineral
state. Animals derive their nourishment, both directly and indirectly,
from vegetables. Their incapacity to change inert into living matter is
one of the most characteristic distinctions between the animal and
vegetable kingdoms.

Protoplasm was shown to be rudimentary formative vegetable matter: so
Sarcode, or rudimentary flesh, forms the whole or part of every animal
structure. It is a semi-fluid substance, consisting of an albuminous
base, mixed with particles of oil in a state of very fine division. It
is tenacious, extensile, contractile, and diaphanous, reflecting light
more than water, but less than oil. It is rendered perfectly transparent
by citric acid, and is dyed brown by iodine. This substance, in a
homogeneous state, constitutes the whole frame of the lowest grade of
animal life; but when gradually differentiated into cell-wall and
cell-contents, it becomes the origin of animal structure from that which
has little more than mere existence to man himself; in fact, cellular
origin and cellular structure prevail throughout every class of animal
life. Unicellular plants and animals live for themselves independently
and alone; but the cells which form part of the higher and compound
individuals of both kingdoms, may be said to have two lives, one
peculiarly their own, and another depending on that of the organized
beings of which they form a part.

Flesh or muscle, which is organized sarcode, consists of two parts,
namely, bundles of muscular fibre imbedded in areolar tissue. Nervous
matter also consists of two parts, differing much in appearance and
structure, the one being cellular, the other fibrous. The vital activity
of the nerves far surpasses that of every other tissue; but there is an
inherent irritability in muscular fibre altogether independent of
nervous action: both the nervous and muscular tissues are subject to
decay and waste.

The blood, which is the ultimate result of the assimilation of the food
and respiration, conveys nourishment to all the tissues during its
circulation; for with every breath, with every effort, muscular or
mental, with every motion, voluntary or involuntary, at every instant of
life, asleep or awake, part of the muscular and nervous substances
becomes dead, separates from the living part, is returned to the
circulation, combines with the oxygen of the blood, and is removed from
the system, the waste being ordinarily in exact proportion to the
exertion, mental and physical. Hence food, assimilated into blood, is
necessary to supply nourishment to the muscles, and to restore strength
to the nervous system, on which all our vital motions depend; for, by
the nerves, volition acts upon living matter. Waste and repair is a law
of nature, but when nature begins to decay, the waste exceeds the
supply.

However, something more than food is necessary, for the oxygen in the
blood would soon be exhausted were it not constantly restored by
inspiration of atmospheric air. The perpetual combination of the oxygen
of the air with the carbon of the blood derived from the food is a real
combustion, and the cause of animal heat; but if the carbonic acid gas
produced by that chemical union were not continually given out by the
respiratory organs, it would become injurious to the animal system. Thus
respiration and the circulation of the blood are mutually dependent; the
activity of the one is exactly proportional to that of the other: both
are increased by exercise and nervous excitement.

External heat is no less essential to animals than to vegetables; the
development of a germ or egg is as dependent on heat as that of a seed.
The amount of heat generated by respiration and that carried off by the
air is a more or less constant quantity; hence, in hot countries, rice
and other vegetable diet is sufficient, but as the cold increases with
the latitude, more and more animal food or hydrocarbon is requisite for
the production of heat.

The waste of the tissues, and the aëration of the vital juices, that is,
the exchange of the respiratory gases, are common to all animals. The
heart, upon whose expansions and contractions the circulation of the
blood depends, is represented in the lower animals by propelling organs
of a variety of forms; and the organs of respiration differ exceedingly,
according to the medium in which the animals live. Water, both fresh and
salt, though a suffocating element to land animals, contains a great
deal of air, not only in the state of gas, but also in solution, the
quantity in solution being directly as the pressure; so that animals
living in the deepest recesses of the ocean breathe as freely as those
that live on land, but with respiratory organs of a very different
structure. In the lowest classes, which have no respiratory organs at
all, the gases are exchanged through their thin delicate skins.

The mechanical forces act within the living being according to the same
laws as they do in the external world: the chemical powers too, which
are the cause of digestion, heat, and respiration, follow the same laws
of definite and quantitative proportion as they do in inert matter; but
neither the mechanical forces, nor the physical powers, could create a
germ; nor could they even awaken its dormant state to living energy,
unless a vital power existed in it, the origin of which is beyond the
reach of man.

Animals are endowed with nerve-force, in addition to mechanical force
and the physical powers which are common to them and vegetables; a force
which constitutes their prime distinction, which is superior to all the
other powers from its immediate connection with mind, and which becomes
more evident, and more evidently under the control of the animal, in
proportion as the animal approaches the higher grades of life, and only
attains its perfect development in the human race.

The bones of man and the higher animals are clothed with a system of
muscles, so attached that the head, eyes, limbs, &c., can be moved in
various directions. In each of these muscles the fibres of two sets of
nerves ramify, namely, the sensory and the motor nerves.

The sensory nerves convey external impressions to the brain, and by them
alone the mind is rendered conscious of external objects. The
impressions made by light and sound upon the eye and the ear, or by
mechanical touch on the body, are conveyed by the sensory nerves to the
brain, where they are perceived, though the impressions take place at a
distance from it. Conversely, the mind or will acts through the brain on
the motor nerves, which by alternately contracting, relaxing, and
directing the muscles, produces muscular motion. Thus the motor nerves
convey the emotions of the mind to the external world, and the sensory
nerves convey the impressions made by the external world to the mind. By
these admirable discoveries, Sir Charles Bell has proved that ‘we are
placed between two worlds, the invisible and the material;’ our nervous
system is the bond of connection. The connection, however, between the
mind and the brain is unknown: it has never been explained, and is
probably inexplicable; yet it is evident that the mind or will, though
immaterial, manifests itself by acting on matter; that is, as a power
which stimulates the nerves, the nerve-force acting on the muscles.
Mental excitement calls forth the most powerful muscular strength, and
an iron will can resist the greatest nervous excitement. The nervous and
muscular forces are perpetually called into action, because, for
distinct perception, the muscles require to be adjusted. Mind is passive
as well as active: we may see an object without perceiving it, and we
may hear a sound without attending to it. We must look in order to see,
listen in order to hear, and handle in order to feel; that is, we must
adjust the muscular apparatus of all our senses, of our eyes, ears, &c.,
if we would have a distinct perception of external exciting objects: and
that is accomplished by the power of mind acting upon matter.

Dr. Carpenter has shown that it is by a series of forces acting upon
matter that man conveys his ideas to man, the sonorous undulations of
the atmosphere being the medium between the two. On one side the will,
or power of mind, acts upon the nerves, nerve-force acts upon the
muscles of speech, and these muscles, while in the act of speaking,
produce sonorous undulations in the atmosphere. On the other side, these
undulations are communicated by the mechanism of the ear to the auditory
nerves, exciting nerve-force, and nerve-force acts upon the mind of the
hearer. ‘Thus the consciousness of the speaker acts upon the
consciousness of the hearer by a well-connected series of powers.’

Nerve-force generates, directly or indirectly, light, heat, chemical
power, and electricity. When the optic nerve is pressed in the dark, a
luminous ring is seen round the eye, and a blow on the face excites a
flash of light. Nervous excitement, by accelerating respiration,
increases the chemical combination of the oxygen of the air with the
carbon of the blood, and thus produces animal heat. But the development
of electricity by nervous and muscular force, is one of the most
unexpected and singular results of physiological research.

MM. Matteucci and Du Bois Reymond have proved that the intensity of the
nervous and muscular forces is at a maximum when the muscles are
contracted; and that if each arm of a man be put in contact with a wire
of a galvanometer so as to form an electric circuit, an instantaneous
deviation of the needle will take place, now in one direction and now in
the other, according as he contracts his right arm or his left. The
electricity thus evolved, when conveyed to the needle through several
miles’ length of coiled insulated wire, will cause a deflection
amounting to sixty or seventy degrees, according to the strength of the
man—that is, according to his muscular and nervous force; the amount of
the electricity being exactly in proportion to the amount of muscular
force.

It appears that the electric currents in the nerves are eight or ten
times stronger than those in the muscles. M. Helmholtz found that the
time required to contract a muscle, together with the time required to
relax it again, is not more than the third of a second, and is a
constant quantity, for the compensation of energy prevails also in
organic nature. He also found that the motion or velocity of the
electric current in a man is at the rate of 200 feet in a second. The
electric equivalent, as determined by M. Helmholtz, is equal to the
electricity produced in a voltaic battery by the seven millionth part of
a milligramme of zinc consumed in the ten-thousandth part of a second, a
milligramme being the 0·015432 part of a grain.

The contraction and muscular action or mechanical labour produced by the
passage of an electric current through a nerve is 27,000 times greater
than the mechanical labour which results from the heat disengaged by the
oxidation of that small quantity of zinc requisite to generate the
electricity; that is to say, the mechanical labour really produced by
the contraction of the muscles is enormously greater than the labour
corresponding to the zinc oxidized. In fact, the electric excitement of
a nerve is analogous to an incandescent particle or electric spark that
sets fire to a great mass of gunpowder. This result, and the association
between the greatest activity of respiration and the intensity of the
muscular energy, led M. Matteucci to suspect that a chemical action must
take place in the interior of a muscle during its contraction; and he
found by experiment that there actually is what he calls a muscular
respiration, namely, that the muscles themselves absorb oxygen, and give
out carbonic acid gas and nitrogen when contracted. This kind of
respiration is more or less common to all animals; if impeded, the blood
is imperfectly oxygenized, and loss of animal heat is the consequence.
The heat that is perpetually escaping from animals is replaced, by the
combustion of the carbon of the tissues or of the food with the oxygen
inhaled by the lungs and the skin.

In the highest class of animal life the brain is at once the seat of
intelligence and sensibility, and the origin of the nervous system. In
the lower animals intelligence and sensibility decrease exactly in
proportion to the deviation of their nervous system from this high
standard. The forms of the nervous system are more and more degraded as
the animals sink in the scale of being, till at last creatures are found
in which nerves have only been discovered with the microscope; others
apparently have none, consequently they have little or no sensibility.

The brain and the spinal cord enclosed in the vertebræ of the backbone
form a nervous system, which in the vertebrated creation is equal to all
the contingencies and powers of these animated beings, but is beyond all
comparison most perfect in the human race. The brain alone is the seat
of consciousness, for the spinal cord, though intimately connected with
it, and of a similar ‘mysterious albuminous electric pulp,’ appears to
have no relation to the faculties of perception and thought, yet it is
essential to the continuance of life. It is a distinct nervous centre
which generates muscular energy in man and animals corresponding to
external impressions, but without sensation, and is entirely independent
of the will; the vegetative functions of respiration, the contractions
of the heart, circulation of the blood, and digestion, are carried on
under every circumstance, even during sleep. The reason of their being
independent of sensation and the will is, that the nerves in the organs
performing these functions never reach the brain, which is the seat of
intelligence and sensation, but they form what is called the reflex
system; for any impressions made upon them are carried to the upper part
of the spinal cord alone, and are reflected back again to the muscles of
the heart, lungs, &c., which, by their contractions, produce these
involuntary motions. For instance, the flow of blood into the cavities
of the heart while dilating, acts upon the nerves, and these excite a
rhythmical movement in the muscular fibres of the heart. For there is a
vital contractility in muscular tissue which is one of the most
universal attributes of living beings, and is probably the sole cause of
motion in the lowest grades of life, and the movements produced by it in
the higher grades are in all cases the most directly connected with the
vegetative functions. The involuntary reflex system of nerves
constitutes the chief locomotive power in a number of the lower animals;
but it forms a continually decreasing portion of the whole nervous
system in proportion as animals rise in the scale of life, till in man
its very existence has been overlooked. If the spinal cord were
destroyed, instant death would be the consequence; whereas infants born
without brain have sucked and lived for a day or two.

There are numerous actions, especially among the lower animals, as
little under the influence of the will or intelligence as the reflex
nerves, which nevertheless depend upon sensation for their excitement.
The sensation may call the muscular apparatus into action without any
exertion of reason or will, in such a manner as to produce actions as
directly and obviously adapted to the well-being of the individual as
the reflex system. For example, a grain of dust irritates the nostrils,
and involuntarily excites the complicated muscular movements concerned
in the act of sneezing. This class of actions, which is called
sensori-motor, or consensual, includes most of the purely instinctive
motions of the lower animals, which, being prompted by sensations,
cannot be assigned to the reflex group.

Purely emotional movements are nearly allied to the preceding. Sensation
excites a mental feeling, or impulse, which reacts upon the muscular
system without calling either the will or the instinct into exercise.
These emotional movements are often performed in opposition to the
strongest efforts of the will, as when a sense of something ridiculous
may excite irresistible laughter at an improper time. It is probable
that the strong emotions exhibited by many of the lower animals, which
have been ascribed to instinct, are referable to this group.[1]

The movements of such animals as have no nerves are merely owing to the
vital contractility of muscular fibre.

In the highest province of animal life, which includes the mammalia,
birds, reptiles, and fishes, the general structure of the nervous system
consists of a double lobed brain, from whence a spinal cord proceeds,
protected by articulated bones, which extend along the back of the
animals, and from thence nerve-fibres extend to every part of the body.
But in order to suit a great variety of forms, this system undergoes
many modifications. In all the lower grades of life that have nerves,
the system chiefly consists of small globular masses, or nuclei, of
nervous matter, technically called ganglia, which are centres of nervous
energy, each of which is endowed with its own peculiar properties; the
nervous cords and filaments proceeding from them are merely organs of
transmission. The arrangement of these centres of nerve-force is
symmetrical, or unsymmetrical, according to the form of the animal.

In the lower portion of Articulated animals, such as insects, crustacea,
annelids, worms, &c. &c., there is a double cord extending along the
ventral side of the animal, united at equal intervals by double
nerve-centres, or ganglia. These two cords diverge towards the upper
end, surround the gullet, and unite again above that tube to form a
distinct bilobed principal nerve-centre or brain. A third form of the
nervous system is only a ring round the gullet; the points in it from
whence the nerves radiate are swollen nerve-centres, or ganglia. Those
on the sides and upper parts of the ring represent the brain, and supply
the eyes, mouth, &c., with nerves: other centres, connected with the
lower side of the ring, send nerves to the locomotive organs, viscera,
and respiratory organs. In animals of a still lower grade there are
single nuclei irregularly scattered, but in every case they are centres
of energy from whence filaments are sent to the different parts of the
creature. The last and lowest system consists of filamentous nerves,
chiefly microscopic.

Intelligence, or the mental principle, in animals differs in degree,
though not in kind, from that in the human race. It is higher in
proportion as the nervous system, especially the brain, approximates in
structure to that of man; but even in many of the lower orders may be
traced the dawn of that intelligence which has made man supreme on
earth. Every atom in the human frame, as well as in that of other
animals, undergoes a periodical change by continual waste and
renovation; but the same frame remains: the abode is changed, not the
inhabitant. Yet it is generally assumed that the living principle of
animals is extinguished when the abode finally crumbles into dust, a
tacit acknowledgment of the doctrine of materialism; for it is assuming
that the high intelligence, memory, affection, fidelity, and conscience
of a dog, or elephant, depend upon a combination of the atoms of matter.
To suppose that the vital spark is evanescent, while there is every
reason to believe that the atoms of matter are imperishable, is
admitting the superiority of matter over mind: an assumption altogether
at variance with the result of geological sequence; for Sir Charles
Lyell observes, that ‘sensation, instinct, the intelligence of the
higher mammalia bordering on reason, and lastly the improvable reason of
man himself, presents us with a picture of the ever-increasing dominion
of mind over matter.’

The physical structure of a vast number of animals has been investigated
from such as are a mere microscopic speck to the highest grade of animal
life; but very little is comparatively known of their intelligence and
means of communication. We know not by what means a pointer and
greyhound make an agreement to hunt together; nor how each dog is not
only aware that his companion possesses a property which he has not, but
that by their united talents they might accomplish their purpose, which
is merely sport, for they never eat the game.[2] The undulations of the
air and water are no doubt the means by which most animals communicate;
but there is reason to believe that many inhabitants of the earth, air,
and water are endowed with senses which we do not possess, and which we
are consequently incapable of comprehending.




                              SECTION II.

                               PROTOZOA.


THE Protozoa are the very lowest forms of animal existence, the
beginning and dawn of living things. They first appear as minute
shapeless particles of semi-fluid sarcode moving on the surface of the
waters. The pseudopodia, or false feet, with which they move, are merely
lobes of their own substance which they project and retract. In
creatures of a somewhat higher grade the form is definite, the
pseudopodia, numerous and filamental, serving for locomotion and
catching prey; and from the resemblance they bear to the slender roots
of plants are called Rhizopods.[3] The microscopic organisms possessing
these means of locomotion and supply, are of incalculable multitudes,
and of innumerable forms. Thus the waters, as of old, still ‘bring forth
abundantly the moving creature that hath life;’ in them the lowest types
of the two great kingdoms have their origin, yet they are diverse in the
manifestation of the living principle, that slender but decided line
which separates the vegetable from the animal Amœba.


                          CLASS I.—RHIZOPODA.

The Amœba, which is the simplest of the group, is merely a mass of
semi-fluid jelly, ‘changing itself into a greater variety of forms than
the fabled Proteus, laying hold of its food without members, swallowing
it without a mouth, digesting it without a stomach, appropriating its
nutritious material without absorbent vessels or a circulating system,
moving from place to place without muscles, feeling (if it has any power
to do so) without nerves, multiplying itself without eggs, and not only
this, but in many instances forming shelly coverings of a symmetry and
complexity not surpassed by those of any testaceous animal.’

[Illustration: Fig. 86. Amœba princeps.]

Such is the description given by Dr. Carpenter of the Amœba and its
allies. The Amœba princeps, which is the type of the naked group, fig.
86, is merely a shapeless mass of semi-fluid sarcode, coated by a soft,
pellucid and highly contractile film, called diaphane by Mr. W. J.
Carter, and in many forms of Amœba the whole is inclosed in a
transparent covering. It is in the interior semi-fluid sarcode alone,
that the coloured and granular particles are diffused, on which the hue
and opacity of the body depend, for the ectosarc or external coat is
transparent as glass. These creatures, which vary in size from the
1/2800 to the 1/70 of an inch in diameter, are found in the sea, but
chiefly in ponds inhabited by fresh-water plants. They move irregularly
over the surface of the water, slowly and continually changing their
form by stretching out portions of their gelatinous mass in blunt
finger-like extensions, and then drawing the rest of it into them; thus
causing the whole mass to change its place. Before it protrudes these
pseudopodia or false feet, there is a rush of the internal semi-fluid
matter to the spot, due to the highly contractile power of the diaphane,
which is often so thin and transparent as to be scarcely perceptible.

When the creature in its progress meets with a particle of food, it
spreads itself over it, draws it into its mass, within which a temporary
hollow or vacuole is made for its reception; there it is digested, the
refuse is squeezed out through the external surface; the nutritious
liquid that is left in the vacuole seems to be dispersed in the sarcode,
for the vacuole disappears. An Amœba often spreads itself over a Diatom,
draws it into a vacuole newly made to receive and digest it; the
siliceous shells of the diatom are pushed towards the exterior, and are
ultimately thrust out; then the vacuole disappears, either immediately
or soon after. These improvised stomachs are the earliest form of a
digestive system.

Besides the vacuoles of which there may be several at a time, the slow
and nearly rhythmical pulsations of a vesicle containing a subtle fluid
may be seen, which changes its position in the interior of the sarcode
with every motion of the Amœba. It gradually increases in size, then
diminishes to a point, and as some of the digestive vacuoles nearest the
surface of the animal are observed to undergo distension when the
vesicle contracts, and to empty themselves gradually as it fills, Dr.
Carpenter thinks it can hardly be doubted that the function of the
vesicle is to maintain a continual movement of nutritious matter, among
a system of channels and vacuoles excavated in the substance of the
body. It is the first obscure rudiment of a circulating system.

In all the Amœbæ the semi-fluid sarcode, with the numerous bodies
suspended in it, rotates at a varied rate within the pellucid coat; a
motion presumed to be for respiration, that is to exchange carbonic acid
gas for oxygen, so indispensable for animal life.[4]

Although like other animals, the Amœba cannot change inorganic into
organic matter, as the vegetable Amœba can do, these two Protozoa are
similar in one mode of reproduction; for portions of the animal Amœba or
even one of the pseudopodia separate from the gelatinous mass, move to a
little distance on the surface of the water, and become independent
Amœbæ.

With a high microscopic power, many bodies besides the digesting
vacuoles and pulsating vesicles may be seen imbedded in the sarcode of
the Amœba princeps; namely, coloured molecules, granules, fat-globules,
and nuclei. All these bodies were seen by Mr. Carter, in certain Amœbina
he found at Bombay, together with what he believed to be female
reproductive cells, and motile particles similar to spermatozoids, or
male fertilizing particles.

[Illustration: Fig. 87. Actinophrys sol.—A, ordinary form; B, act of
division or conjugation; C, process of feeding; D, discharge of fæcal
matter, _a_ and _b_; _o_ _o_, contractile vesicles.]

The Actinophrys, a genus of the order Radiolaria, differs from the Amœba
princeps in having a definite nearly spherical form with slender
root-like filamental pseudopodia radiating from its surface in all
directions as from a centre. They taper from the base to the apex, and
sometimes end in knobs like a pin’s head, but vary much in length and
number, and can be extended and retracted till they are out of sight.
They are externally of a firmer substance than the sarcode of the body,
which is merely a viscid fluid inclosed in a pellucid film. The
Actinophrys sol, which is the type of the genus, is a sphere of from
1/1300 to 1/650 of an inch in diameter, with slender contractile
filaments the length of its diameter extending from its surface as rays
from the sun. It can draw them in and flatten its body so as to be
easily mistaken for an Amœba. This creature, which is common in
fresh-water pools where aquatic plants are growing and even in the sea,
has little power of moving about like the Amœba; it depends almost
entirely on its pseudopodia for food. They have an adhesive property,
for when any animalcule or diatom comes in contact with one of them,
they adhere to it; the filament then begins to retract, and as it
shortens the adjacent filaments apply their points to the captive,
enclose it, coalesce round it, the whole is drawn within the surface of
the Actinophrys, the captive is imbedded in the sarcode mass, and passes
into a vacuole where it is digested, and then the pseudopodia thrust out
the undigested matter by a process exactly the reverse of that by which
the food was taken in (D fig. 87). The pseudopodia are believed by
Professor Rupert Jones to have the power of stunning their prey, for if
an animalcule be touched by one of them, it instantly becomes
motionless, and does not resume its activity for some time. The
pulsations of the contractile vesicle are very regular, and its duty is
the same as in the Amœba princeps.

The Actinophryna are propagated like the lowest vegetables by gemmation
and conjugation, shown in B fig. 87; moreover Mr. Carter saw the
production of germ-cells and motile particles in the Actinophrys exactly
after the mode already described in the Amœba.

Mr. Carter mentions an instance in which the Actinophrys sol showed what
may possibly be a certain degree of instinct. An individual was in the
same vessel with vegetable cells charged with particles of starch; one
of the cells had been ruptured and a little of the internal matter was
protruded through the crevice. The Actinophrys came, extracted one of
the starch-grains, and crept to a distance; it returned, and although
there were no more starch-grains in sight, the creature managed to take
them out from the interior of the cell one by one, always retiring to a
distance and returning again, showing that it knew its way back, and
where the starch-grains were to be found. On another occasion Mr. Carter
saw an Actinophrys station itself close to the ripe spore cell of a
plant, and as the young zoospores came out one after another, the
Actinophrys caught every one of them even to the last and then retired
to a distance as if instinctively conscious that no more remained. Like
Amœbæ these animals select their food, but notwithstanding the superior
facility and unfailing energy with which they capture prey larger and
more active than themselves, they are invariably overcome even by a very
small Amœba which they avoid if possible. When they come into contact
the Amœba shows unwonted activity, tries to envelope the Actinophrys
with its pseudopodia, but failing to capture the whole animal it tears
out portions and conveys them to improvised vacuoles to be digested. Dr.
Wallich mentions that he had seen nearly the half of a large Actinophrys
transferred piecemeal to the interior of its enemy, where it was quickly
digested.

_Fig. 88, p. 19._

[Illustration: ACANTHOMETRA BULBOSA.]

As every part of the body of the Actinophrys is equally capable of
performing the part of nutrition, respiration, and circulation; and as
in the absence of muscles and nerves they may be presumed to have no
consciousness, the marks of apparent intelligence can only be attributed
to a kind of instinct, and their motions to the vast inherent
contractility of the sarcode and its enclosing film, which is also the
case with the Amœbæ.

The Acanthometræ (see fig. 88, Acanthometra bulbosa) are all marine
animals; their skeleton consists of a number of long spicules which
radiate from a common centre, tapering to their extremities. These
spicules are traversed by a canal with a furrow at the base through
which groups of pseudopodia enter, emerging at the apex. Besides, there
are a vast number of pseudopodia not thus enclosed, resembling those of
the Actinophrys in appearance and action. The body is spherical, and
occupies the spaces left between the bases of the spicules. The exterior
film covering the body seems to be more decidedly membranaceous than
that of the Actinophrys, but it is pierced by the pseudopodia which
radiate through it. This exterior film itself is enclosed in a layer of
a less tenacious substance, resembling that of which the pseudopodia are
formed. There is a species of Acanthometra (echinoides) extremely common
in some parts of the coast of Norway, which, to the naked eye, resembles
merely a crimson point.

_Fig. 90, p. 20._

[Illustration: DICTYOPODIUM TRILOBUM.]

[Illustration: Fig. 91. Podocyrtis Schomburgi.]

The Polycystina are an exceedingly numerous and widely dispersed group
of siliceous rhizopods. They are inhabitants of the deep waters, having
been brought up from vast depths in the Atlantic and Pacific oceans.
Their bodies are inclosed in siliceous shells, which have either the
form of a thin hollow sphere perforated by large openings like windows,
or of a perforated sphere produced here and there into tubes, spines,
and a variety of singular projections: so they have many varied but
beautiful microscopic forms. The animal which inhabits these shells is a
mouthless mass of sarcode, divided into four lobes with a nucleus in
each and covered with a thick gelatinous coat. It is crimson in the
Eucyrtidium and Dictyopodium trilobum of Haeckel (figs. 89 and 90): in
others, as the Podocyrtis Schomburgi, it is olive brown with yellow
globules (fig. 91). These creatures extend themselves in radiating
filaments through the perforations of their shells in search of food,
like their type the Actinophrys sol, to whose pseudopodia the filaments
are perfectly similar in form, isolation, and in the slow movements of
granules along their borders. The Polycystine does not always fill its
shell, occasionally retreating into the vault or upper part of it, as in
the Eucyrtidium (fig. 89, frontispiece to vol. i.). Sometimes the shell
is furnished with radiating elongations, as in the Dictyopodium trilobum
(fig. 90). In both of these shells the animal consists of four crimson
lobes. These beautiful microscopic organisms are found at present in the
Mediterranean, in the Arctic and Antarctic seas, and on the bed of the
North Atlantic. They had been exceedingly abundant during the later
geological periods; multitudes are discovered in the chalk and marls in
Sicily, Greece, at Bermuda, at Richmond in Virginia and elsewhere; in
all 282 different fossil forms have been described, grouped in 44
genera.

_Fig. 92, p. 21._

[Illustration: AULOCANTHA SCOLYMANTHA.]

_Fig. 93, p. 21._

[Illustration: ACTINOMMA DRYMODES.]

_Fig. 94, p. 21._

[Illustration: HALIOMMA ECHINASTER.]

In certain Polycystina, the perforations of the shell are so large and
so close together, that the sarcode body of the animal appears to be
covered by a siliceous net. This connects them with the Thalassicollæ,
minute creatures found passively floating on the surface of the sea. Th.
morum, which is one of the most simple of the few forms known, has a
spherical body of sarcode covered with a siliceous net, through which
the pseudopodia radiate in all directions, as in the Actinophrys, but it
is studded at regular distances with groups of apparently radiating
siliceous spicules.

The Aulocantha scolymantha (fig. 92), found by M. Haeckel in the
Mediterranean, may be taken as an example of the most general form of
the Thalassicolla. The siliceous skeleton of some of the Radiolaria
resembles the Chinese ivory toy of ball within ball. That of the
Actinomma drymodes (fig. 93) consists of three perforated concentric
spheres, with six strong spicules attached to the outer surface,
perpendicular to one another and prolonged in the interior to the
central sphere. Hundreds of finer bristle-like spicules radiate from the
surface. The animal is chiefly contained in the central sphere, and from
it a perfect forest of fine, long pseudopodia radiate in thick tufts
through the apertures of the exterior sphere.

The skeleton of the Haliomma (fig. 94) consists of only two concentric
spheres. In many species of Haliomma and Actinomma the animals are of
the most vivid vermilion or purple colour. Little or nothing is known of
the reproduction of these microscopic organisms.

The Actinomma drymodes and the Haliomma are two of the most beautiful
microscopic rhizopods discovered by M. Haeckel.

There is a family of fresh-water testaceous rhizopods of which one group
secretes its shell and the other builds it. The horny shell secreted by
the group of the Arcella presents various degrees of plano-convexity,
the convexity in some cases amounting to a hemisphere. They rarely, if
ever, have mineral matter on their surface, which is studded with
regular but very minute hexagonal reticulations. The aperture or mouth
of the shell is small, and invariably occupies the centre of the plane
surface, its margins being more or less inverted. The form of the shell
is exceedingly varied, sometimes it even has horns indefinite in number,
sometimes symmetrical, sometimes not; when its test or covering becomes
too small for its increasing size, it quits it, and secretes a new one.
The filamental pseudopodia proceed from the mouth of the shell only, and
by means of these it creeps about on its mouth in search of food.

[Illustration: Fig. 95. Simple Rhizopods.—A, B, Difflugiæ; C, D,
Arcellæ.]

The Difflugia build their own shells, which are usually truncated
spheres, ovate, or sometimes elongated into the form of a pitcher or
flask. The most minute recognisable of these shells is about the 1/1000
of an inch in diameter, but they are constructed with the most perfect
regularity. The Difflugia pyriformis or symmetrica has the form of an
egg with an aperture at the small end. It is entirely made up of
rectangular hyaline plates, arranged with the greatest regularity in
consecutive transverse and longitudinal rows, the smaller ones being at
the extremities, while the larger ones occupy the central and widest
portion of the structure. The inhabitant of this abode is an Amœba with
a sarcode body covered with a thin film, from whence it sends off
pseudopodia through the mouth of its shell. The Difflugia is propagated
by conjugation, but before that takes place it becomes densely charged
with chlorophyll-cells and starch-grains. The former disappear during
the subsequent changes, and are replaced by a mass of colourless cells
full of granules which are supposed to be the elements of a new
generation. The embryo or earliest form is a minute truncated sphere,
but the animal builds up its habitation very much according to local
circumstances.

The greater number of the Difflugiæ secrete a substance which forms a
smooth layer in the interior, which the animal covers with sarcode from
its mouth, and then it drags itself with its pseudopodia to the
particles which it selects, and they adhere to it. The particles
selected are invariably mineral matter. ‘The selective power is carried
to such an extent that colourless particles—sometimes quartzose,
sometimes felspathic, sometimes micaceous—are always chosen.’ ‘The
particles seem to be impacted into the soft matter, laid on the exterior
in the same way that a brick is pressed into the yielding mortar, and
that too, in so skilful a manner as to leave the smallest possible
amount of vacant area; whilst in the specimens of Difflugia in which
tabular or micaceous particles are used, they are sometimes disposed
with such nicety that there is no overlapping, but the small fragments
are placed so as to occupy the space left between the larger ones. These
excellent architects seem to know that in the valves of the Diatoms are
combined the properties best suited to their wants, that is,
transparency and form, capable of being easily arranged.’

Both the Difflugia and Arcella are Amœbæ in the strictest sense of the
word; their bodies consist of sarcode, which sends out finger-like lobes
from the mouth of the shell at one end, while the other end has an
adhesive property, which fixes it to the bottom. The nucleus and
contractile vesicles are identical in character with those of the Amœbæ,
and exhibit the same tendency to subdivision at certain periods of the
creature’s history that is witnessed on a large scale in the Amœba
proper; and the reproductive process is the same.[5]

The Difflugiæ are found in rivulets and pools containing aquatic plants;
the condition of the water and the nature of the soil have a great
influence on the form of their shell.

The Euglyphæ is the third group of fresh-water rhizopods. They are
extremely minute, and there are no mineral particles whatever on their
shells, the axes of which do not coincide with the aperture. The
interior of the animal is like that of the Arcella and Difflugia, but it
differs from them in as much as the pseudopodia and ectosarc, or
external coat, are finely granular, and the whole mass of the body
possesses a decided degree of adhesive viscidity. The pseudopodia are
filiform, tapering, radiating, and readily coalesce; and ‘as if to
compensate for the restricted power of locomotion, compared with that of
the Amœba proper, the pseudopodia of the Euglyphæ are much more active.
The rapidity with which they admit of being projected outwards, and
withdrawn into the shell, is unequalled in any other form, presenting
the most wonderful example of inherent contractility in an amorphous
animal substance, that is to be met with in either of the great organic
kingdoms.’[6]

The order Reticularia, with a very few exceptions, are animals dwelling
in calcareous microscopic shells, and differing essentially in
constitution from all the preceding Rhizopods. The ectosarc or
surface-layer of the sarcode in the Amœba and Actinophrys has so much
consistence, that their pseudopodia, which are derived from it, have a
decidedly firm outline and never coalesce; whereas in the order
Reticularia, the sarcode is merely a semi-fluid protoplasm or colourless
viscid fluid, without the smallest surface-layer or film, so that their
pseudopodia possess no definiteness either in shape, size or number.
Sometimes they are cylindrical, and sometimes form broad flat bands,
whilst they are often drawn into threads of such extreme tenuity, as to
require a high magnifying power to discern them. They coalesce and fuse
into each other so freely and so completely when they meet, that no part
of their substance can be regarded as having more than a viscous
consistence. Their margins are not defined by continuous lines, but are
broken by granules irregularly disposed among them, so that they appear
as if torn; and these granules, when the animal is in a state of
activity, are in constant motion, passing along the pseudopodia from one
end to the other, or passing through the connecting threads of this
animated network from one pseudopodium to another, with considerable
rapidity, analogous to the movement of the particles in the cells of the
hairs of the Tradescantia and other plants.[7]

The sarcode body of the Gromiæ is inclosed in a yellowish brown horny
envelope or test of an oval shape, with a single round orifice of
moderate size, through which the pseudopodia extend into the surrounding
water, some forms of the animal being marine, others inhabitants of
fresh water. When the animal is at rest all is drawn within the test,
and when its activity recommences, single fine threads are put out which
move about in a groping manner until they find some surface to which
they may attach themselves. When fixed, sarcode flows into them so that
they rapidly increase in size, and then they put forth finer
ramifications, which diverging come in contact with those from other
stems, and by mutual fusion form bridges of connection between the
different branching systems; for the protoplasm spreads over the
exterior of the test, and from it pseudopodia extend and coalesce,
wherever they meet, so that the whole forms a living network, extending
to a distance of six or eight times the length of the body. Fig. 96
represents the Gromia oviformis with its pseudopodia extended.

[Illustration: Fig. 96. Gromia oviformis.]

In the Gromiæ the granular particles in the semi-fluid protoplasm are in
constant motion. In the finer filaments there is but one current, and a
particle may be seen to be carried to the extremity, and return again
bringing back with it any granules that may be advancing; and should
particles of food adhere to the filament they take part in the general
movement. In the broader filaments two currents carrying particles pass
backwards and forwards in opposite directions at the same time, and the
network in which these motions are going on is undergoing continual
changes in its arrangements. New filaments are put forth sometimes from
the midst of the ramifications, while others are retracted; and
occasionally a new centre of radiation is formed at a point where
several threads meet. The food consists of diatoms and morsels of
vegetable matter; but the Gromiæ have no vent, so that the indigestible
matter collects in a heap within them. However, as the form of the test
is such that the animal cannot increase its size, it leaves it when it
becomes too small for its comfort and forms another, and it is supposed
to get rid of the effete matter at the same time. The Gromiæ have no
nucleus or contractile vesicle.


                        CLASS II.—FORAMINIFERA.

The geological importance of the Foraminifera, their intrinsic beauty,
the prodigious variety of their forms, their incredible multitude, and
the peculiarity of their structure, have given these microscopic
organisms the highest place in the class of Rhizopods. The body of these
animals consists of a perfectly homogeneous sarcode or semi-fluid
protoplasm, showing no tendency whatever to any film or surface-layer.
It is inclosed in a shell; and the only evidence of vitality that the
creature gives, is a protrusion and retraction of slender threads of its
sarcode, through the mouth or pores of the shell, or through both
according to its structure. Fig. 97 shows some of their forms.

By far the greater number of the Foraminifera are compound or
many-chambered shells. When young, the shell has but one chamber,
generally of a globular form; but as the animal grows, others are
successively added by a kind of budding in a definite but different
arrangement for each order and genus of the class. When the creature
increases in size, a portion of its semi-fluid sarcode projects like a
bud from the mouth of its shell. If it be of the one-chambered kind, the
bud separates from its parent before the shelly matter which it secretes
from its surface consolidates, and a new individual is thus produced.
But if the primary shell be of the many-chambered kind, the shelly
secretion consolidates over the sarcode projection which thus remains
fixed, and the shell has then two chambers, the aperture in the last
being the mouth, from which, by a protrusion of sarcode, a third chamber
may be added, the new chamber being always placed upon the mouth of its
predecessor, a process which may be continued indefinitely, the mouth of
the last segment being the mouth of the whole shell.

[Illustration: Fig. 97. Various forms of Foraminifera:—A, Oolina
claxata; B, Nodosaria rugosa; C, Nodosaria spinicosta; D Cristellaria
compressa; E, Polystomella crispa; F, Dendritina elegans; G, Globigerina
bulloïdes; H, Textularia Mayeriana; I, Quinqueloculina Bronniana.]

By this process an ovate shell with a mouth at one extremity may have a
succession of ovate chambers added to it, each chamber being in
continuity with its predecessor so that the whole shell will be straight
and rod-like, the last opening being the mouth. If the original shell be
globular, and if all the successive gemmæ given out be equal and
globular, the shell covering and uniting them will be like a number of
beads strung upon a straight wire. Sometimes the successive gemmæ
increase in size so that each chamber is larger than the one which
precedes it; in this case the compound shell will have a conical form,
the primary shell being the apex, and the base the last formed, the
aperture of which is the mouth of the whole shell; a great many
Foraminifera have this structure. The spiral form is very common and
much varied. A series of chambers increasing in size may coil round a
longitudinal axis, like the shell of the snail; but if each of the
successive chambers, instead of being developed exactly in the axis of
its predecessor, should be directed a little to one side, a curved
instead of a straight axis would be the result; there is a regular
gradation of forms of Foraminifera between these two types. The
convolutions are frequently flat and in one plane, but the character of
the spiral depends upon the successive enlargement or not of the
consecutive chambers; for when they open very wide and increase in
breadth, every whorl is larger than that which it surrounds; but more
commonly there is so little difference between the segments after the
spiral has made two or three turns, that the breadth of each whorl
scarcely exceeds that which precedes it.

However varied the forms may be, the mouth of the last shell is the
mouth of the whole, either for the time being or finally. For all the
chambers are connected by narrow apertures in the partitions between
them. Each chamber is occupied by a segment of the gelatinous sarcode
body of the animal, and all the segments are connected by sarcode
filaments passing through the minute apertures in the partitions between
the chambers, so that the whole constitutes one compound creature.

Although the character and structure assumed by the semi-fluid bodies of
the known Foraminifera have been determined in most cases with admirable
precision, it is still thought advisable to arrange them according to
the substance of the shell: consequently they form three natural orders;
namely, the Porcellanous or imperforate, which have calcareous shells
often so polished and shining that they resemble porcelain; secondly,
the Arenaceous Foraminifera, consisting of animals which secrete a kind
of cement from their surfaces, and cover themselves with calcareous or
siliceous sand-grains; and lastly, the Vitreous and Perforated order,
which is the most numerous and highly organized of the whole class, has
siliceous shells transparent as glass, but acquires more or less of an
opaque aspect in consequence of minute straight tubes which perforate
the substance of the shell perpendicularly to its surface, and
consequently interfere with the transmission of light.


                 _Order of Porcellanous Foraminifera._

The Miliolidæ constitute the porcellanous order, which consists of
twelve genera and many species, varying from a mere scale to such as
have chambered shells of complicated structure.

The genus Miliola has minute white shells resembling millet seeds, often
so brilliantly polished that they are perfectly characteristic of the
porcelain family to which they belong. No Foraminifera are better suited
to give an idea of the intimate connection between the shell and its
inhabitant than the Miliola, the fundamental type of this genus. The
shell is a spiral (I, fig. 97), which is made up of a series of half
turns arranged symmetrically on its two sides. Each half turn is longer
and of greater area than that on the opposite side, so that each turn of
the spire has a tendency to extend itself in some degree over the
preceding one, which gives a concave instead of a convex border to the
inner wall of the chamber. The sarcode body of the Miliola consists of
long segments which fill the chambers, connected by threads of sarcode
passing through the tubular constrictions of the shell. As the animal
grows, its pseudopodia extend alternately now from one end, and now from
the other extremity of the spiral, and by them it fixes itself to
seaweeds, zoophytes, and other bodies, for these Foraminifera never
float or swim freely in the water. The genus Miliola is more extensively
diffused than almost any other group of Foraminifera; they are most
abundant between the shore and a depth of 150 fathoms, and are
occasionally brought up from great depths. Beds of miliolite limestone
show to what an extent the Miliola abounded in the seas of the Eocene
period; but the type is traced back to the Lias.

The genus Peneroplis is distinguished by a highly polished opaque white
shell; its typical form is an extremely flat spire of two turns and a
half opening rapidly and widely in the last half whorl. It is strongly
marked by depressed bands which indicate the septa or shelly partitions
between the chambers in the interior. The polished surface of the shell
is striated between and transversely to the bands by parallel
platted-looking folds 1/1400 of an inch apart. But the peculiarity of
this shell and its congeners is, that the partitions between the
chambers in its interior are perforated by numerous isolated and
generally circular pores which in this compressed type are in a single
linear row. Their number depends upon the length of the partition
between the chambers, which increases with the age of the animal and
size of the shell. There is but one pore in each of the consecutive
partitions from the globular centre to the fourth chamber. From the
fourth to the seventh chamber the communication is by two pores; after
this the number is gradually increased to three, four, six, &c., up to
forty-eight, so that the last segment may send out forty-eight
pseudopodia from the mouth of the shell. In its early youth one
pseudopodium appears to have been sufficient to find food for the
animal, but as the shell increased in size and the segments in number, a
greater supply of food was requisite and a greater number of pseudopodia
were necessary to fish for it. Moreover when an addition to the shell is
required the pseudopodia coalesce at their base and form a continuous
segment upon which the new portion of the shell is moulded.

In varieties of the Peneroplis where the spire is less compressed there
are sometimes two rows of pores in the partitions between the chambers.
The Dendritine variety deviates most from that described. It is
characterised by a single large aperture in each partition which sends
out ramifications from its edges. The form of these openings depends
upon that of the spire; when compressed the aperture is linear and less
branched at its edges; but in shells which have a very turgid spire it
is sometimes broader than it is long, and much branched; but these
extremes are connected by a variety of forms. The shells of this variety
of the Peneroplis are strongly marked by the depressed bands and striæ,
as in the Dendritina elegans (F, fig. 97). The segments of the animal
inhabiting these shells must be more intimately connected than in most
of the other Foraminifera; and the pseudopodia sent through these large
apertures out of the mouth of the shell must be comparatively quite a
mass of sarcode. The Dendritinæ are inhabitants of shallow water and
tropical seas, while the other members of the genus Peneroplis abound in
the Red Sea and the seas of other warm latitudes, especially in the zone
of the great laminarian fuci. They do not appear in a fossil state prior
to the beginning of the Tertiary period.

The last whorls of some of the compressed spiral Foraminifera of the
Porcellanous order so nearly encompass all their predecessors, that the
transition from a flat spiral to the Orbitolite with its flat disk of
concentric rings is not so abrupt as might at first appear. The gradual
change may be distinctly traced in the species of the genus Orbiculina.
The exteriors of the shells of the genus Orbitolites have less of the
opaque whiteness than many others of its family. In its simplest form it
is a disk about the 1/500 of an inch in diameter, consisting of a
central nucleus surrounded by from ten to fifteen concentric circular
rings. The surface is usually plane, though sometimes it is concave on
both surfaces in consequence of the rings increasing in thickness
towards the circumference. The rings or zones are distinctly marked by
furrows on the exterior of the shell, and each of these zones is divided
by transverse furrows into ovate elevations with their greatest diameter
transverse to the radius of the disk, so that the surface presents a
number of ovate elevations arranged in consecutive circles round the
central nucleus. The margin of the disk exhibits a series of convexities
with depressions between them; in each of these depressions there is a
circular pore surrounded by a ring of shell: these pores are the only
means the animal possesses of communicating with the water in which it
lives.

[Illustration: Fig. 98. Simple disc of Orbitolites complanatus.]

[Illustration: Fig. 99. Animal of Orbitolites complanatus.]

Fig. 98 is a horizontal section of the simple Orbitolite showing the
internal structure of the disk. A pear-shaped chamber with a
circumambient chamber forms a nucleus which is surrounded by series of
concentric rings of ovate cavities. The chambers of the nucleus and all
the cavities are filled with segments of homogeneous semi-fluid sarcode,
which constitute the body of the animal (fig. 99). The segments in the
rings are connected circularly by gelatinous bands of sarcode extending
through passages which connect the cavities laterally. The segments are
also connected radially by similar sarcode bands, which originate in the
mass of sarcode filling the nucleus, and extend to the pores in the
margin of the disk. The cavities of each zone alternate in position with
those of the zones on each side of it. The animal sends out its
pseudopodia through the marginal pores in search of food, which consists
of Diatoms and Desmidiaceæ; they are drawn in, digested without any
stomach, and the nutritious liquid is conducted by the gelatinous bands
from segment to segment and from zone to zone, even to the innermost
recesses of the shell.

It is supposed that during the growth of the Orbitolite, when the animal
becomes too large for its abode, its pseudopodia coalesce and form a
gelatinous massive coat over the margin of the exterior zone, which
secretes a shelly ring with all its chambers and passages, each ring
being a mere vegetative repetition of those preceding it. That
vegetative property enables the animal to repair its shell or add a part
that is wanting. For, if a small portion of a ring be broken off and
separated from the living animal, it will increase so as to form a new
disk, the want of the central part or nucleus not appearing to be of the
smallest consequence; indeed, the central rings are very often
imperfect. The sarcode of these animals is red, and although the shell
is of a brownish-yellow by transmitted light, it is so translucent that
the red tint is seen through it.

The simple Orbitolite has many varieties. Sometimes it begins its life
as a spiral which changes to a circular disk as it advances in age. It
varies in thickness, and some of its very large varieties may be said to
consist of three disks or stories of concentric chambers and many
marginal pores instead of one. The upper and base stories of concentric
chambers are alike, the intermediate one very different, but the sarcode
segments in all the three are so connected as to form a very complex
compound animal.[8] Different as this structure is from that of the
simple Orbitolite, they are merely varieties of the same species; for it
has been shown by Dr. Carpenter that, although many pass their lives in
the simple one-storied state, they may change into the complex form at
any stage of their growth; and as an equally extensive range of
variation has been proved by Professor Williamson and Mr. Parker to
prevail in other groups of Foraminifera, the tendency to specific
variation seems to be characteristic of that type of animal life, and
consequently the number of distinct species is less than they were
supposed to be.

The Orbitolites are found in the dredgings of all the warmer seas, in
vast multitudes at the Philippine Islands, but those from Australia are
the most gigantic, being sometimes the size and thickness of a shilling.


                  _Order of Arenaceous Foraminifera._

In the numerous family of Lituolidæ the abode of the animal consists of
a cement mixed with very fine particles of sand with larger ones
imbedded in the surface. The order includes a wide range of forms
divided into three genera, the simplest of which consists of a
cylindrical tube twisted into a spiral gradually increasing in diameter,
and attached to a foreign substance by one of its surfaces. The creature
which lives in it is a uniform cord of sarcode, which sends its
pseudopodia out through a large aperture at the extremity of its tube in
search of food. Although the tube consists of sand imbedded in an
ochreous-coloured cement secreted by the animal, its surface is smooth
as a plastered wall. The spiral tubes of this genus take various forms,
and in some cases are divided into chambers.

The members of the genus Lituola exude from their surfaces a thick coat
of cement with a quantity of siliceous particles roughly imbedded in it,
but in some instances the particles are so uniform in size and shape,
and are so methodically arranged, that the surface resembles a
tesselated pavement. The usual form of the Lituola is a mere string of
oval convex chambers increasing gradually in size, and fixed to shells
and corals by their flat surfaces. In some instances the shells, or
rather the substitutes for shells, take a nautiloid form, and become
detached from the foreign bodies to which they were attached. In the
highest forms of this genus the chambers are divided by secondary
partitions.

The typical form of the genus Valvulina is a three-whorled, three-sided
pyramidal shell, with three chambers in every turn of the spire. The
aperture is large and round, with a valve of smaller size attached by a
tooth of shell to its rim. The creature itself has an exceedingly thin
perforated vitreous shell, covered by an incrustation of calcareous
particles, which so entirely blocks up the perforations that it can only
extend its pseudopodia through the mouth of its shell.


                   _Order of Vitreous Foraminifera._

Nearly all the Foraminifera on the British coasts belong to the Vitreous
or Perforated order, which consists of three natural families and many
genera. Their shells are vitreous, hyaline, and generally colourless,
even although the substance of the animal is deeply coloured; in some
species both the animal and its shell are of a rich crimson. The glassy
transparency of the shells would be perfect were they not perforated by
numerous tubes running from the interior of the chambers straight
through the shell, and ending in pores on its surface. According to
microscopic measurement the tubes in the Rotalia, which are the largest,
are on an average the 1/1000 of an inch in diameter, and as they are
somewhat more than that apart, the transparency of the shell appears
between them and gives the surface a vitreous aspect. The pseudopodia of
the animal have been seen to pass through every part of the wall of the
chambers occupied by it; the apertures of the tubuli in this case are
wide enough to permit particles of food to be drawn into the interior of
the shell. But threads of sarcode of extreme tenuity alone could pass
through the tubuli of the Operculina, which are not more than the
1/10000 of an inch in diameter, and the distance between them not much
greater, which gives the shell an opaque appearance. Particles of food
can hardly be small enough to pass through such tubes into the interior
to be digested. Dr. Carpenter, however, is almost certain, from the
manner in which the animal repairs injuries done to its shell, that the
semi-fluid sarcode extends itself at certain times, if not constantly,
over the exterior of the shell, as in the Gromia; and therefore it is by
no means impossible that the digestive process may really be performed
in this external layer, so that only the products of digestion may have
to pass into the portion of the sarcode occupying the body of the shell.

In such many-chambered shells as are pierced by tubuli wide enough to
permit particles of food to be drawn into the interior, each segment of
the animal, being fed within its own chamber, has a life of its own, at
the same time that it shares with all the others in a common life
maintained by food taken in through the mouth of the shell. There are
many instances of this individual life combined with a common life among
the lowest tribes of animals.

Although the Perforated order contains types widely apart, they are
always connected by intermediate forms; but there is no such connection
between the two great natural orders, which are not only separated by
the tubuli in the shell, but in many instances by the structure of the
interior and the corresponding character of the animal.

In the Lagenidæ, which form the first family of the Perforated order,
the vitreous shell possesses great hardness, and is pierced by numerous
small tubuli. It is very thin, and of glossy transparency. The first
four shells in fig. 97 represent some of its forms.

The genus Nodosaria has a very extensive range of forms, from the
elongated structure to the nautiloid spiral, depending upon the relative
proportions and arrangement of the segments. The segments are separated
by constrictions transverse to the axis of growth, or by bands as in the
Nodosaria rugosa, B, fig. 97. It frequently happens that parts of the
shell are not perforated; and there are generally longitudinal ribs
which sometimes have spines projecting from every part of the interior,
as in Nodosaria spinicosta, C, fig. 97.

In the genus Nodosaria, the axis of growth changes from a straight line
to that of a spiral, so that the septa or divisions between the segments
cross the axis obliquely, and the aperture instead of being exactly
central becomes excentric. Between these extremes there is a numerous
series of gradations. The Cristallaria is the highest type; the form is
a nautiloid spiral, more or less compressed (D, fig. 97), of which each
whorl has its chambers extended by winged projections so as to reach the
centre, and entirely encloses the preceding whorl. The number of
chambers in each whorl is much smaller than in most of the nautiloid
spirals, not being more than eight or nine. The divisions are always
strongly marked externally by septal bands, varying in character
according to the species. The margin of the shell runs into a keel,
which is sometimes extended into a knife-edge. Nearly all the Lagena
family are found in the North Atlantic and Mediterranean, especially in
the Adriatic, which is rich in species. In the Nodosaria the cells which
compose the shell have so little connection one with another that they
may be easily detached; which gives reason to believe that the
separation of the parts may be a means of reproduction and dispersion.

The Globigerinidæ are the most numerous family of the perforated series,
and the most remarkable in the history of the existing Foraminifera.
They are distinguished by the coarseness of the perforations in their
shells, and by the crescentic form of the aperture by which the chambers
communicate with each other.

The genus Globigerina consists of a spiral aggregation of globose
segments, which are nearly disconnected from each other although united
by mutual cohesion. The segments are always somewhat flattened against
one another in their planes or junctions, and sometimes the flattening
extends over a pretty large surface as in G, fig. 97. The entire series
of segments shows itself on the upper side, but on the lower side only
the segments forming the latest convolution are prominent; they are
usually four in number, and are arranged symmetrically round a deep
depression or vestibule; the bottom of which is formed by the segments
of the earlier convolutions. In this vestibule each segment opens by a
large crescent-shaped orifice, the several chambers having no direct
communication with each other. The entire shell of the ordinary type may
attain the diameter of about 1/30 of an inch, but it is usually much
smaller; the typical form, however, is subject to very considerable
modifications. In newly formed segments of Globigerina, the hyaline
shell substance is perforated by tubuli varying from 1/10000 to 1/5000
of an inch in diameter, arranged at pretty regular distances; but in
deep seas the surface of the shell is raised by an external deposit into
tubercles or ridges, the orifices of the pores appearing between them.

_Fig. 100, p. 41._

[Illustration: ROSALINA ORNATA.]

Each chamber of the shell is occupied by a reddish-yellow segment of
sarcode, from which pseudopodia are seen to protrude; and it is supposed
that the sarcode body also fills the vestibule, since without such
connecting band it is difficult to understand how the segments which
occupy the separate chambers can communicate with each other, or how new
segments can be budded off. In the Globigerina the slight cohesion gives
reason to believe that the separation of the parts may be a means of
reproduction.

The Rosalina ornata, one of the most beautiful specimens of this group,
and remarkable for the size of its pores, is represented in fig. 100
with its pseudopodia extended, and coalescing in some parts.

The shells of the genus Textularia consist of a double series of
chambers disposed on each side of an axis, so that they look as if they
were mutually interwoven. As the segments for the most part increase
gradually in size, the shell is generally triangular, the apex being
formed of the first segment, and its base of the two last (H, fig. 97).

The aperture is always placed in the inner wall of each chamber, close
to its junction with the preceding segment on the opposite side. In the
compressed shells it is crescent-shaped, but it is semilunar in the less
compressed, and may even be gibbous. The shell is hyaline, with large
pores not very closely set, though in some varieties they are minute and
near to one another. Sometimes the pores open on the surface in deep
hexagonal pits. The older shells are frequently incrusted with large
coarse particles of sand, and some specimens from deep water are almost
covered with fine sand, but with a good microscope the pores may be seen
between them.

The sarcode segments of the animal perfectly correspond in shape and in
alternate arrangement with the segments of the shell, and are connected
by bands of sarcode passing through the crescent-shaped apertures by
which each chamber communicates with that which precedes and follows it.

The Textulariæ are among the most cosmopolitan of Foraminifera; some of
their forms are found in the sands and dredgings from all shores, from
shallow or moderately deep water. In time they go back to the Palæozoic
period.

The Rotalia Beccarii, common on the British coast, affords a good
example of the supplemental skeleton, a structure peculiar to some of
the higher vitreous Foraminifera. It has a rather compressed turbinoid
form with a rounded margin. Its spire is composed of a considerable
number of bulging segments gradually increasing in size, disposed with
great regularity, and with their opposed surfaces closely fitted to each
other. The whole spire is visible on the exterior, with all its
convolutions, and on account of the bulging form of the segments, their
lines of junction would appear as deep furrows along the whole spire,
were they not partly or wholly filled up with a homogeneous
semi-crystalline deposit of shell-substance, which is very different in
structure and appearance from the porous shell wall of the segments.

The genus Calcarina is distinguished by a highly developed intermediate
skeleton with singular outgrowths, which is traversed by a system of
canals; through these the animal sends its pseudopodia into the water
for food to nourish the whole.

A homogeneous crystalline deposit invests almost the whole of the minute
spiral shell of a Calcarina, and sends out many cylindrical, but more
generally club-shaped spines in all directions, though they usually
affect more or less that of the equator, as in the typical form
Calcarina calcar, which is exactly like the rowel of a spur. The spines
are for the most part thick and clumsy, and give the shell a very
uncouth appearance, especially when their extremities are forked. The
turbinoid spire of the shell has a globose centre surrounded by about
five whorls progressively increasing in size, and divided by perforated
septa into chambers. Each whorl is merely applied to that preceding it,
and does not invest it in the least degree. Internally the turns of the
spire are separated from each other by the interposition of a solid
layer of shell-substance quite distinct from the walls of the chambers.
A crystalline deposit begins at the very centre of the spire in a thin
layer gradually increasing in thickness as it proceeds, and sending off
club-shaped spines from time to time so that the spines are of later and
later production, and become thicker and longer. From this it is evident
that the intermediate skeleton grows simultaneously with the turns of
the spire, but strange as it may seem, their growth is independent,
though both are nourished and increased by the sarcode in the interior
of the chambers. For the intermediate skeleton is traversed in every
part by an elongated network of canals, which begin from irregular
lacunæ or openings in the walls of the chambers, and extend to the
extremities of the spines. Through these canals threads of the sarcode
body of the animal within the chambers have access to the exterior, and
provide nourishment for the intermediate skeleton; while pseudopodia,
passing into the water through pores in the last partition of the shell,
provide for its growth and procure nourishment for the animal. The
communication between the adjacent chambers in the whorls, is by means
of a series of pores in the septa, or partitions; and it is through the
pores of the last septum that the pseudopodia of the animal have access
to the water to provide for the growth of the spire, for the punctures
on the surface are merely the terminations of some of the branching
canals. On approaching the surface the canals become crowded together in
some parts, leaving columns of the shelly skeleton unoccupied which
either appear as tubercles on the surface, or, if they do not rise so
high, form circular spots surrounded by punctations which are the
apertures of the canals.

The Rotaline series of the Globigerina family is one of the most
numerous and varied of the whole class of Foraminifera; but varied as
their forms are, they all bear the characteristic marks which
distinguish their order, with this essential difference, that in the
genus Globigerina each chamber of the spire has a communication with the
central vestibule by a crescent-shaped aperture, while in the Rotalinæ
each chamber only communicates by a crescentic aperture with that which
precedes and follows it.

In the Rotaline group the internal organization rises successively from
the simple porous partition between the chambers, to the double
partition with the radiating passages, and from the latter to the double
partitions, intermediate skeleton, and complicated system of canals. To
these changes the structure of the compound animal necessarily
corresponds, for it may be presumed that not only the chambers but all
the passages and canals in the interior of the shell are either
permanently or occasionally filled with its sarcode body.

However, it is in the Nummuline family that the Foraminifera attain the
highest organization of which they are capable. This family surpasses
all the Vitreous tribe in the density and toughness of the shell, the
fineness of its tubuli, and in the high organization of its canal
system. Their forms vary from that resembling a nautilus or ammonite to
a flat spiral or cyclical disk, like an Orbitolite, though vastly
superior to it in organization both with regard to the animal and to the
structure of the shell.

All the species of the genus Nummulite are spiral; in the typical form
the last turn of the spire not only completely embraces, but entirely
conceals, all that precede it. In general, the form is that of a double
convex lens of more or less thickness; some are flat, lenticular, and
thinned away to an acute edge, while others may be spheroidal with a
round, or obtuse edge. They owe their name to their resemblance to
coins, being, in general, nearly circular. Their diameters range from
1/16th of an inch to 4-1/2 inches, so that they are the giants of their
race; but the most common species vary from 1/2 an inch to 1 inch in
diameter.

[Illustration: Fig. 101. Section of Faujasina.]

Fig. 101 represents a section of the Nummulite Faujasina near and
parallel to the base of the shell. It shows a series of chambers
arranged in a flat spiral, and increasing in size from the centre to the
last turn of the spire, which embraces and conceals all that precede it.
Every segment of the animal is enclosed in a shell of its own, so that
they are separated from one another by a double wall and space between;
however, they are connected in the spiral direction by narrow passages
in the walls.

The segments of the animal in the exterior whorl have direct
communication with the water by means of a shelly marginal cord, _a_,
fig. 101, perforated by multitudes of minute tubes, less than the
1/10000 of an inch in diameter, through which threads of sarcode finer
than those of a spider’s web can be protruded. These tubuli are so very
fine and numerous, that they characterize the Nummuline family.

[Illustration: Fig. 102. Interior of the Operculina.]

Fig. 102 represents the interior of the Operculina, which is an existing
representation of the Nummuline type. Every segment of the animal is
enclosed in a shell of its own, but all the segments are connected in
the spiral direction by narrow passages in the walls as in the
Faujasina.

Although each of the interior whorls has its perforated marginal band,
the segments can have no direct access to the water; however, they are
indirectly brought into contact with it by means of a system of
branching shelly canals, radiating from the central chamber, ending in
conspicuous pores in the external surface of the shell. During this
course the canals send small tubes into the chambers on each side of
them; through these the internal segments of the animal can fill the
canals with cords of sarcode, and protrude them into the water, whence
they are supplied with food.

The genus Polystomella is distinguished by the high development of the
intermediate skeleton and the canal system that maintains it. The
Polystomella crispa (fig. 97, E), a beautiful species common on the
British coasts and in other temperate seas, has a lenticular form, the
1/16 to the 1/12 of an inch in diameter. It consists of a small number
of convolutions winding round the shorter axis of the lens, increasing
rather rapidly in breadth, and each one almost entirely enclosing its
predecessor, so that the shell is exactly alike on both sides, and only
the last convolution is to be seen. At the extremities of the axis there
is a mass of solid shell-substance, perforated by orifices which are the
apertures of a set of straight, parallel canals. In the figure only the
last convolution is visible, upon which the convex septal bands are very
conspicuous, dividing the surface into well marked segments, upon the
exterior edge of each of which there are strong transverse crenulations.
The only communication which the chambers have with the exterior, is by
means of a variable number of minute orifices near the inner margin of
the sagittate partition-plane, close to its junction with the preceding
convolution; a very high microscopic power is required to see them, as
well as the minute tubercles with which the surface of the shell is
crowded, more especially on the septal bands and in the rows of
depressions between the segmental divisions.

The sarcode animal itself corresponds exactly with the form and spiral
arrangement of the chambers so strongly marked on the exterior of the
shell. The segments form a spiral of crescents, smooth on the convex and
crenulated on the concave side; and from the latter threads of sarcode
proceed, which pass through pores in the inner margins of the
partitions, and unite them into one animal.

The Polystomella lives in tropical seas; P. crispa in temperate
latitudes, and P. striato-punctata inhabits the polar waters; the genus
is found everywhere.

Although variety of form without specific difference is characteristic
of the Foraminifera, it sometimes happens that identity of external form
is accompanied by an essential difference in internal structure. Of this
the Cycloclypeus is an instance; it is a rare species of nummuline,
dredged up from rather deep water off the coast of Borneo. The shell is
gigantic, some specimens being two and a half inches in diameter; but
its mode of growth is the same with that of the most complicated
Orbitolite. It consists of three superposed stages of circular discs,
each circle of chambers enclosing all those previously formed. However,
each segment of the animal being enclosed in its own shelly envelope, a
supplemental skeleton, and a radial, vertical and annular system of
canals, prove that the two animals belong to essentially different
families of Foraminifera. There are many instances, especially in the
Rotaline group, of isomorphism accompanied with generic difference; thus
no reliance can be placed on variety of external form, unaccompanied by
change of internal structure.

An attempt has been made in the preceding pages to describe a few
species most characteristic of some of the genera of this multitudinous
class; and of those selected a mere sketch of the most prominent
features of the animal and its abode is given, that some idea may be
formed of the wonderfully complicated structure of beings, which are
mostly microscopic specks. Yet the most minute circumstances in the
forms of the animals and their shells, with their varieties and
affinities, have been determined with an accuracy that does honour to
microscopic science.

They are now arranged in a natural system by William B. Carpenter, M.D.
F.R.S. assisted by William K. Parker, Esq., and T. Rupert Jones, Esq.,
and published in the Transactions of the Ray Society in 1862. To this
admirable work, the author is highly indebted.

It was known that different types of Foraminifera abound at different
depths on the coasts of the ocean; but it was long believed that no
living creature could exist in its dark and profound abyss. By deep-sea
sounding, it has been ascertained that the basin of the Atlantic Ocean
is a profound and vast hollow or trough, extending from pole to pole; in
the far south, it is of unknown depth, and the deepest part in the north
is supposed to be between the Bermudas and the Great Banks of
Newfoundland. But by a regular series of soundings made by the officers
of the navies of Great Britain and the United States, for the purpose of
laying a telegraphic cable, that great plain or steppe was discovered,
now so well known as the telegraphic plateau, which extends between Cape
Race in Newfoundland, and Cape Clear in Ireland. From depths of more
than 2,000 fathoms on this plateau, the ooze brought up by the sounding
machine consisted of 97 per cent. of Globigerinæ. The high state of
preservation of these delicate shells was no doubt owing to the perfect
tranquillity which prevails at great depths; for the telegraphic plateau
and the bed of the deep ocean everywhere is covered by a stratum of
water unruffled by the commotion raised by the hurricane which may be
raging on the surface. The greater number of the Globigerinæ were dead
empty shells; but although in many the animal matter was quite fresh,
Professor Bailly of New York could not believe that such delicate
creatures could live on that dark sea bed, under the pressure of a
column of water more than 2,000 fathoms high, a weight equal to rather
more than that of 340 atmospheres or 5,100 lbs. on every square inch of
sea-bed; wherefore he concluded that the tropical ocean and the Gulf
Stream, which absolutely swarm with animal life, must have been the
birth-place and home of these minute creatures, and that this mighty
‘ocean river,’ which divides at the Great Banks of Newfoundland, and
spreads its warm waters like a fan over the north Atlantic, deposits
their remains over its bed, which has thus been their grave-yard for
unknown periods, and which, in the lapse of geological time, may be
raised above the waves as dry land.

Professor Ehrenberg on the contrary concluded that residentiary life
exists at the bottom of the ocean, both from the freshness of the animal
matter found in the shells, and from the number of unknown forms which
are discovered from time to time at various and often great depths along
the coasts. This opinion has been confirmed beyond a doubt on several
occasions, especially by Dr. Wallich, who accompanied an expedition sent
under the command of Sir Leopold M‘Clintock, to sound the North Atlantic
for laying a telegraphic line.

In doing that two operations are requisite. The first is to ascertain
the depth: when that is known, the nature of the sea-bed must be
determined, and on that account a sample of it is then sounded for; but
owing to the difficulty of ascertaining the exact time at which the
ground is struck, a quantity of rope in excess of the depth is given
out, which lies on the bottom of the sea while the machine is being
drawn up, which occupies a considerable time when the depth is great.
About midway between Greenland and the north of Ireland, when the
machine was hauled up from a depth of a mile and a half, several
starfish were clinging with their long spiny arms to fifty fathoms of
the rope that had been lying on the surface of the sea-bed while the
machine was being drawn up, and to that part of the rope alone. They
continued to move their limbs energetically for more than a quarter of
an hour after they were out of the water. They certainly had not been
entangled in the line while swimming, because star-fishes are invariably
creeping animals. The deposit on which they had rested at the bottom of
the ocean contained ninety-five per cent. of Globigerinæ. Abundance of
these minute Foraminifera were found in the stomachs of the starfish;
which seemed to prove not only that the starfish were caught on their
natural feeding ground, but that their food was living organisms whose
normal abode is the surface of the bed of the deep ocean.

Dr. Wallich also discovered in the ooze brought up from a depth of
nearly two miles and a quarter a number of small bodies from 1/16 to 1/4
of an inch in length and about a line in breadth. They consisted of
equal globes arranged in a straight line like the Nodosaria, or built
up, each lying on part of the one below it, and increasing in size from
the uppermost about 1/1250 to the undermost about 1/450 of an inch in
diameter. Both of these forms, called coccospheres, consisted of sarcode
enclosed in a calcareous deposit; and were studded at nearly regular
distances by minute round or oval bodies concave below, and with an
aperture on their convex surface sometimes single, sometimes double.
These coccospheres were also found free in the ooze, and had been seen
previously by Capt. Dayman. They have likewise been seen as free
organisms living on the surface of the ocean.

The ooze in the bed of the Atlantic ocean, as well as of the
Mediterranean and Adriatic contains fifty per cent. of Globigerinæ; they
exist in the Red Sea, in the vicinity of the West Indian Islands, on
both sides of South America and near the Isle of France, but not in the
Coral Sea which is occupied by different genera. Though in utter
darkness, at the bottom of a deep ocean, these little creatures can
procure food by means of their pseudopodia, whose extreme sensibility
makes up for the want of sight; and the very excess of pressure under
which they live insures them a supply of oxygen at depths to which free
air cannot penetrate, for it is believed that the quantity of dissolved
air that water contains is in proportion to the pressure.

Fossil Foraminifera enter so abundantly into the sedimentary strata,
that Buffon declared ‘the very dust had been alive.’ 58,000 of these
fossil shells have been computed in a cubic inch of the stone of which
Paris and Lyons are built. The remains of these Rhizopods are for the
most part microscopic. M. D’Orbigny estimated that an ounce of sand from
the Antilles contained 1,800,000 shells of Foraminifera. A handful of
sand anywhere, dry sea-weeds, the dust shaken from a dry sponge, are
full of them.

When the finer portions of chalk amounting to one half or less are
washed away, the remaining sediment consists almost entirely of the
shells of Foraminifera, some perfect, others in various stages of
disintegration. In some of the hard limestones and marbles, the relics
of Foraminifera can be detected in polished sections and in thin slices
laid on glass. It is now universally admitted that some crystallized
limestones which are destitute of fossil remains, had been originally
formed by the agency of animal life, and subsequently altered by
metamorphic action; the opinion is gradually gaining ground among
geologists that such is the history of the oldest limestones.

At certain geological periods circumstances favoured the development of
an enormous multitude of individual animals. In the earlier part of the
Tertiary period the Nummulites acquired an extraordinary size. They were
like very large coins two or more inches in diameter, and were
accumulated in such quantities as to constitute the chief part of the
nummulitic limestone; a formation in some places 1,500 feet thick, which
extends through southern Europe, Libya, Egypt, Asia Minor, and is
continued through the Himalayan mountains into various parts of the
Indian peninsula, where it is extensively distributed. The Great Pyramid
of Egypt is built of this limestone, which gave rise to singular
speculations with regard to the Nummulites in very ancient and even in
more recent times. Although this is incomparably the greatest, it is by
no means the only instance of an accumulation of the fossil shells of
individual animals. The ‘Lingula flags,’ a stratum in the upper Cambrian
series of North Wales, was so named from the abundance of the Brachiopod
Lingula that it contains.

Professor Ehrenberg discovered that the shells of the Foraminifera
sometimes undergo an infiltration of silicate of iron, which fills not
only the chambers, but also their canal-system even to its minutest
ramifications, so that if the shell be destroyed by dilute acid, a
perfect cast of the sarcode matter remains. The greensands in the
different geological strata from the Silurian formation upwards, are
chiefly composed of these casts; and Professor Baily of the United
States more recently discovered that a process of infiltration is even
now taking place in some parts of the ocean bed, and that beautiful
casts of Foraminifera may be obtained by dissolving their shells with
dilute acid.

A most extensive comparison of the Foraminiferous group of Rhizopods,
recent and fossil, has been made by Messrs. Parker and Rupert Jones from
almost every latitude on the globe, from the arctic and tropical seas,
from the temperate zones in both hemispheres, and from shallow as well
as deep-sea beds. They have also reviewed the fossil Foraminifera in
their manifold aspects as presented by the ancient geological faunas
throughout the whole series from the Tertiary down to the Carboniferous
strata inclusive; and have come to the astonishing conclusion that
scarcely any of the species of the Foraminifera met with in the
secondary rocks have become extinct. All that they had seen have their
counterparts in the recent Mediterranean deposits. Throughout that long
series of geological epochs even to the present day, the Foraminifera
show no tendency to rise to a higher type; but variety of form in the
same species prevailed then as it does now.

Subsequently to this investigation, a gigantic Orbitulite twelve inches
in diameter, and the third of an inch thick, has been found in the
Silurian strata in Canada. The largest recent species Dr. Carpenter had
seen was about the size and thickness of a shilling.

The lowest stratum of the Cambrian formations has been regarded as the
most ancient of the Palæozoic rocks; now, however, strata of
crystallized limestone near the base of the Laurentian system, which is
50,000 feet thick in Canada, are discovered by Sir W. E. Logan to have
been the work of the Eozoön Canadense, a gigantic Foraminifer, at a
period so inconceivably remote that it may be regarded as the first
appearance of animal life upon the earth. In a paper published by Dr.
Carpenter, in May 1865, he expressed his opinion that the Eozoön would
be found in the older rocks of central Europe; and in the December
following he received specimens from the fundamental quartz rocks of
Germany, in which he found undoubted traces of the Eozoön. Here the
superincumbent strata are 90,000 feet thick; the transcendent antiquity
of the Eozoön is therefore beyond all estimation.

The fossil Eozoön consists of a succession of parallel rows or tiers of
chambers, in which the sarcode of the living animal had been replaced by
a siliceous infiltration, so that when the calcareous shell was
destroyed by dilute acid, the cast was found to be precisely like that
of a Nummulite; thin slices of it taken in different directions being
examined with a microscope, it was found that the siliceous matter had
not only filled that portion of the chambers which had been occupied by
the sarcode-body of the animal and the canal-system, but had actually
taken the place of the pseudopodial threads, the softest and most
transitory of living substances, which were put forth through tubuli in
the shell-walls of less than the 1/10000 part of an inch in diameter.
‘These are the very threads themselves turned into stone by the
substitution which took place, particle by particle, between the sarcode
body of the animal and certain constituents of the water of the ocean,
before the destruction of the sarcode by ordinary decomposition.’[9] The
shell had an intermediate skeleton, but the minute tubes in the walls of
the chambers are so characteristic of the Nummulites, that they were
sufficient alone to determine the relationship of the Eozoön to its
modern representative.

The external shape and limits to the size of the individual Eozoön have
not been determined with certainty, on account of its indefinite mode of
growth, and the manner in which the fossilized masses are connected with
the highly crystalline matrix in which they are imbedded; there is no
doubt, however, that they spread over an area of a foot or even more,
and attained a thickness of several inches. As they seem to have
increased laterally by buds which never fell off, they formed extensive
reefs; at the same time they had a vertical growth, for in some of the
reefs the older portions appear to have been fossilized before the newer
were built up on them as a base, exactly like the coral reefs in the
tropical ocean of the present day,[10] with this difference however,
that shells and other crustaceans are associated with the corals, while
no organic body has been found in the Eozoön reefs; nevertheless the
Eozoön must have had food. It may therefore be inferred that parts at
least of that primeval ocean swarmed with animal life, whose remains
have been obliterated by metamorphic action. Carbon (which in the form
of graphite both constitutes distinct beds, and is disseminated through
the siliceous and calcareous strata of the Laurentian series, as well in
Norway as in Canada), may indicate the existence of vegetation in the
Eozoön period.

The Eozoön is by no means confined to Canada and central Europe. The
serpentine marble of Tyree which forms part of the Laurentian system on
the west of Scotland, and a similar rock in Skye, when subjected to
minute examination, are found to present a structure clearly identical
with that of the Canadian Eozoön. And the like structure has been
discovered by Mr. Sanford in the serpentine marble of Connemara, known
as Irish green. The age of that rock however, is doubtful: for when it
was discovered to contain Eozoön, Sir Roderick Murchison who had
previously studied its relations was at first inclined to believe it
belonged to the Laurentian series; now however, he considers the
Connemara marble to be of the Silurian age. ‘If this be the case it
proves that the Eozoön was not confined to the Laurentian period, but
that it had a vast range in time, as well as in geographical
distribution; in this respect corresponding to many later forms of
Foraminifera which have been shown by Messrs. Parker and Rupert Jones to
range from the Trias to the present epoch.’[11]

The Carpenteria found in the Indian seas forms a link between the
Foraminifera and Sponges. The shell is a minute cone adhering to the
surface of corals and shells, by its wide base which spreads in broad
lobes. Double-walled chambers and canals form a spiral within it, and
are filled with a spongy sarcode of a more consistent texture than the
sarcode of the Foraminifera, which in the larger chambers is supported
by siliceous spicules similar to those which form the skeletons in
sponges.


                          CLASS III.—SPONGES.

According to the observations of Mr. Carter, sponges begin their lives
as solitary Amœbæ which grow by multiplication into masses, and assume
endless forms according to the species; turbinate, bell-shaped, like a
vase, a crater, a fan, flat, foliaceous and lobed or branching and
incrusting the surface of stones. All the Amœbæ are so connected as to
form one compound animal. The whole substance of a sponge is permeated
by innumerable tubes which begin in small pores on the surface, and
continually unite with one another as they proceed in their devious
course to form a system of canals increasing in diameter and ending in
wide openings called oscula, on the opposite side of the mass. Currents
of water enter through the pores on the surface, and bring minute
portions of food which are seized upon by a vast multitude of Amœbæ with
long cilia which form the walls of the tubes and canals; and after they
have extracted the nutritious part, the offal is carried into the sea
through the oscula, by the current of water whose flux is maintained by
the vibrations of the cilia. In the compressed and many of the tubular
sponges the water passes through them in a straight line; in branched
and encrusting sponges, the afferent and efferent openings are on the
same surface. The water is inhaled continuously and gently like an
animal breathing, but it is rapidly and forcibly ejected; and in its
passage it no doubt furnishes oxygen to aërate the juices of the
compound animal, whose flesh or sarcode is irritable while alive, and
which has the power to open and shut the pores and oscula of the canals,
for the whole sponge forms one compound creature whose mass is nourished
by the myriads of Amœbæ of which it is constituted.

Within the animated sarcode mass of the sponges there is in most cases a
complicated skeleton of fibrous network, either horny, calcareous, or
siliceous, which supports the soft mass, and determines its form.

Besides the skeleton, the mass of sponges is for the most part
strengthened and defended by siliceous, and more rarely by calcareous,
spines or spicules, either imbedded among the fibres of the skeleton, or
fixed to them by their bases. The fibres of the skeleton network always
unite, whether they be horny, calcareous, or siliceous; the spicules
never, though they often lie in confused heaps over one another. They
are of innumerable forms and arrangements. Some are like long needles
lying close together in bundles, pointed or with a head like a pin at
one or both ends; a great number are stellate with long or short rays;
there may even be several different forms in the same sponge. Many
calcareous sponges have cavities full of organic matter; and when the
calcareous matter is dissolved by dilute acid, the organic base is left.

The common commercial sponges have a skeleton which consists of a
network of tubular, horny, tough, and elastic fibres which cross in
every direction. They have no spicules or very few; and when such do
project from the horny skeleton, they are generally conical, attached by
their bases, and their surface is often beset with little spines
arranged at regular intervals, which gives them a jointed appearance.
The common sponge which is so abundant in the Mediterranean has many
forms; those from the coast of North America are no less varied, but
that most used in the United States is turbinate, concave, soft, and
tomentose.

[Illustration: Fig. 103. Section of Sponge.]

In the calcareous sponges a mass of three-rayed spicules surround the
interior canals, where they are held together by a cartilaginous
substance which is wanting in the horny sponges, but which remains in
this order after the destruction of the more delicate matter when the
sponge is dried.[12] The pores are also occasionally defended by the
projecting points of half buried spines.

In nearly every species of this order the pores on the surface are
protected by spicules; and they are also projected from the surface of
the large cloacal cavity, and curved towards its opening, to defend it
from Annelids and other enemies.[13] Some species have a long ciliary
fringe at the orifice of the cavity, through which the water may pass
out, but no animal can come in.

The spicula and skeleton of most of the marine sponges are siliceous and
singularly beautiful; the skeleton of the Dactylocalyx pumiceus of
Barbadoes is transparent as spun glass; and a species from Madagascar
has numerous simple transparent and articulated spicules implanted in
the siliceous fibres of the skeleton. The Cristata, Papillaris, Ovulata,
and many more have siliceous skeletons, some garnished with spicules of
various forms, and the surface occasionally covered with a layer of
siliceous granules.

The variety in the size, structure, and habits of the marine sponges is
very great: temperate and tropical seas have their own peculiar genera
and species; some inhabit deep water, others live near the surface,
while many fix themselves to rocks, sea-weeds, and shells, between high
and low water mark. There are very few dead oyster, whelk, scallop, and
other shells that escape from the ravages of the Cliona, an extremely
minute burrowing sponge of the simplest structure, which has a coat of
siliceous spicules supposed to be the tools with which it tunnels a
labyrinth through the mid-layer of the shell, in a pattern that varies
with the species of the sponge. A communication is formed here and there
with the exterior by little round holes, through which the sponge
protrudes its yellow papillæ. From the force exhibited by this little
sponge, it may perhaps be inferred to possess a rudimentary muscle and
nerve.[14]

Sponges are propagated twice in the year by minute ciliated globules of
sarcode, detached from the interior of the aquiferous canals, which swim
like zoospores to a distance, come to rest, and lay the foundation of
new sponges. The little yellow eggs of Halichondria panicea are lodged
in the interstices between the interior canals; when mature, they are
oval and covered with cilia, and are carried out by the currents; and
after swimming about for some days fix on a solid object, become covered
with bristles, spread out into a transparent film, charged with
contractile vesicles of different sizes in all degrees of dilatation and
contraction, as well as with sponge ovules. Spicules are developed at
the same time, and these films ultimately become young sponges, and if
two happen to meet they unite and are soldered together.[15] Besides
eggs, larger bodies covered with radiating spicules are produced,
containing granular particles of sarcode, each of which when set free by
the rupture of the envelope, becomes an Amœba-like creature, and
ultimately a sponge.

Fresh-water sponges are sometimes branched, and sometimes spread over
stones, wood, and other substances; and one species covers an earthy
mass some inches thick formed by its own decayed matter. The skeleton of
such species as have one, consists of bundles of siliceous spicules,
held together and mixed with groups of needles, the rods of which
project through the surface of the sponge and render it spinous. The
motions in the gelatinous sarcode mass are the most remarkable feature
in the fresh-water sponges, which all belong to the genus Spongilla. Mr.
Carter observed that portions of the surface of some individuals of the
Spongilla fluviatilis in his aquarium had long cilia by means of which
they rapidly changed their places during the spring, but when winter
came they emitted processes from such parts of their surfaces as were
free from cilia and retracted them again just like Amœbæ. These portions
often had cells, and when the Amœba-like motions ceased, a nucleus and
nucleolus appeared within them, and at last the whole gelatinous sarcode
mass consisted of these cells or globules. Some had no nucleus, but were
filled with green or colourless granules.

At certain seasons of the year, whatever the form of the fresh-water
sponges may be, a multitude of minute hard yellow bodies are produced in
their deeper parts. They consist of a tough coat containing radiating
spicules like a pair of spoked wheels united by an axle with a pore in
its surface. Within this last there is a mass of motionless granular
cells, and when put into water the cells come out at the pore and give
rise to new sponges.

Insulated groups of germs covered with cells called swarm-cells seem to
form parts of the sponges; they lie completely within the mass of the
living sponge. They have the form of a hen’s egg, are visible to the
naked eye, and when they come into the water they swim in all directions
for a day or two; become fixed; a white spot within is enlarged; and the
constituents of young sponges appear.[16]

The generic forms of fossil sponges augment in number and variety from
the Silurian to the Cretaceous beds, where the increase is rapid; but
all the sponges which had a stony reticulated form without spicules
passed away with the Secondary epoch, so that the family has no
representatives in the Tertiary deposits or existing seas. The
calcareous sponges which abound in the Oolite and Cretaceous strata, and
attain their maximum in the Chalk, are now almost extinct, or are
represented by other families with calcareous spicules. Siliceous fossil
sponges are particularly plentiful. In England extensive beds of them
occur in the Upper Greensand, and in some of the Oolitic and
Carboniferous Limestones; and some beds of the Kentish Rag are so full
of their siliceous spicules, that they irritate the hands of the men who
quarry them. Since every geological formation except the Muschelkalk is
found in England, the number and variety of fossil sponges are very
considerable. The horny sponges are more abundant now than they were in
the former seas. According to M. D’Orbigny the whole number of fossil
sponges known and described amount to thirty-six genera and 427 species,
which is probably much below the real number.[17]


                          CLASS IV.—INFUSORIA.

The Infusoria, which form the second group of the Protozoa, are
microscopic animals of a higher grade than any of the preceding
creatures, although they go through their whole lives as isolated single
cells of innumerable forms. They invariably appear in stagnant pools and
infusions of animal and vegetable matter when in a state of rapid
decomposition. Every drop of the green matter that mantles the surface
of pools in summer teems with the most minute and varied forms of animal
life. The species called Monas corpusculus by the distinguished
Professor Ehrenberg, has been estimated to be 1/2000 part of a line in
diameter. ‘Of such infusoria a single drop of water may contain
500,000,000 of individuals, a number equalling that of the whole human
species now existing upon the face of the earth. But the varieties in
size of these animalcules invisible to the naked eye are not less than
that which prevails in almost any other natural class of animals. From
the Monad to the Loxades or Amphileptus, which are the fourth and sixth
part of a line in diameter, the difference in size is greater than
between a mouse and an elephant; within such narrow bounds might our
ideas of the range in animal life be limited if the sphere of our
observation was not augmented by artificial aid.’[18]

This singular race of beings has given rise to the erroneous hypothesis
of equivocal or spontaneous generation, that is to say, the production
of living animalcules by a chemical or even fortuitous combination of
the elements of inert matter. That question has been decided by direct
experiment, for Professor Schultz kept boiled infusions of animal and
vegetable matter for weeks in air which had passed through a red-hot
tube, and no animalcules were formed, but they appeared in a few hours
when the same infusions were freely exposed to the atmosphere, which
shows clearly that the germs of the lowest grade of animal life float in
the air, waiting as spores do, till they find a nidus fit for their
development.

M. Pasteur, Director of the Normal School in Paris, in a series of
lectures published in the ‘Comptes Rendus,’ has not only proved that the
atmosphere abounds in the spores of cryptogamic fungi and moulds, but
with infusoria of the form of globular monads, the Bacteria, and
vibrios, which are like little rods round at their extremities and
extremely active. The Bacteria mona and especially the Bacteria terma,
are exceedingly numerous. These minute beings are the principal agents
in the decomposition of organic matter. They are more numerous in dry
than in wet weather, in towns than in the country, on plains than on
mountains.

In a memoir read at the Academy of Sciences, Paris, Mr. J. Samuelson
mentions that he had received rags from Alexandria, Japan, Melbourne,
Tunis, Trieste and Peru. He sifted dust from the rags from each of these
localities respectively through fine muslin into vases of distilled
water. Life was most abundant in the vases containing dust from Egypt,
Japan, Melbourne, and Trieste. The development of the different forms
was very rapid, and consisted of protophytes, Rhizopods and true
Infusoriæ. In most of the vases monads and vibrios appeared first, and
from these Mr. Samuelson traced a change first into one then into
another species of infusoria. In the dust from Japan he followed the
development of a monad into what appeared to be a minute Paramœcium,
then into Lexodes cucullus, and finally into Colpoda cucullus. From
these and other experiments it is proved that many infusoria now classed
as distinct types are really one and the same animal in different states
of development. That appears to be the case also with the Amœbæ. In the
dust from Egypt Mr. Samuelson found a new Amœba whose motions were very
rapid; as to shape and mode of motion he compared it to soap bubbles
blown with a pipe. He traced the gradual changes of the globular form of
this Amœba until its pseudopodia were in full action, its increase by
conjugation, and other circumstances of its life. In the same dust and
in that only, the development of the Protococcus viridis was seen, and
that in such abundance that at last the water was tinged green by that
plant. In the dust from Egypt a vibrio was changed into a vermiform
segmented infusoria of an entirely new type. Its length varied from the
1/150 to 1/100 of an inch, each ring was ciliated, and the whole series
of cilia extending along the body acted in concert; a circlet of them
surrounded the anterior segment; a canal seemed to extend throughout the
body. It was propagated by bisection; the two parts remained attached to
one another; an independent ciliary motion was observed in each which
did not interfere with the motion of the whole. It was supposed to be a
larval form or series of forms. Mr. Samuelson’s observations show, that
the atmosphere in all the great divisions of the globe is charged with
representatives of the three kingdoms of nature, animal, vegetable, and
mineral: that the animal germs not only include the obscure types of
monads, vibrios, and Bacteria, but also the Glaucoma, Cyclides,
Vorticella, and other superior Infusoriæ, and occasionally though very
rarely germs of the Nematode worms.

It has been already mentioned that many of the microscopic fungi are
ferments, aiding greatly in the decomposition of organic matter. They
however are by no means the only agents in decomposition. The moment
life is extinct in an animal or vegetable, Infusoria of the lowest grade
seize upon the inanimate substance, speedily release its atoms from
their organic bond, and restore them to the inorganic world whence they
came. The ferment which transforms lactic acid into butyric acid is a
species of vibrio which abounds in the liquid, isolated or united in
chains; they glide, pirouette, undulate, and float in all directions,
and multiply by spontaneous division. Vibrios possess the unprecedented
property of living and propagating without an atom of free oxygen; they
not only live without air, but air kills them. This singular property
forms an essential difference between the Vibrios and the Mycoderms: the
former cannot live in oxygen; the latter cannot live without it, and as
soon as it is exhausted within the infusion, they go to the surface to
borrow it from the atmosphere.

There are also two groups of Infusoria which possess these opposite
characters, one being unable to live in oxygen, while the other cannot
live without it; sometimes they even inhabit the same liquid. When the
tartrate of lime is put into water along with some ammoniacal and
alkaline phosphates, a Monad, the Bacteria terma, and other Infusoria
appear after a time. These little animals bud rapidly in an infusion of
animal matter, then a slight motion is produced by the appearance of the
Monas corpusculum and the Bacterium terma, which glide in wavy lines in
all directions in quest of the oxygen dissolved in the liquid, and as
soon as it is exhausted they go to the surface in such numbers as to
form a pellicle, where by aid of the oxygen they form the simple binary
compounds water, ammonia, and carbonic acid. In the meantime the
Vibrios, which are without oxygen, are developed below, and keep up the
fermentation, and between the two, the work of decomposition is
completed.

It is not the worm that destroys our dead bodies; it is the Infusoria,
the least of living beings. The intestinal canal of the higher animals,
and of man, is always filled during life not only with the germs of
vibrios, but with adult and well-grown vibrios themselves. M.
Leewenhoeck had already discovered them in man, a fact which has since
been confirmed. They are inoffensive as long as life is an obstacle to
their development, but after death their activity soon begins. Deprived
of air and bathed in nourishing liquid, they decompose and destroy all
the surrounding substances as they advance towards the surface. During
this time, the little Infusoria, whose germs from the air had been
lodged in the wrinkles and pores of the skin, are developed, and work
their way from without inwards, till they meet the vibrios, and after
having devoured them, they perish, or are eaten by maggots.

Of all the Infusoria and ferments the Vibrios are the most tenacious of
life; their germs resist the destructive effect of a temperature of 100°
Cent. The spores of the Mucedines are still more vivacious; they grow
after being exposed to a heat of 120° Cent., and are only killed by a
temperature of 130° Cent. As neither spores of the fungi nor the germs
of the Infusoria are ever exposed to so high a temperature while in the
atmosphere, they are ready to germinate as soon as they meet with a
substance that suits them.

M. Ehrenberg has estimated that the Monas corpusculum is not more than
the 1/24,000th part of an inch in diameter; whence Dr. M. C. White,
assuming that the ova of the Infusoria and the spores of minute fungi
are only the 1/10th part in linear dimensions of their parent organisms,
concludes that there must be an incalculable amount of germs no larger
than the 1/240,000th or 1/100,000th part of an inch in diameter; and
since according to MM. Sullivant and Wormley, vision with the most
powerful microscope is limited to objects of about the 1/80,000th part
of an inch in diameter, we need not be surprised if Infusoria and other
organisms appear in putrescible liquids in far greater numbers than the
germs in atmospheric dust visible by the aid of microscopes would lead
us to expect.

The ferments are the least in size and lowest in organization of all the
Infusoria. The higher group which abounds in stagnant pools and ditches
are exceedingly numerous, and their forms are varied beyond description.
They are globular, ovoid, long and slender, short and thick, many have
tails, one species is exactly like a swan with a long bending neck, but
whatever the form may be, all have a mouth and gullet. Although the skin
of the Infusoria is generally a mere pellicle, that of the red
Paramœcium and some others resembles the cellulose covering of a
vegetable cell, engraved with a pattern; but in all cases respiration is
performed through the skin.

Whatever form the cell which constitutes the body of the Infusoria may
have, the highly contractile diaphanous pellicle on its exterior is
drawn out into minute slender cilia which are the locomotive organs of
these creatures. Vibrating cilia form a circlet round the mouth of some
of these animalcules, a group of very long ones are placed like whiskers
on each side of it, as in the Paramœcium caudatum, and in some cases
there is a bunch of bristles in front. Certain Infusoria have cilia in
longitudinal rows, and in many the whole body is either partially or
entirely covered with short ones. In some Infusoria their vibrations are
constant, in others interrupted, and so rapid that the cilia are
invisible. These delicate fibres which vary from the 1/500th to the
1/13,000th part of an inch in length, move simultaneously or
consecutively in the same direction and back again, as when a fitful
breeze passes over a field of corn. These animalcules seize their prey
with their cilia, and swim in the infusions or stagnant pools, in which
they abound, in the most varied and fantastic manner; darting like an
arrow in a straight line, making curious leaps and gyrations, or fixing
themselves to an object by one of their cilia and spinning round it with
great velocity, while some only creep. These motions, which bring the
animalcules into fresh portions of the liquid, are probably excited by
the desire for food and respiration.

[Illustration: Fig. 104. Paramœcium caudatum. _a_ _a_, contractile
vesicles; _b_, mouth.]

[Illustration: Fig. 105. Kerona silurus.—_a_, contractile vesicle; _b_,
mouth; _c_ _c_, animalcules which have been swallowed by the Kerona.]

None of the Infusoria have regular jointed limbs, but certain families
of the higher genera have peculiar and powerful organs of locomotion
partly consisting of strong ciliary bristles placed on the anterior in
rows, used for crawling or climbing, and partly consisting of groups of
strong processes which serve as traction feet, generally trailing behind
the animal while swimming, or used to push it forward. When the bristles
or cilia of this high group of Infusoria are used for crawling their
motions may be traced to the contraction of the skin; but in the
Infusoria that are never fatigued though their cilia vibrate incessantly
night and day, it may be presumed that these motions are altogether
independent of the will of the animal, in as much as there are
innumerable cilia in the human frame that are never at rest during the
whole course of our existence, nor do their vibrations cease till a
considerable time after death—a striking instance of unconscious and
involuntary motion.

The cell which constitutes the body of the Infusoria is filled with
sarcode, which is the receptacle of the food, and in that substance all
the internal organs of the animalcule are imbedded. In the higher genera
it is full of granular particles of different sizes and forms, and it
contains a nucleus in its centre, characteristic of cellular protozoa
generally. The nucleus is of a dull yellow colour, and is enclosed in a
transparent capsule, which in the smaller Infusoria reflects light
brilliantly. It is generally of an ovoid form and single, but in several
species the nucleus is double, and in others there are several nuclei.

The Infusoria have a distinct mouth and gullet, and for the most part
another aperture for ejecting the indigestible part of their food,
though some discharge it by the mouth, others through any part of their
surface. A few of the larger Infusoria devour the smaller; others feed
on minute vegetable particles, chiefly diatoms. Solid substances that
are swallowed are collected into little masses mixed with water, and
enter into clear spherical spaces called vacuoles in various parts of
the sarcode, where they are partially digested. When the animal has not
had food for some time, clear spaces only filled with a very transparent
fluid are seen, variable both in size and number. It was on account of
the digestive vacuoles that the Infusoria were called Polygastria by
Ehrenberg.

Transparent contractile vesicles of a totally different nature from the
vacuoles are peculiarly characteristic of such Infusoria as have a
digestive cavity. They exist either singly or in even numbers, from 2 to
16, according to the species, and never change their places; but they
dilate and contract rhythmically at pretty regular intervals. When
dilated, they are filled with a clear, colourless fluid, the product of
the digestive process which they are supposed to diffuse through the
body of the animal.

The Euglena, a very extensive genus of Infusoria, have smooth bodies and
green particles imbedded in the sarcode, which fills their interior; and
M. Wöhler discovered that the green mantle covering the saline springs
at Rodenberg and Königsborner, which consists of three species of these
green Infusoria, gives out bubbles of pure oxygen; thus indicating a
respiratory process in these animals, the same with that in plants,
namely, fixing the carbonic acid of the atmosphere and exhaling oxygen,
a singularly close analogy, if not identity, of action. The Euglenæ are
also distinguished by an irregular oblong space in the head filled with
a red liquid; but, as it does not contain a crystalline lens, it can
only be regarded as the very earliest rudiment of an eye, totally
incapable of distinguishing objects, though probably sensible to the
influence of light. They swim with a smooth gliding and often rotatory
motion, producing a kind of flickering on the surface of the water by
the lashing of a long filament attached in front, and supposed to be
their only organ of locomotion; nevertheless, Mr. Gosse thinks that they
are covered with most minute cilia from their manner of swimming. The
Euglena acus is one of the prettiest of these little animals; it is long
and slender, of a sparkling green with colourless extremities, a
thread-like proboscis, and a rich crimson spot. When it swims it
rotates, and a series of clear, oblong bodies are seen towards the head,
and another at the tail, as if they were imbedded in the flesh round a
hollow.

The Loxades bursaria, which is a giant among its fellows, has an ovoid
body with green particles imbedded in its interior. The outer skin is
spirally grooved, so as to form a kind of network, the elevated points
of which support the cilia with which its body is beset. It has a mouth
and gullet lined with cilia, which force the food in balls into the soft
matter in the interior, where both the food and the green particles
circulate, being carried along by a gyration of the gelatinous matter in
which they are imbedded.

A species of Peridinium, which is luminous at night, and occasionally
covers large portions of the Bay of Bengal with a scarlet coat by day,
nearly approaches the character of the unicellular Algæ. Mr. J. H.
Carter observed that at first, when these animalcules were in a state of
transition, their nearly circular bodies were filled with translucent
green matter, closely allied, if not identical with, chlorophyll, which
disappeared when the animal approaches its fixed state, and a bright red
took its place: the Infusoria were then visible to the naked eye, and
the sea became scarlet. The scarlet state only lasts for a few days, for
each of these innumerable Infusoria becomes encysted or capsuled, and
either floats on the water, or sinks to the bottom and remains
motionless. The Euglena sanguinea has a scarlet state analogous to that
of the Peridium. It is so minute and versatile that it is difficult to
ascertain its true form, which, however, seems to be a spindle shape,
with a pointed and blunt round head. In general it is of a rich emerald
green, with perfectly clear, colourless extremities; but it sometimes
occurs of a deep red, and in such multitudes as to give the water the
appearance of blood.[19]

[Illustration: Fig. 106. Noctiluca.]

The Noctiluca miliaris, a luminous inhabitant of the ocean, and the most
beautiful of the Infusoria, is distinguished by its comparatively
gigantic size, and by its brilliant light, which makes the sea shine
like streams of silver in the wake of a ship in a warm summer evening,
when they come to the surface in countless multitudes. It is a globular
animal like a minute soap bubble, consisting of gelatinous matter, with
a firmer exterior, and being about the thirtieth of an inch in diameter,
it is visible to the naked eye, when a glass in which it is swimming is
held to the light. On one side of the globe there is an indentation,
from whence a tail of muscular fibre springs striped with transverse
rings, which aids the animal in swimming. At the root of the tail lies
the mouth, bordered on one side by a hard dentile lip leading into a
funnel-shaped throat, from whence a long flickering cilium is protruded,
supposed to be connected with respiration. The throat leads into a large
cavity in the gelatinous substance, from whence the rudiments of an
alimentary canal descend. From the internal surface of the globe sarcode
fibres extend through the gelatinous matter, so as to divide it into a
number of irregular compartments, in which vacuoles are often seen. They
give buoyancy to the animal, and enable it to rise and sink in the
water, but seem to disappear when the food is digested. The sarcode
fibres constantly change their form and position, and the electric light
emitted by a direct exertion of nerve power, which seems to be constant
to the naked eye, really consists of momentary scintillations that
increase in rapidity and intensity by the dash of an oar or the motion
of the waves.

The Noctiluca is propagated by spontaneous division, a line appears
bisecting the globe, which becomes more and more constricted till the
animal is like a dumb-bell; the slender thread separating the two parts
is then broken by their efforts to get free; the two new creatures swim
off in different directions, and soon assume their adult form. But in
many individuals there are clear, yellow globules with a well-defined
nucleus, of a rich reddish-brown, which are the germs of the animal.

Most of the Infusoria multiply by continuous bisection, like the
unicellular Algæ. The division generally begins with the nucleus, and is
longitudinal or across, according to the form and nature of the animal,
and is accomplished with such rapidity, that, by the computation of
Professor Ehrenberg, 268,000,000 of individuals might be produced from
one single animalcule of the species Paramœcium in a month. The
Paramœcia are reproduced too by gemmation, and, as they are male and
female, they are reproduced also like the higher classes.

The Infusoria have another mode of increasing. The animalcules either
draw in or lose their cilia, and consequently come to rest. The animal
then assumes a more globular form, and secretes a gelatinous substance
from its surface, which hardens into a case or cyst, in which its body
lies unattached and breaks up into minute ciliated gemmules, which swim
forth like zoospores as soon as they come into the water by the thinning
away of part of the cyst. In fact the animal is resolved into its
offspring, which, as soon as free, gradually acquire the parent’s form,
though at first they may bear no resemblance to it. The scarlet Peridium
seen by Mr. Carter in the Bay of Bengal is propagated in this manner.
For the parent Peridium is broken up within its cyst into from two to
four new ones, each of which when set free and grown up might undergo
the same process.

The Loxades bursaria increases by three distinct methods, and sometimes
by two at a time. In autumn, or the beginning of winter, six or eight
germs containing granular matter and one or more hyaline nuclei are
formed within the animal, each enclosed in two contractile cysts: they
lie freely in the cavity of the body, and come one by one into the water
through a canal ending in a protuberance in the skin. During this time
the pulsations of the vesicles within the Loxades are continued, but the
gyration of the green particles is suspended till all the germs are
excluded and swim away, and then it is renewed as vigorously as ever. At
first the young are totally unlike their parent, but by degrees acquire
its form. The Loxades is also increased by division, sometimes across,
sometimes longitudinally, and, in the latter case, one half is
occasionally seen to contain germs which have been excluded before the
other half had separated, so that the two distinct systems of
propagation are simultaneous.[20]

The Vorticella nebulifera and some others of the Infusoria are
remarkable for the diversity of their reproductive powers; for, besides
division and gemmation, they are reproduced by a kind of alternate
generation, accompanied by singular metamorphoses. The Vorticella, one
of the most beautiful animals of its class, lives in pools of fresh
water: groups of them are found on almost every mass of duckweed like
little blue bells upon slender stalks, creating active currents in the
water by the vibrations of long and powerful cilia with which the margin
of the bell is fringed. The lip or edge of the bell is bent outwards
into a permanent rim, and a deep groove cleaves the rim on one side, in
which a wide cavity forming the mouth is placed. The mouth, the short
throat or gullet, and the whole bell, are bristled with vibratile cilia.

[Illustration: Fig. 107. Vorticellæ.]

The Vorticellæ feed on vegetable organisms, chiefly diatoms, and are
exceedingly voracious. The cilia round the rim of the bell entangle the
food, draw it into the mouth, and those in the gullet force particle
after particle mixed with water into vacuoles which they make in the
interior of the soft sarcode which fills the bell, and there the
particles undergo rotation till digested and absorbed, and, if refuse
remain, it is ejected through a softer part in the outer layer of the
bell.

[Illustration: Fig. 108. Acineta.]

The stem that fixes the animal to a solid object is a tubular
continuation of its outer membrane, containing a highly contractile
filament; and, as the creature is extremely sensitive to external
impressions, it folds up the ciliated rim of its bell, and its stalk
shrinks down in a spiral on the slightest alarm, but the bell opens and
the stalk stretches out again as soon as the alarm is over. When a
Vorticella is reproduced by division, the bell separates longitudinally
into two parts; one is often smaller than the other, and separates from
its parent, swims about till it gets a stem, and fixes itself to an
object. When the two parts are of equal size, the division extends to a
greater or less distance down the stalk, and as each of these become
perfect bells, and do not fall off but subdivide in the same manner, it
follows that, by successive divisions, a whole group of these beautiful
animals may spring from the same stem, as in fig. 107.

The Vorticella has a most wonderful mode of reproduction common to a few
other Infusoria. A gelatinous substance is secreted by the bell, which
hardens and envelopes it in a cyst; the encysted bell then separates
from its stalk, and is transformed into an infusorial animal called an
Acineta (fig. 108), closely resembling the Actinophrys sol with
radiating filaments which it continually stretches out and draws in. A
motile ciliated embryo, or Vorticella bud, is then formed within the
Acineta, which, after a time, comes out at a slit in its side, swims
about, gets a stem, fixes to some object, and is developed into a
Vorticella. The slit closes again, and the Acineta keeps moving its
filaments as usual, and another motile embryo is formed within it, which
is emitted by a slit in the same manner, and is also developed into a
Vorticella. As these young Vorticellæ, or bell animals, may undergo the
same transformations, there may be an indefinite alternation of the two
forms. The Vorticella-bud, when it issues from the slit in the Acineta,
has an oval form, with a circlet of long cilia at its narrow end, a
mouth at the more obtuse, a nucleus, and contractile vesicles, and,
after swimming about till it finds a suitable place, it becomes fixed by
one end of its oval body, a style or stem is formed, which rises
rapidly, and the adult shape is developed. The Acinetæ are said to live
upon Infusoria: they apply the dilated apex of their rays as sucking
discs to the animal, and suck its contents till it dies. The Tricoda
linceus undergoes metamorphoses analogous to those of the Vorticella,
but more numerous and complicated.[21]

Most of the Vorticellæ, and probably the majority of Infusoria, remain
unchanged for a time within their cysts, being then in a state analogous
to the hybernal sleep of some of the reptiles. The cyst shelters them
from cold and draught, and, when heat and moisture are restored, they
resume their active vitality. The motions of the Infusoria are probably
automatic, and in some instances consensual; they have neither true
eyespecks, though their whole body seems to be conscious of light and
darkness; nor have they ears; and, with the exception of touch, which
the Vorticellæ have in a marvellous degree, it may be doubted whether
the Infusoria have any organs of sense whatever, though they avoid
obstacles and never jostle one another. The vibrations of their cilia
are involuntary as in plants, an instance of the many analogies which
perpetually occur between the lowest tribes of the two great kingdoms of
nature. In both there are examples of propagation by bisection,
conjugation, budding, and the alternation of generation, which occurs
more frequently among Protozoa than among any other class of animals.
There is a perfect resemblance between Zoospores and Protozoa; they both
cease to move, the Zoospore when it secretes its cellulose coat and
becomes a winter or resting spore, the Protozoon previous to encysting,
a process presumed to be universal among that class of animals, before
subdivision or reproduction begins. It is the dried cysts or germs of
the Infusoria that float in the atmosphere as winter spores do, and it
is believed that, like the fungi, the same germs may develope themselves
into several different forms according to the nature of the liquid into
which they may chance to be deposited; consequently, it is not necessary
that the variety of germs should be very great, although the Infusoria
themselves are of numerous forms.[22]

The Infusoria, the smallest of beings, apparently so insignificant, and
for the most part invisible to the unaided eye, have high functions
assigned to them in the economy of nature. They ‘are useful for
devouring and assimilating the particles of decaying animal and
vegetable matter from their incredible numbers, universal distribution,
and insatiable voracity—they are the invisible scavengers for the
salubrity of the atmosphere. They perform a still more important office
in preventing the gradual diminution of the present amount of organic
matter upon the earth. For, when this matter is dissolved or suspended
in water in that state of comminution and decay, which immediately
precedes its final decomposition into the elementary gases, and its
consequent return from the organic to the inorganic world, these wakeful
members of Nature’s invisible police are everywhere ready to arrest the
fugitive organic particles, and turn them back into an ascending stream
of animal life. Having converted the dead and decomposing matter into
their own living tissues, they themselves become the food of larger
Infusoria, as the Rotifera and numerous other small animals, which, in
their turn, are devoured by larger animals as fishes, and thus a pabulum
fit for the nourishment of the highest organized beings is brought back
by a short route from the extremity of the realms of organized
matter.’[23]




                              SECTION III.

                          HYDROZOA, ZOOPHYTES.


ZOOPHYTES are animals of a much higher organization than the Protozoa,
inasmuch as they are furnished with special organs of prehension,
offence and defence, of attachment, and in many of locomotion. For the
most part they consist of numerous individuals called Polypes, united in
a community, and living together in intimate sympathy and combined
action, so as to form one single compound animal.

Zoophytes are divided into two groups, namely the Hydrozoa, whose type
is the common fresh-water Hydra, and the Actinozoa, which are composite
animals, including the reef-building corals, whose polypes are formed
according to the type of the Actinia, or common Sea Anemone. The
Hydrozoa consist of seven orders, the first of which are the Hydridæ,
inhabitants of fresh water; the next constitute the oceanic Hydrozoa,
some of which, though extremely varied in form, are connected by the
most wonderful relations.

The solitary Hydra that lives in fresh-water pools and ditches, consists
of a soft cylindrical muscular bag, capable of being stretched into a
slender tube, shrunk into a minute globe, or widely distended at will.
At one end there is a circular mouth, which is highly sensitive,
opening, closing, or protruding like a cone, and surrounded at its base
by six long flexible arms called tentacles, arranged symmetrically. The
mouth opens into a cavity extending throughout the length of the body,
which is the stomach; the other end of the sac is narrow, and terminates
in a disk-shaped sucker, by which the Hydra fixes itself to aquatic
plants, or floating objects, from whence it hangs down, and the
tentacles float in the water.

[Illustration: Fig. 109. Thread-cells and darts.—A, B, C, D,
Thread-cells at rest; E, F, G, H, appearance of the darts when
projected.]

The sac or body is formed of two layers, an inner and an outer layer, of
firmer texture, formed of cells imbedded in a kind of sarcode, and the
space between the two layers is filled with a semifluid substance, mixed
with solid particles and full of vacuoles. The inner and outer layers
are united at the mouth, and the tentacles are closed tubes in
communication with the cavity of the stomach. The exterior layer of the
tentacles is beset with wart-like excrescences, formed of clusters of
cells, with a larger one in the centre filled with a liquid. In all of
them a long spicula, or sting, often serrated at the edge, is coiled up
like a thread, and fixed by one end to a kind of tube, like the inverted
finger of a glove, that the animal can dart out in an instant.

Thus armed, the tentacles are formidable weapons; they are highly
contractile and wonderfully strong, tenaciously adhering to the small
worms and aquatic insects on which the Hydræ feed, and they are aided by
the roughness of their surface. They transfix their prey, and are
believed to infuse a liquid poison from the dart, or thread-cells, into
the wound, then twisting their other tentacles round the victim, it is
instantly conveyed to the mouth, and slowly forced into the digesting
cavity, where it is seen through the transparent skin to move for a
short time, but as soon as the nutritious juice is extracted, the animal
ejects the refuse by its mouth. In the inner layer, enclosing the cavity
of the stomach, there are cells containing a clear liquid with coloured
particles floating in it, which is supposed to perform the part of a
liver; and, as the Hydræ have no respiratory organs, their juices are
aërated through their skin. They have no perceptible nerves nor nerve
centres, yet they are irritable, eminently contractile, and are
attracted towards the light—all these being probably sympathetic
motions.

Though in general stationary, the Hydra can change its place; it bends
its body, stretches to a little distance, and fixes its anterior
extremity firmly by its tentacles; then it detaches its sucker and
brings it close to its mouth, fixes it, and again stretches its fore
part to a little distance along its path, and repeats the same process,
so that it moves exactly after the manner of certain caterpillars. It
can even move along the water by attaching the expanded disk of its
sucker to the surface, where it soon dries on being exposed to the air,
and becomes a float, from whence the Hydra hangs down with its tentacles
extended like fishing lines, as in fig. 110; or it can use them as oars
to row itself along under the surface of the water.

On account of their simple organization, the Hydræ are endowed with the
most astonishing tenacity of life. As the whole animal is nourished from
the surface of the digestive cavity, they appear to suffer no
inconvenience from being turned inside-out, the new cavity performing
all the functions of digestion as well as the old one. They may be cut
into any number of pieces, and, after a little time, each piece becomes
a perfect Hydra. The head may be cut off and they get a new one; or it
may be split into two or three parts or more, and the animal becomes
many-headed; and, what is still more marvellous, two Hydræ may be
grafted together direct, or head and tail, and they combine into one
animal.

[Illustration: Fig. 110. Hydra fusca.]

These singular and voracious creatures increase like plants by budding.
A little protuberance rises on the body by the bulging out of the double
skin or wall, so that the interior of the bud is a clear cavity in
communication with the stomach of the Hydra (fig. 110, _b_). The bud
increases in length, opens at its extremity into a mouth, and gradually
acquires the size and form of its parent (fig. 110, _c_); the
communication is then by degrees closed, and at last the matured bud
drops off and becomes an independent Hydra. Dr. Carpenter observed that
this process, which so closely resembles the budding of plants, must be
regarded as a modification of the ordinary nutritious process. The same
may be said of the power of reparation, which every animal body
possesses in a greater or less degree, but which is most remarkable
among the lower tribes, for when an entire member is renewed, or even
when the whole body is regenerated from a small fragment, which is the
case in many polypes, it is by a process exactly analogous to that which
takes place in the reparation of the simplest wound in our own bodies,
and which is but a modification of the process that is constantly
renewing, more or less rapidly, every portion of our frame.

There is but one species of the single colourless Hydra, but there are
four compound fresh-water Hydræ in England—the rubra, viridis, vulgaris,
which is of an orange brown, and the fusca. They have coloured
particles, either imbedded in their external coat, or immediately under
it. The Hydra viridis and H. vulgaris have short tentacles, whilst H.
fusca, which is a rare animal, has arms from seven to eight inches long,
and so contractile, that they can shrink into the space of small
tubercules. All these four Hydræ are compound and permanently
arborescent animals; each springs from one individual hydra of its own
race, which increases in length and forms the stem, while young ones
spring from it and from one another consecutively, like the compound
branches of a tree. The numerous tentacles that hang down like fishing
lines, thickly covered with thread-cells and their envenomed darts,
catch prey for the whole colony, because the communication between the
stomachs of the young polypes or Hydræ and that of their parent is never
cut off, as it is when the offspring is deciduous; but tubes from the
base of each individual Hydra or polype, passing through the stalks and
branches of the living tree, unite their stomachs with the stomach or
assimilating cavity in the main stem. Each individual polype, sometimes
to the number of nineteen, after having digested its food or prey,
ejects the refuse from its mouth, and the nutritious juice traverses the
labyrinth of tubes to that general reservoir.

Since every portion of the bodies of the Hydræ is nearly of the same
kind, and as every part of their surface inside and outside is in
contact with the water in which they live, and from whence they derive
oxygen to aërate their juices, no circulation is necessary in these
simple animals, either for nutrition of their tissues, or to furnish
them with oxygen.

If the Hydræ only produced deciduous buds which are developed into
facsimiles of their parent, their race would become extinct, since they
die in winter, unless kept artificially in water of mild temperature;
but the animals are hermaphrodite, so that each individual produces
fertilized eggs in autumn, which are hatched in spring, so that the
Hydra is alternately propagated by deciduous buds and by eggs. The
fresh-water hydræ are the only hydroids that are locomotive, all the
others being fixed to some solid substance.

The oceanic Hydrozoa comprehend the three families of Corynidæ,
Tubulariidæ, and Sertulariidæ. They are chiefly compound animals,
numerous in genera and species, and have great variety of form. They may
be simple and slender, they may be creeping or like a bush or tree, more
or less compound and regularly branched according to the form of the
polypary or tubular substance which unites their numerous hydra-form
polypes into one animal. In general they are exceedingly small; three or
four inches in height is quite gigantic. There is scarcely a still clear
pool left by the retiring tide among the rocks along the British coasts,
that does not abound with these beautiful creatures attached to stones,
old shells, or sea-weeds. But they must be sought for amidst the
luxuriant marine vegetation and profusion of animal life which adorn
these rocky pools, otherwise they would escape notice; and even when
large enough to be conspicuous, the eye must be aided in order to see
the wonderful minuteness and delicacy of their structure. The aquaria
have furnished an opportunity to study their forms, habits, and the
marvellous circumstances of their lives and reproduction.

The compound oceanic Hydrozoa are essentially the same in structure as
the compound fresh-water Hydræ. They differ, however, from them in often
having a greater number of tentacles, and in being defended by a firm
and flexible horny coat; notwithstanding which they increase in size by
budding from the base of a single primary polype. The horny coat covers
the bud and grows with it; but as soon as the polype is formed within
it, the top of the bud opens and the young polype protrudes itself, so
that a separation is effectually prevented; and while the stem and
branches are being formed, and increase by the continual development of
new buds, the communication between the stomachs of the whole brood of
polypes with that in the parent stem is maintained by tubes from their
bases passing through the interior fleshy matter in the branches.

In short these marine Hydrozoa consist of a ramified tube of sensitive
animal matter, covered by an external flexible and often jointed and
horny coat or skeleton, and they are fed by the activity of the
tentacles and the digestive powers of frequently some hundreds of
hydra-formed polypes, as in the Sertularia cupressina. The common
produce of their food circulates as a fluid through the tubular
cavities, for the benefit of the whole community, while the indigestible
part is ejected from the mouth of each individual. The stomach of each
polype has a more or less ciliated lining, containing cells with
nutritive juices, which are supposed to perform the part of a liver. The
liquid which circulates in these animals is colourless, with solid
particles floating in it; and there is reason to believe that sea-water
is admitted into the tubes, and that, mixed with the juices prepared by
the polypes, it circulates through the ramified cavities, is sent into
the hollow prehensile tentacles, and returns back into the digesting
cavity after having contributed to respiration by its oxygen. The
movements of this fluid appear to depend upon the delicate ciliated
fibre which lines the cavities of the tentacles and those of the stem
and branches of the compound animal, possibly aided by vital
contraction. The soft skin of the tentacles contains cells full of
liquid, with a thread and its sting or dart coiled up within it. These
thread stings are protruded when the skin is irritated, which frequently
gives the tentacles the appearance of being beset with bristled warts.
In many instances these kinds of Hydrozoa are covered with a gelatinous
substance, either as a film or thick coat.

The reproduction of many of these arborescent or compound Hydrozoa is
one of the most unexpected and extraordinary phenomena in the
life-history of the animal creation. For besides the system of
consecutive budding from a single polype which builds up the compound
animal, peculiar buds are formed and developed, which bear no
resemblance whatever to the polype buds: on the contrary, when mature,
they assume an organization exactly the same as that of the common
jelly-fish or Medusoid Acalephæ, and swim freely away from their fixed
parent as soon as they are detached. These medusiform zooids, which are
extremely small, consist of a cup or umbrella-shaped bell of colourless
transparent matter, which is their swimming apparatus; it is contracted
and expanded by a muscular band under the rim, the water is alternately
imbibed and forcibly ejected, and by its reaction the zooid is impelled
in a contrary direction. From the centre of the bell a stomach hangs
down in the form of a proboscis, with a mouth at its extremity, either
with or without tentacles and sting-cells. Four canals, or a greater
number, which begin in the stomach, radiate through the transparent
matter of the bell, and are united by a circular canal round the rim;
they convey the nutritious liquid from the stomach throughout the
system. This general structure may be traced in the zooids of the three
great families of the oceanic hydraform-zoophytes, in a greater or less
degree, from deciduous perfect medusæ to such as are imperfect and
fixed.

These medusiform zooids are male and female, and when detached from
their parent they are independent creatures, each of them being
furnished with nutrient and locomotive organs of its own. They produce
fertilized eggs, which are developed into ciliated locomotive larvæ;
after a time these lose their cilia and acquire a rayed sucking disc,
with which they fix themselves permanently to a solid object, and, after
various changes, each gets a mouth and tentacles and becomes a perfect
young hydra. Thus a brood of young hydræ is produced, each of which
acquires the compound form of its parent by budding, and as each of
these compound animals in its turn gives off medusa-buds, there is a
cycle of the alternate forms of hydra and medusa or jelly-fish, showing
a singular connection between two animals which seem to have nothing in
common. The analogy which so often prevails between plants and animals
obtains here also, for the medusa-buds bear the same relation to the
hydra or polype-buds that the flower-buds of a tree do to the leaf-buds:
the flower-buds contain the germs of future generations of the tree,
while the leaf-buds contain only the undeveloped stems, stalks, and
leaves of the individual plant on which they grow.

The Corynidæ form the first of the three families of the oceanic hydra
zoophytes. They comprise six genera, and many species of compound
animals of various forms, each derived from a single animal by budding;
and although they possess a thin flexible coat, the polypes are sheathed
either in a thin membrane or bone. Their club-shaped tentacles form
either a single or double circlet round the base of their conical mouth,
and are also scattered over their bodies when bare.

[Illustration: Fig. 111. Syncoryna Sarsii with Medusa-buds.]

The zooids are developed at once in the Syncoryna Sarsii, which is a
long, thinly branched, and horny zoophyte, with a single naked,
spindle-shaped polype at the extremity of each branch, as in fig. 111,
A. The bodies of the polypes are studded with numerous tentacles, among
which buds appear (fig. 111, _a_, _b_); these gradually expand into
bell-shaped medusa-zooids (fig. 111, _c_), some being masculine and
others feminine. They drop off their parent, swim away by the
contraction of their bell, and their fertilized eggs are developed into
single hydræ, which become arborescent like their parent by budding.

The family of the Sertulariidæ take branching forms, sometimes of
perfect symmetry: they have a firm, horny coat, which not only covers
the stem and branches, but becomes a cup for the protection of the
polype. The most common form of the family of the Tubularia has no
branches: it has an erect, hollow stem like a straw, sometimes a foot
high, coated by a horny sheath. The polype which terminates each plant
has a mouth surrounded by alternately long and short tentacles. The
stomach of the polype is connected with the hollow in the stem by a
muscular ring, by whose alternate dilatation and contraction, at
intervals of eighty seconds, the fluid is forced up from below, enters
the stomach, and is again expelled. Another liquid carrying solid
particles circulates in a spiral through the whole length of the stem.
Some of this family are propagated by perfect deciduous medusæ, others
by imperfect fixed ones; both are developed on the polypes or among
their tentacles. Like the fresh-water Hydræ, these creatures can restore
any part of their bodies that is injured.

Numerous instances might be given to show that the minute medusiform
zooids are only a stage or phase in the life of an oceanic hydra:
conversely it will now be shown, that the single simple hydra is but a
stage in the life-history of the highly organized medusa, jelly-fish, or
sea-nettle of sailors, the Acalepha of Cuvier.

The medusæ vary in size, from microscopic specks that swim on the
surface of the sea in a warm summer day to large umbrella-shaped jelly
fish almost a yard in diameter. They abound in every part of the ocean
and in all seas, often in such shoals that the surface of the water is
like a sheet of jelly. Their substance is transparent, pure, and nearly
colourless; chiefly consisting of water, with so little solid matter,
that a newly caught medusa, weighing two pounds, dries into a film
scarcely weighing thirty grains.

The Pulmograde Medusæ, which swim by the contractions of their
umbrella-shaped respiratory disc, form two distinct groups, the
naked-eyed medusæ and the covered-eyed group. Both are male and female;
each has its own form of thread-cells; and the stinging power or
strength of the poison is nearly in proportion to the size of the animal
and the coarseness of its threads.

The disk, or umbrella-shaped swimming organ, in both groups consists of
a large cavity included between two layers of gelatinous matter, which
unite at the rim. The interior membrane, called the sub-umbrella, is
encircled at its edge by a ring of highly contractile muscular fibre
like the iris of our eyes, by which this swimming organ is expanded and
contracted. From the centre of the sub-umbrella a stomach, in the form
of a proboscis, is suspended, which is of a very different structure in
the two groups.

[Illustration: Fig. 112. Thaumantia pilosella.]

The Thaumantia pilosella, a member of the naked-eyed group, is like an
inverted watch-glass (fig. 112), less than an inch in diameter. The roof
of this umbrella is much thicker than the sides, and gradually thins off
towards the rim. The proboscis, or stomach, descends from the centre of
the sub-umbrella, but not so far as to the edge of the rim: it ends in a
mouth with four sensitive fleshy lips. Four slender canals, which
originate in the cavity of the stomach, radiate from the centre of the
roof of the umbrella and extend to its margin, where they unite at the
quadrants with a canal which encircles the rim, and are prolonged beyond
it in the form of tentacles armed with numerous thread-cells containing
poisonous darts. These tentacles must be formed of muscular fibre, for
they are very irritable: each of them may be extended and contracted
separately or along with the others; they guide the medusa through the
water, and can anchor it by twisting round a fixed object.

The prey caught is digested in the stomach, the refuse is ejected by the
mouth, and the nutritious fluid that has been extracted is carried up
through the base of the stomach into the four radiating canals, to
supply the waste and nourish the system. The digestive cavity and canals
are lined with a soft membrane, covered with cilia, whose vibrations
maintain the circulation of the juices and perform the duty of a heart;
for the medusæ have none, nor have they any special respiratory system:
their juices are aërated through the under-surface of the rim of the
umbrella, while passing through the circular canal lying either within
the water or on its surface.

A fringe of filamental tentacles hangs down into the water from the rim
of the disc or umbrella, which is studded at equal distances by fleshy
bulbs, each of which has a group of fifty dark eye-specks, being the
rudiment of an eye; and if the animal be disturbed when in the dark,
each eye-speck shines with a brilliant phosphoric light, and the
umbrella looks as if it were begirt with a garland of stars.

[Illustration: Fig. 113. Otolites of Magnified Thaumantias.]

Close to the edge of the canal which encircles the margin of the
umbrella, there are eight hollow semi-oval enlargements of the flesh,
two in each quadrant formed by the four radiating canals: they are the
eight ears of the medusa, for in these hollow organs there are from
thirty to fifty solid, transparent, and highly refractive spheres,
arranged in a double row, so as to form a crescent, those near its
centre being larger than the more remote. The solid spheres are
analogous to the otolites in the ears of the more highly organized
animals. Mr. M‘Cready has discovered nerve-centres behind each tentacle,
and under each marginal coloured speck in several species of the
open-eyed medusæ, which places this group of Acalephæ in a higher grade
than any of the preceding orders. The medusæ swim by the muscular energy
of their umbrellas: at each rhythmical contraction the water, which
enters by the mouth and fills the great central cavity within the
umbrella, is forced out again through an orifice at the other end, and
by its reaction the medusa is impelled with considerable velocity in the
contrary direction, so that the top of the umbrella goes first, and all
its tentacles are dragged after it.

The medusæ are diœcious: in the males four reproductive cells full of
reddish or purple granular matter surround the cavity of the stomach,
and appear like a coloured cross through the top of the gelatinous
umbrella. In the females, at a point just before the four radiating
canals enter the marginal canal, the flesh on the exterior of the
umbrella swells out into bulbs, containing vessels full of clear eggs
with minute globular yolks. These eggs, when fertilized, are hatched,
and the young are developed within these ovaries, so that they come into
the water as a kind of infusorial ciliated animalcule destitute of a
mouth. One end of the creature acquires a suctorial disc, fixes itself
to an object, and uses its cilia. The other end opens into a mouth,
round which tentacles like fishing lines spring forth; the central part
is converted into the cavity of the stomach, and thus a perfect hydra is
formed, capable of being propagated naturally by budding, or
artificially by being cut in pieces, each piece becoming a perfect
hydra, differing in no respect from a common simple fresh-water Hydra.

[Illustration: Fig. 114. A, B, C, D, development of Medusa-buds; _a_,
polype-body; _b_, tentacles; _c_, a secondary circle of tentacles; _d_,
proboscis; _e_, new polype-bud.]

From one of these, numberless successive generations of simple hydræ may
be produced by budding, all catching their prey with their tentacles and
digesting it in their stomachs. The limits to this budding-system seems
to be indefinite: years may pass in this stage, but at length it ceases,
and either the original hydra, or one of its descendants, undergoes a
series of remarkable changes. The body of the hydra lengthens into a
cylinder; it is then marked transversely by a number of constrictions
beginning at the free end; these become deeper and deeper, till at
length they break up the body into a pile of shallow cups, each lying in
the hollow of the other, and leaving a kind of fleshy wall at the point
of suspension or fixture. The edges of the cups are divided into lobes
with a slit in each, in which the coloured rudiment of the eye is sunk.
The cups are permanent, and characteristic of the group of naked-eyed
medusæ. After a time, the cups begin to show contractile motions, which
increase till the fibre of their attachment is broken, and then the
superimposed cups are detached from the pile one after another, and swim
freely away by the contractions of their lobes as young medusæ, leaving
what remains of the parent hydra to repair its loss and again repeat
this singular process. However, the young medusæ are not yet perfect. As
they increase in size the divisions on the edge of the cup fill up; a
proboscis-shaped stomach, with its four coloured cells and its square
mouth, is developed from the centre of the sub-umbrella; the radiating
canals extend from the central cavity, the encircling canal and fringe
form round the umbrella-shaped cups, and the result is a highly
organized Thaumantia pilosella, in whose life-history a simple hydra
forms a singular stage.

Thus hydræ produce medusæ whose offspring are hydræ, and perfect medusæ
produce hydræ whose offspring are perfect medusæ. However, the law of
the alternation of generation is by no means peculiar to the Thaumantiæ.
Many species of medusæ are subject to it, as the Turris neglecta, a
beautiful little medusa not larger than a hempseed, common on the
British coasts. It has a white muscular pellucid umbrella, a large
proboscis of a rich orange colour at its upper part: in the
orange-coloured flesh of it there are ovaries containing rose-coloured
eggs, which are hatched within them, and come into the water as ciliated
gemmules, which, after swimming about for a time, become fixed and are
developed into small hydræ of a rich purple colour with sixty-four
tentacles. From these hydræ others bud off indefinitely till the time
comes when one of them becomes lengthened, constricted, divided into
cups which drop off, and finally become a brood of the Turris neglecta.

The naked-eyed medusæ are extremely numerous. There are six orders of
them and many genera, chiefly distinguished by the position and nature
of their ovaries and the number of canals which radiate through their
swimming organs. Both of the medusæ that have been described have four
radiating canals; yet they belong to different orders, for the ovaries
of the Thaumantia are in the edge of the umbrella, while those of the
Turris are in the substance of the proboscis. Neither of these kinds
have more than four ovaries, but some other kinds have eight ovaries and
eight radiating canals. Most of the canals are simple, but in one genus
they are branching. All are furnished with tentacles, some of them
having stings, others none.

The covered-eyed group consists only of two natural divisions—the
Rhizostoma, or many-mouthed medusæ, and the Monostoma, or one-mouthed
medusæ. In both the coloured eye-specks at the margin of the umbrella
are larger and more numerous, than in the naked-eyed group, and they are
covered with a hood. The proboscis of the one-mouthed order terminates
in a square mouth, the four angles of which are prolonged into tentacles
with a solid hyaline axis. They have a fringed membrane along their
under-surface, containing numerous stinging thread-cells. Sixteen
canals, connected with the stomach or cavity of the proboscis, radiate
over the flattish, cup-shaped umbrella; eight of these are branched, and
terminate in the circular canal which runs round its fringed edge, and
they form the nutrient and respiratory system of the animal, while the
eight simple and alternate canals terminate in eight openings at the rim
of the umbrella, through which the refuse or indigestible part of the
food is discharged, thus forming an exception to the other pulmograde
medusæ, and indeed to the Hydrozoa in general, which eject it at the
mouth. All the canals are lined with cilia, whose vibrations maintain
the circulation of the fluids, and perform the duties both of a heart
and respiratory apparatus. Dr. A. Krohn has observed that in three
species of the genus Pelagia belonging to the covered-eyed medusæ, the
young are at once developed as medusæ without the intervention of the
hydra form.

[Illustration: Fig. 115. Rhizostoma.]

The disk of the Rhizostoma, or root-mouthed medusæ, is rather flat, and
the large proboscis is unlike any other of the tribe. In the naked-eyed
medusæ digestion is performed in the cavity of the proboscis; but in
this order the proboscis is divided into four very long branches ending
in club-shaped knobs (fig. 115), and nutrient tubes extend to their
extremities from the great central cavity in the umbrella. Their
broadish frilled borders are divided and subdivided along their whole
lengths, and the nutrient canals, which follow all their ramifications,
end in numerous fringed pores upon their edges and upon the club-shaped
ends of the quadrifid proboscis. These numerous pores are mouths; they
absorb minute animalcules, which are digested while passing through the
united canals to the great central cavity of the umbrella, which
receives the products of digestion. Eight canals radiate from that great
cavity and traverse the umbrella; and the nutrient fluid, mixed with the
sea-water, passes from the great cavity through these canals into an
elegant network of large capillary tubes spread on the under-surface of
the margin of the umbrella, which is always in contact with the water;
and in this beautiful respiratory organ the carbonic acid gas is
exchanged for the oxygen in the water of the sea. The indigestible part
of the food is discharged through the mouths or pores, whose edges are
prolonged into solid tentacles containing thread-cells, with their usual
weapons of offence and defence. Besides these armed tentacles, which are
very numerous in the covered-eyed group, the gelatinous umbrella has a
multitude of oval thread-cells on its external coat, in each of which a
very long filament is spirally coiled, which darts out to a considerable
distance on the smallest touch, and stings severely.

A few only of the British pulmonigrade medusæ sting: the Cyanea
capillata, one of the single-mouthed covered-eyed family, is most
formidable. It has very long tentacles, which it can throw off if they
get entangled, but they continue to sting, even after they are detached
from the medusa.

This is one of the most remarkable instances of the inherent
irritability of muscular fibre still in full force after the tentacles
have been separated from the living animal. In many of the lower
animals, as in the Hydra itself, vitality is so far from being
extinguished in the severed members that it repairs the injury. Since
the covered-eyed medusæ have eyes, ears, and very sensitive tentacles,
it may be inferred that they possess nerves of sight, hearing, and
touch, though none have been discovered, probably on account of the
softness and transparency of their tissues. The stinging power by which
they kill their prey and defend themselves may be classed among the
consensual powers prompted by the sympathetic sensations of hunger or
danger.

In all latitudes the medusæ are highly luminous, especially in warm
seas. Professor Vogt remarked that flashes of light passed over their
disk when they touched one another in swimming, and they appear at
intervals like globes of fire among the lesser lights of the Noctilucæ;
if from involuntary nervous contraction, as is most likely, the light
must be electric.

The medusæ are infested by many parasites. Entozoa are often abundant in
their gelatinous substance, and crustaceans of various kinds and
colours, such as shrimps, sand-hoppers, and a galæmon of glassy
transparency, move about in the substance of their disc and arms,
entering unscathed by the poisonous darts which inflict instant death on
others of their class. The Libanea crab, of gigantic size compared with
its host, is in the habit of taking up its abode between the four
columns of the Rhizostoma. But the most singular intruder is the
Philomedusa Vogtii, which is a polype with twelve thick short tentacles,
its whole body and tentacles being covered with cilia and thread-cells.
These polypes live in the disk, arms, and stomach of the medusæ, and,
when taken out, their stomachs are found to contain fragments of the
tentacles of their host, and even the thread-cells with their stings.
The larger polypes devour the smaller ones, and the latter live for
weeks within the larger ones without apparent inconvenience to
either.[24]

Mr. M‘Cready mentions that the larvæ of the medusa Cunina octonaria swim
as parasites in the cavity of the bell of the medusa Turritopsis
nutricula, which not only furnishes a shelter and dwelling-place to the
larvæ during their development, but it also serves as a nurse, by
permitting the parasites, which adhere by their tentacles, to take the
food out of its mouth by means of their long proboscides. They undergo
many transformations, and become nearly perfect medusæ while within
their nurse.

Medusæ of different species are met with in every sea from the equator
to the poles. They are eminently social, migrating in enormous shoals to
great distances. The largest shoal of young sea nettles on record was
met with in the Gulf Stream, off the coast of Florida, by a vessel bound
for England. The captain likened them to acorns; they were so crowded as
completely to cover the sea, giving it the appearance from a distance of
a boundless meadow in the yellow leaf. He was five or six days in
sailing through them, and in about sixty days afterwards, on returning
from England, he fell in with the same school, as the sailors call it,
off the Western Islands, and was three or four days in sailing through
them again. Mr. Piazzi Smyth, when on a voyage to Teneriffe in 1856,
fell in with a vast shoal of medusæ. With a microscope he found part of
the stomach of one of these creatures so full of diatoms of various
forms—stars, crosses, semicircles, embossed circles and spirals—that he
computed the whole stomach could not have contained less than 700,000.
The flinty shells of the diatoms ejected in myriads by the medusæ,
accumulate in the course of ages into siliceous strata, which, heaved up
by subterranean fires, at length become the abode of man. Thus
gelatinous transparent beings indirectly aid in forming the solid crust
of the earth by means of the microscopic vegetation of the sea.


                         _Ciliograde Hydrozoa._

The ciliograde Acalephæ, which form four orders and many genera, and
which swim by means of symmetrical rows of long cilia, are represented
on the British coasts by the Cydippe pileus and the Beroë Forskalia
(fig. 116), little delicately tinted, gelatinous, and transparent
animals that shine in the dark.

The Cydippe pileus is a globe three-eighths of an inch in diameter, like
the purest crystal, with eight bands of large cilia, stretching at
regular distances from pole to pole. A mouth, surrounded by extremely
sensitive tentacles, is situated at one pole, the vent at the other. The
Cydippes poise and fix themselves to objects by means of two very long
tentacles, fringed on one edge by cirri, that is, short curled
tentacles. These cirrated tentacles, which in some species stretch out
to more than twenty times the length of the animal, can be
instantaneously retracted into cavities at the posterior end of the
body, while, at the same time, the marginal filaments are as rapidly
coiled up in a series of close spirals. The whole of these complex
organs are enclosed within the limits of a pin’s head.

[Illustration: Fig. 116. A, Cydippe pileus; B, Beroë Forskalia.]

The manner in which these little gems swim is beautiful; sometimes they
rise and descend slowly, like a balloon, and when they glide along the
surface of the water in sunshine, the cilia on the eight meridional
bands exhibit the most brilliant iridescence. The long cirrated
tentacles follow all their motions in graceful curves, or hang
indolently down, and sometimes they are suddenly stretched to their full
length, and as suddenly retracted, and in all their varied convolutions
the cirri that fringe them are in constant vibration, and exhibit all
the tints of the rainbow. Sometimes these creatures whirl round their
axis with great rapidity, but, active as they are, no nervous system has
yet been discovered in them.

_Fig. 117, p. 103._

[Illustration: PRAYA DIPHYS.]

The common Beroë is like an elongated melon, obtusely octangular, with
eight rows of cilia, extending from a mouth at one end to a kind of
ciliated star at the other. The Beroës are of a gelatinous transparent
substance, which expands and contracts with great facility: it is always
expanded when they swim.

The Cestum Veneris belongs to another genus of the same family. It is
like a blue ribbon, the mouth and vent being on the opposite sides in
the middle of the band, which is furnished throughout its whole length
with active cilia for swimming. The ciliograde Hydrozoa are monœcious,
and do not produce medusa-zoids.


                        _Campanograde Acalephæ._

There is a group of oceanic Hydrozoa, consisting of several families,
which are fed by numerous suctorial organs called polypites, with
tentacula and thread-cells attached to their bodies, so that they are
analogous to the marine hydræ, in being colonies of individuals united
into a compound animal. Some have air-vessels, which enable them to
float on the surface of the water; but the locomotive organs of this
group are bells, so that they may be called Campanograde Acalephæ.

The family of the Diphyidæ are colourless, and of such transparency that
they are all but invisible when in the water, and are gelatinous masses
clear as crystal when taken out of it. They are chiefly inhabitants of
the warmer parts of the Pacific and Atlantic Oceans, but many fine
specimens are found in the Mediterranean. Of these the Praya diphys is
one of the most extraordinary (fig. 117). It has two large
swimming-bells, their mouths turned backwards, with which the whole
community is connected. They are nearly equal in size, soft, gelatinous,
transparent, and colourless, rounded in front, open and truncated
behind. The adjacent sides are parallel, with a groove between them,
into which one end of the long tubular filiform body of the animal is
fixed by slender tubes, through which a nourishing liquid passes into
radiating canals in the bells, and from them into a circular canal at
their margins, which are surrounded by a muscular contractile iris, like
that in our eyes, which shuts and opens the bells. By the alternate
absorption and ejection of the water the bells go head foremost, and
regulate the motions of the whole compound animal. When both bells are
active it goes straight forward; when the right hand bell is alone in
action, it goes to the left, and _vice versâ_; in fact, the bells act as
a rudder.

The slender cylindrical body or axis of the Praya is so transparent,
that the cavity and muscular fibres of its walls are distinctly seen.
These animals are extremely contractile. Professor Vogt mentions an
individual he met with at Nice more than three feet long, when extended
on the surface of the water, which could contract itself into little
more than a finger length. It was said to have had a hundred isolated
groups of polypites with their appendages attached to it; but in general
the Prayæ are not so long, and seldom have more than thirty or forty of
these isolated groups, which are attached to the under-side of the long
flexible body, and hang down like a rich and beautiful fringe. In the
figure, the position of the numerous groups of polypites and their
appendages are merely indicated by round marks and lines.

In the body of the Praya diphys (fig. 117), as in that of the whole
family, there is a nutritious liquid, which, by means of cilia, flows on
its interior surface in two directions: it enters the canals in the two
large bells, and supplies them with nourishment.

The polypites which digest the food are vermiform double sacs
communicating at one end by a valve with the canal in the body of the
animal; and at the free end they are prolonged into a mouth with an
everted lip, and the digesting apparatus lies in the centre. Each
polypite is supplied with food by its own fishing-line descending from a
point close to where the polypite is fixed to the long axis. It is a
long, tubular, branched tentacle, each branch ending in a coloured,
pear-shaped, or fusiform battery of thread-cells with their stings. A
gelatinous plate is placed on the upper side of the common axis
immediately over the isolated groups, to protect and separate them.

Such are some of the most general characters of the family Diphyidæ: the
Praya diphys has something peculiar to itself.

In the Praya, each individual group has a swimming-bell of its own
adjacent to the polypite, and lying parallel to the axis of the animal,
with its mouth turned backwards. It is connected by tubes both with the
general central canal, and with a helmet-shaped protecting plate. On the
other side of the polypite, there is a tuft of vermiform buds with
spiral terminations, bristled with thread-cells. From the centre of this
tuft a tentacle, or fishing-line, descends with numerous branches, the
whole forming a tubular system connected with the common canal in the
axis. Each of the branches of the tentacle terminates in a
vermilion-coloured tendril, coiled up into a minute capsule. The inside
of the tendril is not only bristled with the points of sabre-shaped
darts, but it conceals a filament crowded with thread-cells. On the
slightest touch, the tendril stretches out like a corkscrew of red
coral, and every dart springs forth. Such is, more or less, the
complicated structure of the offensive and defensive weapons of many of
this order of oceanic Hydrozoa, which appear to the naked eye as merely
brightly-coloured points. The use of these tentacles, or fishing-lines,
is the same in all; they seize, kill, and carry their victims to the
mouth of the polypite by contracting their long lines.

In the Praya, the groups are individualized in the highest degree
consistent with union; for, when the animal is at rest, each of the
individual groups, amounting to thirty or forty, swims about by means of
its little bell independent of the rest. Their motions can be compared
to nothing but a troop of jugglers performing gymnastic exercises round
a cord represented by the common body of the animal; except for
adherence to which the life and will of each group are so perfectly
independent, that the mutual dependence of the whole is only seen when
the common trunk contracts to bring all its appendages towards the two
principal bells, which then begin to move.[25]

Thus each group has a special life and motion, controlled by a general
life and motion; strong individual muscular power controlled by general
muscular power; yet no nervous system has as yet been discovered, so
this animal activity must for the present be attributed to a strong,
inherent, contractile power in the muscular fibre. The Praya is seldom
complete, on account of the ease with which it casts off its great
bells.

None of the Diphyidæ have special organs for respiration; their juices
are aërated through their delicate tissues. They are diœcious, and
invariably produce perfect male and female medusiform zooids; they are
situated among the groups of the polypites and their appendages, and are
attached to the axis of the animal. When free, they swim away by the
contraction of their bells; the eggs are fertilized, and produce young
Diphyidæ, male and female; so these animals, like most of the oceanic
Hydrozoa, have two alternate stages of existence.


                         _Physograde Acalephæ._

The Galeolaria lutea (fig. 118, frontispiece) is similar to the Praya
diphys in having a slender, tubular body, with groups of sterile
polypites and their appendages hanging at intervals along its under-side
like a fringe, and also in having two swimming-bells at its anterior
extremity; but it has no special small bells. The large ones differ from
each other in size, form, and position. The largest is nearly
cylindrical, its mouth is turned upwards, and its rim is elevated at one
part into two stiff organs like the blinkers that are put over horses’
eyes: besides these, it has six salient points, which nearly close the
mouth of the bell at each contraction of the muscular iris that lines
the margin of the cavity. The small bell, which goes first in swimming,
is thicker and shorter, and its side rises in a hump, upon which the
closed end of the large bell rests, and in the cavity between the two
the anterior extremity of the filiform body is fixed. Each of the groups
of polypites, with their tentacles, lies immediately under its
spathe-shaped protecting plate. The polypites are very contractile, and
on their protuberant part, containing the digestive cavity, there is a
large circular space, which, as well as the whole tissue of the polypite
and the stinging capsules at the extremities of the tentacles, are of an
orange colour, and are akin in structure to those described.

When very young the Galeolaria and its congeners have only their
swimming-bells and one polypite group affixed to the end of a short
tubular axis; by and by a second group is developed from buds between
the bells and the first group; then a third is developed between the
bells and the second group, and so on; the length of the body and the
number of groups continue to increase indefinitely. It is only when the
animal is full grown and complete in all its parts, that reproductive
organs are developed towards its posterior end. Buds then appear upon
the hollow stems of the polypites towards the posterior end of the body.
But as the Galeolaria is diœcious, male and female buds are never on the
same individual. The female buds become medusiform zooids, like those of
the Praya diphys, only the transparent cup, with which it swims away
from its parent, has two projections like ears on its rim.

The development of the buds in the male Galeolaria is similar. At first
they are pale, but they assume an orange red colour as they advance
towards maturity, and, when complete, the sac which hangs down from the
centre of the transparent cup becomes of a brilliant vermilion. These
male and female medusa-zooids swim about for several days, and the
fertilized eggs are hatched into young Galeolariæ, male and female.

Thus the Galeolaria lutea has two kinds of polypites, both nutritive,
but one is sterile, the other prolific. The latter are similar to the
prolific individuals of the syncorine Hydræ, in which the anterior part
is a digestive organ, while on the base or stalk true medusa-zooids are
found. It is curious that the spathe-protecting plate of the Galeolaria
appears in the egg as a globe of such size that the other parts seem to
be merely the appendages.

_Fig. 119, p. 108._

[Illustration: APOLEMIA CONTORTA.]

The Apolemia contorta (fig. 119) unites the most graceful form to the
utmost transparency and delicacy of tissue. It has a double float, the
first small and globular, the second long and oval. The neck is short,
the rose-coloured body is flat as a ribbon, and covered with thin,
curved, pointed, and imbricated plates, like tiles on the roof of a
house, but so minute that they are only perceptible to the naked eye by
a slight iridescence. At the extremity of the short neck buds,
semi-developed buds, and perfect swimming cups are arranged in vertical
series; and as the flat body is twisted into a spiral to its farthest
end, the cluster of bells forms a perfect cone with the float at its
apex. The bells are flattened; and there is always in their more solid
posterior part a single canal rising directly from the general trunk
which divides into four branches; and these, having traversed the
swimming cavity, unite anew in a circular canal, or iris, destined to
shut and open the cup.

_Fig. 120, p. 109._

[Illustration: PHYSOPHORA HYDROSTATICA.]

The sterile polypites that are attached at intervals by their hollow
stems to the twisted body of the Apolemia, have twelve rows of cells
inserted in the bright lining of their digestive cavities; their
anterior part has a trumpet-shaped mouth full of thread-cells. The
tentacles affixed to their stems and their secondary lines are like
those of the Diphyidæ. Besides these sterile polypites, which serve only
to feed the animal, the Apolemia has a kind of mouthless prolific
organs, which do not contribute to the general nourishment: each group
has a pair of them attached to the extremity of a branching stem. They
resemble polypites in being long and contractile at their extremities;
the interior is full of a substance like sarcode, and encompassed by a
red ring. Female buds yielding eggs appear on the stem of one of these
organs, while male buds are developed into medusiform zooids on the stem
of the other, which become detached, swim away, and the fertilized eggs
yield young Apolemiæ.

The natural position of an individual of the family of the Physophoridæ
when at rest is to hang perpendicularly from its air-vessel. The body,
which begins with a pyriform float, descends in a slender filiform
scarlet tube with a number of hyaline natatory cups or bells attached on
each side. The lower end of the body enlarges into a bulb or disk
supporting various appendages.

The Physophora hydrostatica (fig. 120), common in the Mediterranean, has
a transparent pear-shaped air-vessel tipped with red, from which the
slender cord-like tube of the body descends. Immediately below the
air-vessel, a number of buds and young bells are attached, followed by a
series of perfect three-lobed swimming bells, placed on each side
obliquely one below the other; and as they alternate and embrace the
body with their deeply excavated sides, they give it the appearance of a
crystal cone. Four canals spring from the hollow stalks of the bells,
traverse them, and end in a circular canal close to the membranaceous
iris which surrounds the margin of the internal cavity. Below the cone
the tubular body expands, and is twisted into a flat spiral, so as to
form a hollow disk or bulb, to which three different circlets of organs
are appended. The first and uppermost is a coronet of red, worm-like,
closed sacs, in constant motion, with large thread-cells at their
pointed extremities. They are attached to the upper surface of the bulb
by their broad bases, and communicate with its tubes by a small valve.
Male and female capsules follow either in a circle, or mixed with the
third and undermost circlet of organs, which consist of sterile
nourishing polypites, fixed by hollow stems to the bulb, each of which
has a long branching tentacle fixed to the base of its digesting cavity.

There are as many polypites on the under-side of the bulb as there are
red worm-like sacs on its upper edge. Each polypite consists of three
distinct parts. The posterior part is a hollow red stalk inserted under
the circumference of the disk; the second part is a bright yellow
globular expansion containing the digestive cavity lined with cilia; the
third and anterior part, which ends with the mouth, is quite colourless
and transparent, and assumes various shapes by constant expansion and
contraction.

At the limit between the red stalk and the yellow globular part of the
polypites there is a tuft of cylindrical appendages, from which a long
tentacle descends with its secondary tentacles and red nettle-bulbs. All
the canals of this Physophora are connected, and their walls are lined
with muscular fibres, either circular, longitudinal, or both, which give
a marvellous contractile and motive power. When the animal is suspended
from the surface of the sea by its float, every member is in motion,
especially the numerous tentacles, which are perpetually in search of
food, and are so extremely sensitive that even a sudden motion of the
water makes them shrink under the red worm-like organs on the edge of
the disk. This animal is generally from one to three inches long.

All the preceding members of the physograde group are really
campanograde, for the action of the wind upon the floats of the
Physophoridæ must be small, otherwise they would not be furnished with
so many swimming cups. The Physaliidæ and Velellidæ are the only two
orders that are truly physograde, for the wind is their only locomotive
power.

The Physalia, or ‘Spanish man-of-war’ of sailors, is by far the most
formidable animal of the Acalephæ tribe; its poisonous stings, which
burn like fire, inflict instant death on the inferior animals, and give
painful wounds to man himself. Its body, as it floats, is a long
horizontal double sac (fig. 121), which begins with a blunt point,
gradually enlarges, and becomes cylindrical about the middle; then it
somewhat suddenly widens in a transverse or lateral direction. Along the
upper surface of the pointed half the membrane or wall of the sac is
raised into a transversely placed crest, which dies away at the enlarged
end. The greater part of the body is smooth, but the under-surface of
the transversely enlarged end swells into lobes, from whence numerous
tentacles and other organs descend.

Almost the whole of the body of the Physalia is filled by an air-vessel,
so that it floats on the surface of the sea, and is wafted to and fro by
the wind. The bladder containing the air is enclosed in two membranes,
the outer one dense, thick, and elastic, the inner formed of delicate
fibres and lined with cilia. The air-sac is only attached to one part of
the interior; and there it communicates with the exterior by a small
aperture, which may be seen at about half an inch from the pointed apex
of the animal. The body is several inches long, of a delicate pale green
colour, passing gradually into dark indigo blue on the under-surface;
the ridge of the crest is tipped with dark crimson, and the pointed end
is stained with deep bluish green.

[Illustration: Fig. 121. The Physalia.]

The appendages, which hang down from the inferior and thick part of the
body, are large and small branchless tentacles of various lengths, and
sterile polypites in different stages of development. In some
individuals the tentacles are nine or ten feet long, of a deep blue
colour at their origin, and formed of two distinct parts, which have a
common base. One is a long conical bag, formed by an extension of the
under-surface of the body lined with cilia, and ending in a pointed apex
full of stinging thread-cells. It is flat on one side, attached
throughout its length to the tentacle, and is supposed to furnish poison
for the stings. The tentacle itself is a closed tube whose canal
communicates with the cavity of the long sac, and consequently with that
in the animal’s body. The interior of the tentacle is ciliated, its
upper part is gathered into folds; and the rest, which hangs straight
down, is like a delicate narrow ribbon, highly contractile from muscular
fibres, of which the most conspicuous are longitudinal. The tentacle is
marked at regular intervals by blue kidney-shaped masses, containing
myriads of powerful thread-cells, in which the threads of the darts are
coiled in a spiral, and contain muscular fibres, that serve to contract
and extend them. The smaller tentacles vary as much in length as the
large ones; they are of similar structure, but of a paler colour, and
are indiscriminately mixed with the other appendages.

The polypites, which are direct processes from the under-surface of the
body, are crowded in groups of various sizes round the base of the large
tentacles and mixed with the small; they are of a deep blue at their
base, frequently of a bright yellow at their extremities, and on an
average about three-fourths of an inch long. They are as irritable and
contractile as the tentacles, and are in constant motion. Their mouth is
large, with an everted lip armed with thread-cells; it sucks in the prey
caught and brought to it by the contraction of the tentacles, and which
is speedily dissolved by the powerful solvent juices in its digesting
chamber.

Among the numerous appendages attached to the under-surface of the
Physalia, there are bluish-green velvety masses fixed to extremely small
branching processes from the body of the animal, which seem with a
microscope to consist of tentacles, polypites in various stages of
development, male reproductive capsules which are never detached, and
female buds that are developed into medusiform zooids, and are presumed
to become free as in other cases. The Physaliidæ are social animals,
assembling in numerous shoals in the warm latitudes of the Atlantic and
Pacific. They naturally have their crest vertical, kept steady by their
tentacles, which drag down in the water; but Professor Huxley has seen
them at play, in a dead calm, tumbling over and over. The Physalia does
not possess the power of emptying and refilling its float with air: it
is doubted whether any of the physograde animals have that power, but
the subject is still in abeyance.

The Velellidæ are little sailing members of the physograde group. The
Velella spirans (fig. 122), a Mediterranean species, has a body or deck
consisting of a hollow horizontal disk, of a firm but flexible
cartilaginous substance, surrounded by a delicate membranous fringe or
limb half the width of the body. A triangular vertical crest, formed of
a firm transparent plate, also encompassed by a delicate limb, is fixed
diagonally from one angle of the disk to the other, but not on the
fringe; and as the natural position of the Velella is to float
horizontally on the surface of the water, the crest is exposed to the
wind and acts as a sail.

The float or air-vessel is flat, horizontal, and nearly fills the whole
body of the animal: it consists of two thin, firm, and rather concave
plates joined at their free edges, and united also by a number of
concentric vertical partitions, between which there is a series of
concentric chambers or galleries filled with air. The chambers
communicate with one another by apertures in the dividing membrane; they
also communicate with the exterior by perforations through the surface
of the body. Very long pneumatic filaments, that is tubes filled with
air, descend from the inferior surface of the float, and pass through
the lower plate of the disk into the water.

[Illustration: Fig. 122. Velella spirans:—1, upper side; 2, under-side.]

The disk is transparent, and appears to be white from the air within it;
and it is marked by concentric rings corresponding to the divisions in
the air-vessel below. The fringe-like limb that surrounds it is flat,
flexible, semi-transparent, and of the richest dark blue passing into
green, with a light blue ring; it is very contractile, and moves in slow
undulations. The sail or crest is thin, firm, and transparent, covered
by a bluish membrane; its limb is dark blue, crossed by waving yellow
lines.

An irregular microscopic network of vascular canals, containing yellow
matter, is seen in the soft substance which covers the sail; it ends in
a canal round its margin. A similar system exists both in the upper and
under-surface of the disk. All these systems are connected with one
another, and with organs pending from the inferior side of the disk,
which are hid when the Velella is in its natural horizontal position.
These organs consist of a large central sterile polypite, which supplies
the whole system with elaborated juices; it is surrounded by smaller
polypites, which are both nutritive and reproductive; and the whole is
encircled with a ring of prehensile and armed tentacles fastened to the
rim of the disk, immediately adjoining to the under-side of the limb.
The pneumatic filaments already mentioned are mixed with these different
organs.

In the Velellidæ caught by M. Vogt, he invariably found the stomachs of
the large as well as of the small polypites, full of the carapaces of
minute crustacea, shells, the bones of small fishes, and larvæ, so as
even to be swelled out with them. The indigestible parts are thrown out
at the mouth, and the elaborated juices are transferred to the various
systems of canals to be distributed through all the members of the
animal. The mouths of the small polypites take various forms; sometimes
they are wide and trumpet-shaped, with everted lips, sometimes they are
contracted. These small polypites consist of a double sac, fastened to
the disk by a hollow stem with many rounded elevations on their surface
full of thread-cells. The tentacles of the Velellidæ are strong, thick,
club-shaped tubes, completely closed at their extremities, which abound
in thread-cells; their cavity is filled with a transparent liquid,
supposed to play an important part in their elongation.

Medusiform zooids are formed on the slender stems of the small
polypites. It is presumed that they lay fertilized eggs which yield
Velellidæ, so that this animal has probably alternate states of
existence; but nothing is known of its earliest stages of development.
The youngest form yet discovered is that described by Prof. Huxley, in
his excellent monograph on ‘Oceanic Hydrozoa.’[26] The Velellidæ are
inhabitants of warm and tropical seas, but are occasionally found on the
coasts of Great Britain, being carried by the Gulf Stream to the Bay of
Biscay, and thence wafted northwards by the prevalent winds.

Although the Porpita, a genus of the Velellidæ, has no sail, it is akin
to the Velellæ in size and structure. The body of the Porpita consists
of two circular cartilaginous disks, united at their edges and
surrounded by a blue membranous limb. On the surface of the upper disk
there are beautifully radiating striæ, each of which ends at the
circumference of the disk in a little protuberance, which gives it the
appearance of a toothed wheel. A large sterile polypite occupies the
centre of the under-surface of the body, surrounded by a zone full of
smaller ones; and the space between the zone and the blue limb is
occupied by a narrow area of a reticulated appearance, to which numerous
circles of tentacles are fixed, that spread out and radiate all around
the margin of the animal. The interior circular rows are simple, short,
and fleshy, not extending much beyond the edge of the limb: the
succeeding circles are gradually longer, while the exterior row, which
extends far beyond the limb, are branched and beset with slender
filaments, ending in minute globes, sometimes filled with air, so that a
Porpita is like a floating daisy, though differently coloured. The
Porpita glandifera, a pretty little inhabitant of the Mediterranean,
which only appears in calm weather, is not more than eight lines in
diameter; somewhat convex, white, marked by radiating striæ, and
encompassed by a dark blue limb. The central polypite and those next to
it are whitish, the rest become of a darker blue towards the limb; the
tentacles are pellucid and bluish, and the three last rows have little
dark blue globes attached to them by slender filaments.

The Porpita has a horizontal air-vessel divided vertically into
air-chambers like the Velella, but they are much more numerous. In a
middle-sized Porpita, four or five lines in diameter, there are
twenty-three or twenty-four air-chambers surrounding a central one, and
eighty or ninety pneumatic filaments, so that the animal is extremely
buoyant. Brown matter, supposed to be a liver, lies directly below the
undermost wall of the air-vessel, through which, as well as through the
base of the animal, all the pneumatic filaments penetrate; the greater
number go straight down into the water, but a portion of them terminate
in the walls of the polypites.

A complete system of canals, ciliated internally, traverses all parts of
the animal; and it may be presumed that the cilia maintain its juices in
a state of circulation similar to that in the Velella; and the functions
of the polypites, great and small, that are in connection with the
liver, are also similar to those of the Velella. The Porpita is armed
with thread-cells like all the class. The central polypite is sterile
and nutritive; the small ones are both nutritive and reproductive: buds
spring from their stems, which become independent male and female
medusiform zooids, swim away from their parent and produce abundance of
eggs, whence a new generation of Porpitæ arise.

In this singular class of fresh-water and oceanic Hydrozoa, the internal
cilia, aided by the contraction of the walls of the body, are the sole
means provided by nature for the circulation of the fluids.




                              SECTION IV.

                          ANTHOZOA ZOOPHYTES.


THE life-history of the oceanic Hydrozoa, which may be regarded as one
of the triumphs of microscopic science, would have been incomplete had
it been separated from that of the Pulmograde Acalephæ and the
Physograde groups: but the most important part of that numerous race of
animals are the Anthozoa zoophytes, which include the builders of the
coral reefs and atolls of the Indian and Chinese seas. The coral
polypes, though feeble and inconspicuous individually, when united in
large communities acquire a power which enables them to build the most
stupendous structures in the midst of a tempestuous ocean.

The Anthozoa zoophytes, or living flowers, form two extensive groups—the
Asteroids, or Alcyonian zoophytes, whose polypes have six or eight
hollow prehensile tentacles radiating round their mouth like a star or
the petals of a single blossom, and the Actinian or Helianthoid
zoophytes, which have ten, twelve, or more hollow tentacles encircling
their mouth in several rows, like the blossom of a double sun-flower.


                          _Alcyon Zoophytes._

The Alcyon zoophytes comprise the Alcyons, Gorgons, and Pennatulidæ, or
sea-pens. The polypes are of the same form in all, and are united by a
fleshy or horny substance into large communities, so connected and
mutually dependent as to constitute one compound animal. Figure 123
represents a highly magnified group of Alcyonian polypes in different
stages of expansion. The body of the polype is soft, contractile, and
composed of thin, delicate transparent tissues. It has the form of a
cone resting upon its base, which is generally of a firm material. Its
upper extremity presents a central orifice, which serves both for a
mouth and vent, and is encompassed by six or eight broad, short, hollow
tentacles, enlarged towards their base so as to meet, and their edges
are seen with a lens only to be fringed with minute hollow tubes or
pinnæ closed at their free end.

[Illustration: Fig. 123. Alcyonian polypes highly magnified.]

[Illustration: Fig. 124. Polype of Alcyonidium elegans.]

The narrow slit of the mouth opens into the stomach, which is a flat,
short sac hanging down in the central cavity of the polype’s body, with
an orifice at its lower end. The stomach is fixed to the internal walls
of the body by eight vertical folds forming so many longitudinal
chambers open at their lower extremity. The whole of the surface of the
interior, the walls, the stomach, and the septa or divisions, are
covered with fine cilia, by whose vibrations constant currents are
maintained in the water which bathes every part of the cavity freely
entering at the mouth. The polypes are carnivorous, living upon
infusoria and minute particles of animal matter floating on the water,
which they seize with their mouth, or arrest with their flexible and
contractile tentacles. The food is digested by the solvent juices in the
stomach, and the refuse is ejected at the mouth.

The eggs of these polypes are formed and fertilized among the vertical
folds adjacent to the stomach. When hatched, the larvæ pass through the
stomach and come out at the mouth as active ciliated creatures, so like
eggs that the Alcyon zoophytes were believed to be oviparous. However,
in some of the genera they are discharged through pores between the
bases of the tentacles.

[Illustration: Fig. 125. Spicula of Alcyonium digitatum.]

The Alcyon polypes have multitudes of needle-like spicules, rough with
projecting knots. They are collected into triangular groups at the foot
of each tentacle; the central and largest point runs up into the
tentacle. Towards the lower end of the polype, spicules again occur
scattered through the skin and crowded into groups, as in fig. 125.
These, however, form short thick cylinders, each end being dilated into
a star of five or six short branches. The spicules always contain an
organic base hardened by carbonate of lime, for when Dr. Carpenter
dissolved the lime with dilute acid, a gelatinous substance remained,
which had the form of the spicules. Fig. 125 shows those of the
Alcyonium digitatum, or Dead Man’s Fingers, generally assumed as the
type of this numerous order, which contains sixteen genera and many
species, differing much in form but connected by a similarity of
digitate structure.

The Alcyonium digitatum, when torn from the rock to which these animals
are attached, shrinks into a cream-coloured fleshy mass of somewhat
solid texture, rough and hard to the touch, and studded all over with
hollow depressions or pits. When put into sea-water, these lumps, from
the size of a pea and upwards, expand, become semi-transparent, and from
each depression a polype protrudes its beautifully symmetrical
eight-petalled blossom. Their tentacles are short, broad, and
prehensile; and the slender pinnæ, which fringe their edges arching
outwards, are seen with a high magnifying power to be rough with prickly
rings, discovered by Mr. Gosse to be accumulations of thread-cells with
their darts.

These Alcyons, when expanded, are about an inch-and-a-half high and
two-thirds of an inch thick, but individuals are met with two or three
times as large, and much divided into blunt finger-like lobes. The
sarcode mass of these compound animals is channelled like a sponge, by
branching canals, the orifices of which open into the stomachs of the
polypes; and, by bringing them into communication with each other, unite
the whole into one compound animal, which is maintained by the food
caught and digested by each individual polype. Currents of sea-water
mixed with the nutritious juices are made to circulate through the
branching canals by the vibrations of cilia with which they are lined;
they flow round the stomachs of the polypes, supply their juices with
oxygen, and carry off the carbonic acid gas and refuse of the food. In
this case, as in many others, the cilia may be regarded as respiratory
organs.

The unarmed Alcyons are generally thick, short, and rough; some form a
crust on rocks from whence lobes rise. With the exception of the Xenia,
a tropical species, the polypes of the unarmed Alcyons can retreat
within their polypary, so as to be entirely or partially out of sight.

The polypary, or mass, of the armed Alcyons is either membranous or
leathery, and is entirely bristled with large spicules similar to the
very small ones in the tentacles of its polypes. It forms branching
masses terminated by prominent tubercules thickly beset by spicules. The
polypes retreat into the mass when they are in a state of contraction.

The Gorgons, which form the second family of Alcyonian zoophytes, are
compound animals, consisting of a solid stem or axis either simple or
branched, adhering by its base to a rock or some submarine body, and
coated by a layer of a softer fleshy or horny substance exactly in the
same manner as the bark covers the stem and branches of a tree. This
bark or fleshy substance is filled with polypes similar to those
described; however, they are shorter, their base is a little enlarged,
and is turned towards the axis of the stem and branches of the Gorgon.
The softer substance or bark is much developed between the polypes, and
is full of spicules, of forms varying with the genera. A system of
almost capillary canals traverses the soft coating and opens into the
lower part of the cavity containing the viscera of the different
individuals, thus affording a passage for the circulation of the
nutritive juices.

The larvæ of the Gorgons are like ciliated eggs; they swim with their
thick end foremost, and are perfectly soft. That state, however, is
transitory; for no sooner do they lose their cilia and settle on a
submarine substance than their lower part becomes hard, forms a solid
layer on the substance, and constitutes the base for a Gorgon’s stem. A
small elevation rises on it, and at the same time the upper part of the
larva assumes a fleshy consistence and surrounds the elevation. These
two grow simultaneously; the small elevation rises higher and higher,
and its coat containing the polypes grows proportionally with it, and
continues to cover it whatever form it may take, whether a branching or
plumage stem, or a simple slender rod. The stem and branches are
increased in thickness by successive concentric layers of horny or
calcareous matter between their surface and the soft bark.

The Gorgoniidæ are divided into three natural groups, the Gorgons,
Isidæ, and Corallines, according to the nature of their axis. The two
first agree in having stems either of a substance like cork or horn
entirely or partly flexible; but the stem of the Gorgons has no joints,
while that of the Isidæ is jointed. The stem of the Corallines has no
joints, and is entirely stony and branching.

The Gorgonia verrucosa, so common in the Mediterranean, British Channel,
and the intermediate seas, is like a small shrub a foot high, with
numerous branches: the cup-shaped tubercules inhabited by the polypes
are irregularly distributed, and not very salient, yet enough to give
the white encrusting coat a rough warty surface. In this Gorgon there is
an ovary at the base of each polype: the eggs are discharged through
eight small pores placed between the bases of the eight tentacles. These
animals are wonderfully prolific: a Gorgon, six inches high, produced
ninety eggs in one hour.

The Gorgonia graminea, found on the coast of Algiers, instead of being
arborescent, is thin and cylindrical throughout its whole length. The
covering is white, and nearly smooth; the cups containing the polypes
either have no salient border, or are deeply sunk in the coat.

The Gorgons known as sea-fans live in warm seas, and are of numerous
species. Not only all their branches, but all their branchlets and
twigs, spread in the flat form of a fan, are soldered together so as to
form a net with open meshes; the coating is thin, and the polypes are
placed bilaterally.

The stems and branches of the Isidæ, which form the third group of
Gorgoniidæ, are composed of a series of calcareous cylinders, separated
by either horny or cork-like nodes; the polypes are only born in the
bark of the former. In the genus Isis the calcareous cylinders are
deeply striated by straight or wavy lines. This race of animals are
mostly inhabitants of warm seas; but they once lived in a colder
climate. Some species of them are preserved in a fossil state in the
cretaceous earth in Belgium, and in the plastic clay near London.

There is but one genus of the Coralline Gorgon, and the type of that is
the common red ornamental coral of commerce found in the Mediterranean
Sea only. Dr. Carpenter has discovered that the solid calcareous stem of
the Corallium rubrum is made up of aggregations of spicules closely
resembling those of the other Alcyonian zoophytes, but of an intense
red, sometimes rose colour or whitish. The stem and branches are
delicately striated along their length, and covered with a soft
substance of the same colour as the stem, into which the polypes retreat
when alarmed; but when fishing for food, with their eight white
tentacles expanded, the red stem and branches appear as if they were
studded with stars. Prof. de Lacaze Duthiers, who was appointed by the
French Government to investigate the natural history of the red coral
with a view to the regulation of the fishery at Algiers, found that the
individual polypes are either male or female, but that the males and
females are on different branches of the same coral, one branch being
almost exclusively the abode of male polypes, and another of female. The
eggs are fertilized by the intervention of the water. After an egg is
fertilized, it is transferred to the stomach of the female, which thus
serves both for digestion, incubation, and transformations of the egg.
At first the egg is naked and spherical; afterwards it becomes elongated
and covered with cilia. A cavity is formed in it, which opens
externally, and finally becomes the mouth; it then acquires the form of
a little white worm, and when it comes into the water it is very active,
swimming in all directions, avoiding its comrades when they meet, rising
and descending in the water with its hinder end foremost. It loses its
cilia after a time, fixes itself to a rock, and acquires the form of its
parent in the manner described as to other Gorgons.

[Illustration: Fig. 126. Red Coral Branch.]

The red coral generally grows on the under-side of ledges or rocks, in a
pendent position, and at considerable depths. It is not found at 15 or
20 fathoms; they only begin to fish for it at from 30 to 60 fathoms; and
it is brought up from even 100 or 120, while the strong reef-building
corals cannot exist below 25, or at most 30 fathoms; being immensely
superior in vigour, these require a greater supply of air, light, and
heat. The red coral is generally fished for along the coasts of Algiers
and Tunis; it is also found in the seas round Sicily and Sardinia, and
in the Grecian Archipelago. The red coral is always irregularly
branched. The branches are sometimes white, supposed to be from disease;
the white coral of commerce is a species of Caryophyllia, an Actinian,
and not an Alcyon, zoophyte.

[Illustration: Fig. 127. Red Coral (greatly magnified), from ‘Histoire
Naturelle du Corail,’ par M. Lacaze Duthiers.]

The Corallium Johnstoni, a native of the Atlantic, has a white axis,
with branches spreading flatly and horizontally like a fan from the rock
to which it is attached; it is entirely covered with a yellowish flesh,
but the polypes only inhabit the upper surface, as if they could not
live in shade. The Corallium secundum, a similar zoophyte, was
discovered by Professor Dana near the Sandwich Islands, with a white or
rose-coloured fan-shaped stem and branches, covered by a scarlet coat,
having the polypes also only on the upper surface.

The Pennatulidæ, or sea-pens, which are the third family of the Alcyon
zoophytes, bear a great resemblance to a goose’s feather. The genus
Pennatula has a flatly-feathered, upright, calcareous axis, the bare
part of which is analogous to the quill; but, instead of being fixed
like the stem of a Gorgon, it is merely stuck into sand or mud at the
bottom of the seas, while the upper feathered part, containing the
polypes, remains in the water. The axis decreases in thickness upwards,
and the pinnules, which diverge from it transversely like wings, are
angular, thin, membranaceous, and strengthened by spicules. The whole
animal is covered with a soft fleshy tissue; the polypes, which have
eight pinnated tentacles, are arranged in a single row along the edges
of the pinnules, with their visceral extremities prolonged into the soft
tissue, so as to give it a tubular structure, through which the
nourishing juice prepared by the polypes is carried for the maintenance
of the general envelope, the refuse being thrown out at their mouths.
When the sea-pens leave the mud or sand, they do not swim actively with
their pinnules, but move languidly at the bottom. The Pennatulæ are
phosphorescent; they are of a dull reddish brown during the day, but at
night they shine with the most brilliant iridescence. In the tropical
seas they occasionally exceed a foot in length; in the cool latitudes
they are not more than five or six inches. The Pennatula phosphorea,
found on the British coasts, has a hollow axis, occupied by a
well-developed stylet; long pinnulæ symmetrically disposed on each side
of the middle and upper part of the axis; the polypes, which are very
contractile, are arranged transversely on their upper and anterior
edges; the pinnæ of the wings are scythe-shaped, and furnished with a
vast number of sharp spicules, and these combine in bundles at the base
of the cells, in which the polypes live. The back of the pen, lying
between the feathery wings, is sometimes smooth, sometimes crowded with
scales, arising from the development of the spicules with which it is
filled. The eggs of this animal are yellow, and have the size and form
of poppy seeds. They are developed into ciliated larvæ within the
polypes, which come out at their mouths, and swim away; but their
activity is much diminished when they have acquired their mature form.
These Pennatulæ increase also by a kind of budding. There are species of
phosphorescent sea-pens in all the European seas and Indian Ocean.

The Virgulariæ are sea-pens which have long slender stems, with short
transverse pinnules, on both sides of their extremity: they have no
spicules, and are remarkable for the contractile power both of their
axis and polypes. Mr. Darwin mentions a species he met with during his
voyage in the Southern Ocean, which seems to be akin to the Virgularia
juncea common in the Indian Seas. They were long and slender, projecting
in vast numbers like stubble above the surface of muddy sand. When
touched or pulled, they suddenly shrunk down with such force as to
disappear partly or altogether. Sensitive as these animals are, they
have no nerves; hence their motions must be owing to the irritable
nature of muscular fibre. The eggs of the Virgularia mirabilis, native
on the Scotch and Norwegian coasts, are formed in the fleshy coat at the
base of each polype. As soon as they acquire their yellow colour and
ciliated surface they enter into its body, and revolve in it for a
little time before they come out at its mouth.

The family of the Tubipora, inhabitants of warm seas, are the most
beautiful of the Alcyons. They consist of rounded masses of considerable
size, formed of fragile, hollow, and nearly parallel calcareous tubes.
The tubes do not touch one another, but they are united at intervals by
horizontal plates, formed of an extension of their bases, dividing their
mass into stages. In the Tubipora musica, a native of the Indian Ocean,
there are several superincumbent series of equal and parallel tubes,
exactly like the pipes of an organ. The whole compound fragile mass is
of the richest crimson, and the polypes spread their white tentacles
like stars over the mouths of the uppermost pipes, or retreat into them.
Buds spring from the upper part of the tubes, and the result is the
death of the parents, which are succeeded by a young living race a stage
above them. The Tubipora purpurea lives in the Mediterranean and Red
Seas. The polypes of a species found by Professor Dana, at the Feejee
Islands, have their centre and mouth of a brownish red, and their
tentacles yellow, edged by a double fringe of violet-coloured pinnules.

[Illustration: Fig. 128. Tubipora musica.]


                         _Actinian Zoophytes._

The great family of the Actinian zoophytes abounds in genera and
species. The common Sea Anemone, or Actinia, of which there are more
than seventy species on the British coasts, is the model of the minute
polypes which inhabit the stony corals, and build the coral reefs and
atolls of the tropical Pacific.

The Sea Anemone has a cylindrical body, attached at one end by a sucker
to rocks or stones at no great depth, and a flat circular disk at the
other, with the mouth in its centre: the mouth, which is surrounded by a
series of tubular, smooth-edged, radiating tentacles, resembles a
blossom. The soft smooth body consists of two layers, as may be seen in
the sections of an Actinia (fig. 129). The outer layer generally
contains red matter, the inner one is of muscular fibre, and contains a
great cavity, in which a somewhat globular bag or stomach is suspended.
The space between the stomach and the cylindrical body of the animal is
divided into chambers by perpendicular radiating partitions, consisting
of thin plates or lamellæ. The mouth, which opens at once into the
stomach, imbibes sea-water; and the hollow tentacles surrounding it
being perforated at their extremities, and in communication with the
chambers immediately below them, also imbibe the sea-water and convey it
into the chambers; and the vibrations of the innumerable cilia, with
which all the cavities of the animal are lined, keep them perpetually
bathed with the respiratory medium mixed with nutrient juices from the
coats of the stomach.[27]

[Illustration: Fig. 129. Actinian polype.]

The Sea Anemone is monœcious and oviparous; the eggs are formed and
fertilized in the lower parts of the perpendicular lamellæ or radiant
plates; but they are hatched within the visceral cavity, and the larvæ
issue from the mouth. The Actiniæ are also propagated by buds. They have
as great a power of repairing injuries as the Hydræ, and like them too,
though generally fixed, they can creep about by means of their expanded
suctorial disk, and even float on the surface of the water. In many
species the tentacles, as well as the body, are brightly coloured. The
Actinia sulcata, an inhabitant of the British Channel, is of a deep
crimson, with from 100 to 200 grass-green tentacles. The tints are owing
to coloured particles in minute globules, that lie under the transparent
skin of the animal and its tentacles.

With the exception of some of the Acalephæ, the thread-cells of the Sea
Anemone are more highly developed than in any other animals. They not
only differ in the various Actinian zoophytes, but sometimes even in the
same individual. The complicated structure and action of this warlike
apparatus was unsuspected previous to the microscopic observations of
Mr. Gosse on the Actiniæ in general, and especially on the little
scarlet fringed Sagartia miniata, a native of the British coasts. Like
all the Anemones, it is highly sensitive; on the slightest touch it
draws in its scarlet blossom, and shrinks into the form of a
hemispherical bulb. While in the act of contracting, white filaments
like ribbons shoot out from various parts of its surface, and new ones
appear on every fresh effort, streaming out to the length of several
inches, irregularly twisted and tangled. As soon as the contraction is
finished, these fine white filaments begin to be recalled, and gradually
retire in small irregular coils into the interior chambers between the
stomach and the wall of the body, where they are stored up when not in
activity.

Each filament makes its egress and ingress through an almost
imperceptible transverse slit, discovered by Mr. Gosse, in the middle of
an oval depression in the wall of the animal’s body. The slits, which
are called cinclides, are very numerous, and resemble a pair of inverted
eyelids, which can be opened and shut at pleasure. When the animal is
irritated it contracts, and the water which fills the perpendicular
chambers is forced in a stream through the slits, and carries with it
the white filaments lodged within them; and then these quivers, which
are full of deadly weapons, are ready for action.

Under the microscope, the white filaments are like narrow flat ribbons
with their edges curled in, and thickly covered with cilia. They have
not the slightest trace of muscular fibre, even when viewed with a
microscopic power of 800 diameters; yet they extend, contract, bend, and
coil in every direction; they bring together the margins of the ribbon
so as to form a tube, and open them again; and the filaments perform all
these motions even when severed from the animal, no doubt by the
contractile nature of the clear jelly or sarcode, of which their bases
are composed, as in the tentacles of the Acalephæ.

Innumerable oblong dart or stinging-nettle cells, closely packed
together, lie under the folded edges of the ribbons, throughout their
whole length, especially at their tips.[28]

The polypes of the stony corals, though extremely small, are essentially
the same in structure as the Sea Anemone, but they have no sucker at
their base. The Sea Anemone is of soft tissue throughout its whole body.
In the polypes of the madrepore corals, on the contrary, the whole of
the perpendicular lamellæ which divide the interior of the body into
chambers become hard, from being consolidated by particles of carbonate
of lime; and their upper edges, which appear as rays round the mouth of
the animal, give that starry appearance to the surface of dead
madrepores after the soft part of the polypes has been destroyed.

Most of the coral polypes are unarmed; but in some, as, for example, the
Caryophyllia Smithii, there are multitudes of dart-cells in the
tentacles, besides numerous pellucid filaments or ribbons, full of
thread-cells, lying in coils within the chambers which surround the
stomach.

We are indebted to Mrs. Thynne’s interesting observations on the
Caryophyllia Smithii in her aquarium for the life-history of the animals
armed with this formidable artillery. This madrepore, which inhabits
many parts of the European seas, at various depths, is a species of the
only lamelliform genus of corals which range beyond the tropics. It is a
solitary individual polype, with an external calcareous cylindrical
coat, wider at the base, when it is fixed to a rock; and the mouth,
which has several rows of tentacles, is in the centre of the disk of the
cylinder. The tentacles are delicate, transparent, granular tubes, about
an inch long, tapering to their extremities, and ending in an opaque
white knob full of chambered thread-cells with their darts; but the
thread-cells are of a larger size in the ribbons coiled in the chambers
round the stomach of the animal. These madrepores are described by Mrs.
Thynne as of various tints, from a pure white to a bright apricot
colour. At intervals they eject from the mouth a whitish blue fluid,
resembling wood smoke, in a stream three or four inches long, sometimes
containing a few eggs. But the eggs, though no doubt formed at the base
of the lamellæ, become densely packed like fine dust in the hollows of
the tentacles, from whence they are expelled by contractions, and escape
by the mouth. The eggs lie quiet for a few days in the place where they
are deposited: by and by they begin to rotate, slowly at first, then
more rapidly, and finally they are developed into most minute
madrepores, with the star and colour of the parent. In a few months they
become as large as a crown piece, with a very wide mouth and a
membranous integument or covering, for they do not get their hard
calcareous coat till they are two years old. While in that soft state
they propagate by spontaneous division, which always begins at the
mouth, and is repeated every few weeks during the second year of their
lives. When they split into segments, the broken ends of each segment
bend round and unite; and the mouth, which at first is on one side,
being a portion of the old one, comes to the centre of the disc, and in
addition to the few old tentacles that remain, new ones are added, with
their interior chambers, till they amount to five rows, and in this
manner a brood of young Caryophylliæ is formed.

[Illustration: Fig. 130. Lobophylla angulosa.]

During the second year of their soft state, these madrepores increase by
budding. The buds spring from the base of the membranous covering, they
expand, get a mouth and tentacles, aid in feeding themselves by greedily
taking any small particles of animal food offered to them, and seem also
to share in the sustenance provided by the mother, as they dilate when
she is fed; ultimately they separate from her. These madrepores have
patches of a milk-white fluid substance, which unite and almost cover
the space between the mouths and the rows of tentacles: in others of the
madrepore tribe these patches are purple, green, yellow, or ultramarine
blue. The Caryophylliæ have locomotion while their skin is soft, but no
activity; they merely avoid obstacles, and move away from one another;
but, as soon as they get their hard calcareous coat, they become
permanently fixed, and no longer undergo division or gemmation, but lay
eggs.[29]

The European Caryophylliæ never have more than one star, but sometimes a
great many individuals are united in a spreading bunch, as in the
madrepore Lobophylla angulosa (fig. 130), or in a branched or tufted
mass. Their exterior is invariably striated, and each terminates in a
star, with the polypes, mouth and tentacles in its centre. These
compound madrepores are inhabitants of warm seas.

The number of tentacles possessed by the Actinian polype varies with the
species of the coral. When full grown they have twelve, twenty-four,
even forty-eight, or more. When young, they have only four or six, but
in general the number increases rapidly as they advance in age. The
perpendicular hard lamellæ, which divide the cavity round the stomach of
the polype into perpendicular chambers, as in fig. 129, and form stars
round the mouth, consist of thin sheets or plates, either applied or
soldered together; and for every new tentacle that is produced at the
mouth, a corresponding new chamber is formed immediately below it,
between the sheets or leaves of the lamellæ; so that the number of
chambers and perpendicular plates is always equal to the number of
tentacles, and so the circulation of the fluids is maintained. Since the
upper edges of the lamellæ form the rays of the stars round the mouth of
the polype, it is clear that the number of rays in a star must always be
equal to the number of lamellæ. The new tentacles are always produced
exterior to and between the adjacent old ones, so as to form an outer
circle, and consequently a new circle of rays will be added to the star
round the mouth exterior to the old ones. There may be two, three, or
more concentric circles of tentacles round the mouth of the polype, the
last being the shortest. However, some polypes never have more than
twelve tentacles during the whole course of their lives. The first
formed rays of a star are generally, though not always, the longest and
most prominent; and sometimes the edges of the lamellæ rise high above
the hollow or cup which is the centre of the star, and contains the
mouth of the polype. In some families of corals these edges, which form
the rays, are toothed or spined.[30]

A horny column in the axis of the polype, hardened by sulphate and
carbonate of lime, and called the columella, generally shows its top in
the centre of the star, and varies in structure in the different genera.
Thus the Actinian polypes may be said to possess an internal skeleton,
and as they approach maturity they also acquire an external one in the
form of a cylindrical coat, or stony wall, which surrounds them, and
into which most of them can withdraw the soft upper part of their bodies
and tentacles, so as to be partly or altogether concealed. The
perpendicular lamellæ are sometimes extended through the stony walls of
the polype, so as to form a series of broad, well-developed ribs on its
exterior surface.

The stony substance of corals is chiefly carbonate of lime, which the
polypes have the power of abstracting from the sea-water, combined with
a small quantity of animal matter, and a still smaller quantity of
phosphate of lime, with a trace of silver and magnesia. This stony
substance takes the crystalline form of needles. By the successive
deposition of these needles, a network is formed round the body of the
animal, which by a series of these deposits is condensed into a hard
impervious coat or wall. During this formative process many
characteristic forms may be produced by division and building, depending
upon the genus and species of the polype; but they do not lay eggs till
they come to maturity.

Some corals increase both by budding and division, but by far the
greater number grow in size by budding, as the Astræa, which constitutes
a portion of the reef-building corals of the tropical seas. They form
groups, in which the whole of the polypes, except their starry summits,
are soldered or pasted together by a living viscous substance,
consolidated by carbonate of lime, abstracted from the sea-water, so
that the resulting coral frequently becomes a rounded mass, the surface
of which is more or less covered with stars, which may be circular or
angular, large or small, deeply set or prominent, according to the
genera or species, both of which are exceedingly numerous. In fact the
forms produced vary according as the buds spring from the base of the
polypes, from the sides of the cylindrical body, from the summit or
disk, from the limits of these three parts, or from the whole animal. In
all these varieties the buds are the result of a superabundance of vital
activity in the part. When the buds proceed from the sides of the
polypes the corals are rounded masses; but when they spring from the
disk or cups of the star, the consequence is the death of the parent
polypes, and the development of a new layer of living individuals above
the dead ones. No part of the new polypes is seen except their stars,
their bodies being enclosed in the common tissue. As this process may be
continued indefinitely, the coral may increase to any size; but the size
becomes still greater when successive buds are formed over every part of
the polypes, and when all the successive generations are soldered
together by the common tissue. In every case the polypes are alive only
on the surface where they have free access to light, heat, and air,
which is furnished by the sea-water in which they live.[31]

In the reef-building corals the living viscous substance that covers the
surface and connects the polypes into a mass, is in process of time so
completely consolidated by abstracting the small quantity of carbonate
of lime that the sea-water contains, that little if any animal matter
remains; and as this process is continually repeated, one generation of
polypes perishes after another, the inert matter increases indefinitely,
and the surface at which the consolidation is actually going on is the
only part that is alive.

The surfaces of the dense convex masses of many of these Astræan corals
are entirely covered with deep hexagonal stars, whose rays extend
upwards all round, and end in narrow, sharp, and elevated lines formed
by the junction of the rays of the adjacent stars; in other species the
rays are often crowded together, and the columella only shows a few
points in the deep hollows. Through these deep cups the polypes protrude
their circular disks and tentacles in quest of food, the nutritious
products of which maintain the polypes as well as the general living
fabric which unites them, and the refuse is ejected from their mouths;
for each polype has an independent life of its own besides the
incidental life that it possesses as part of a compound being. In many
of the corals the polypes show great sensibility, shrinking into their
cells on the slightest touch, yet no nervous system has been discovered.

The variety of compact and branching corals far exceeds description: 120
species are inhabitants of the Red Sea alone, and an enormous area of
the tropical Pacific is everywhere crowded with the stupendous works of
these minute agents, destined to change the present geological features
of the globe, as their predecessors have done in the remote ages of its
existence.

Four distinctly different formations are due to the coral-building
polypes in the Pacific and Indian Oceans, namely, lagoon islands or
atolls, encircling reefs, barrier reefs, and coral fringes, all nearly
confined to the torrid zone.

An atoll is a ring or chaplet of coral, enclosing a lagoon or portion of
the ocean in its centre. The average breadth of that part of the ring
which rises above the surface of the sea is about a quarter of a mile,
often less, and it is seldom more than from six to ten or twelve feet
above the waves: hence the lagoon islands are not visible even at a very
small distance, unless when they are covered by the cocoa-nut palm or
the pandanus, which is frequently the case. On the outside, the ring or
circlet shelves down for a distance of one or two hundred yards from its
edge, so that the sea gradually deepens to about twenty-five fathoms,
beyond which the sides of the ring plunge at once into the unfathomable
depths of the ocean with a more rapid descent than the cone of any
volcano. Even at the small distance of some hundred yards no bottom has
been reached with a sounding line a mile and a half long. All the coral
on the exterior of the ring, to a moderate depth below the surface of
the water, is alive; all above it is dead, being the detritus of the
living part washed up by the surf, which is so heavy on the windward
side of the tropical islands of the Pacific and Indian Oceans, that it
is often heard miles off, and is frequently the first warning to seamen
of their approach to an atoll.

On the inside, these coral rings shelve down into the clear calm water
of the lagoon by a succession of ledges of living corals, but of much
more varied and delicate kinds than those on the exterior wall and
foundation of the atoll. The corals known as Porites are the chief
agents in building the exterior face of the ring: they form great
rounded irregular masses, like the Astræa, but much larger, being many
feet in thickness; and as the polypes are only alive on the surface,
numberless generations must have lived and died before they could have
arrived at that size. The rays of the stars are toothed at the edges, so
that they present rows of little points; in some species the rays are
almost invisibly slender, the interstitial matter is full of pores, and
the polypes have twelve tentacles.

The Millepora complanata or palmipora is very commonly associated with
the Porites; it is the largest coral known. It grows in thick vertical
plates, intersecting each other at various angles, and forms an
exceedingly strong honey-combed mass, generally affecting a circular
form, the marginal plates alone being alive. Instead of stars, the
polypes live in simple pores: myriads of these small cylindrical pores
penetrate the surface of the plates perpendicular to their axes;
sometimes they are so minute as to be scarcely visible.

Between the plates, and in the protected crevices of the outer circle of
the ring, a multitude of branching zoophytes and other productions
flourish; but the Porites, Astræans, and Milleporæ seem alone able to
resist the fury of the breakers, essential to the very existence of
these hardy corals, which only obtain their full development when washed
by a heavy sea. The outer margins of the Maldive atolls, consisting
chiefly of Milleporæ and Porites, are beat by a surf so tremendous that
even ships have been thrown by a single heave of the sea high and dry on
the reef. The waves give innate vigour to the polypes by bringing an
ever-renewed supply of food to nourish them, and oxygen to aërate their
juices: besides, uncommon energy is given and maintained by the heat of
a tropical sun, which gives them power to abstract enormous quantities
of solid matter from the water to build their stony homes, a power that
is efficient in proportion to the energy of the breakers which furnish
the supply.

The Porites and Milleporæ, which are the chief reef-building corals,
cannot live at greater depths than fifteen or twenty-five fathoms: not
for want of heat, for the temperature of the ocean in these latitudes
does not sink to 68° Fahr. till a depth of 100 fathoms, but light and
abundance of uncombined air are essential, and these decrease as the
depth increases. The polypes perish if exposed directly to the sun even
for a short time, so they build horizontally between these limits. The
actinian polypes in the corals, which live at different depths in the
crevices of the atolls, have the same general structure; their disks and
tentacles are sometimes tinted with brilliant colours; some sting,
others have a considerable diversity of individual character.

On the margin of the atolls, close within the line where the coral is
washed by the tide, three species of Nullipores flourish; they are
beautiful little plants, very common in the coral islands. One species
grows in thin spreading sheets, like a lichen; the second in stony knobs
as thick as a man’s finger, radiating from a common centre; and the
third species, which has the colour of peach blossom, is a reticulated
mass of stiff branches about the thickness of a crow’s quill. The three
species either grow mixed or separately, and, although they can exist
above the line of the corals, they require to be bathed the greater part
of each tide: hence a layer two or three feet thick, and about twenty
yards broad, formed by the growth of the Nullipores, fringes the circlet
of the atolls and protects the coral below.

The lagoon in the centre of these islands is supplied with water from
the exterior by openings in the lee side of the ring, but as the water
has been deprived of the greater part of its nutritious particles and
inorganic matter by the corals on the outside, the hardier kinds are no
longer produced, and species of more delicate forms take their place.
The depth of the lagoon varies in different atolls from fifty to twenty
fathoms or less, the bottom being partly detritus, partly live coral. In
these calm and limpid waters the corals are of the most varied and
delicate structure, of the most charming and dazzling hues. When the
shades of evening come on, the lagoon shines like the milky way with
myriads of brilliant sparks. The microscopic medusæ and crustaceans
invisible by day form the beauty of the night, and the sea-feather,
vermilion in daylight, now waves with green phosphorescent light. This
gorgeous character of the sea bed is not peculiar to the lagoons of the
atolls; it prevails in shallow water throughout the whole coral-bearing
regions of the Pacific and Indian Oceans.

Encircling reefs differ in no respect from the atoll ring, except in
having islands in their lagoons, surrounded also by coral reefs. Barrier
reefs are of the same structure as the atoll rings, from which they only
differ in their position with regard to the land. They form extensive
lines along the coasts, from which they are separated by a channel of
the sea of variable depth and breadth, sometimes large enough for ships
to pass. A very long one runs parallel to the west coast of New
Caledonia, and stretches for 120 miles beyond the extremities of the
island. But a barrier reef off the northeastern coast of the Australian
continent is the grandest coral formation existing. Rising at once from
an unfathomable depth of the ocean, it extends for a thousand miles
along the coast with a breadth varying from 200 yards to a mile, and at
an average distance of from 20 to 60 or 70 miles from the coast, the
depth of the channel being from 10 to 60 fathoms. The pulse of the
ocean, transcendently sublime, beats perpetually in peals of thunder
along that stupendous reef, the fabric of almost microscopic beings.




                               SECTION V.

                          ANNULOSA, OR WORMS.


THE Annulosa, which are the lowest grade of articulated animals, consist
of four distinct orders: the Entozoa, which are muscle and intestine
parasites; the Turbellariæ, fresh and salt-water animals covered with
cilia; the Annelida, or Worms; and the Rotifers, or Wheel animalcules.


                               _Entozoa._

There are three genera and numerous species of Entozoa. Every animal has
one or more species peculiar to itself; fourteen infest the human race.
They have a soft, absorbent body of a white or whitish colour, in
consequence of being excluded from light, and living as they do by
absorbing the vitalized juices of the animals they infest. Their
nutritive system is in the lowest state of development; yet there are
some of a higher grade. All are remarkable for their vast
productiveness.

The Tænioïdæ, which belong to the inferior group, are intestinal,
many-jointed worms, which have neither mouth nor digestive organs; and
what is called the head has only hooks and suckers to fasten it to the
internal membrane of the animal at whose expense it lives. The common
Tænia, or Tape-worm, sometimes ten feet long, which is the type of the
order, has four suckers and a circle of hooklets round a terminal
proboscis to attach it to its victim. Though destitute of the organs of
nutrition it is extremely prolific, for each segment of its long flat
body is a reproductive monœcious zooid, which forms and lays its own
eggs exactly as if it were a single independent animal, thus furnishing
a very remarkable instance of the law of irrelative repetition, which is
a series of organs performing the same functions independently of one
another. Two pairs of canals containing a clear colourless liquid extend
throughout the body of the worm.

Bags, or vesicles called cysts, had been found in the glands and muscles
of various animals, afterwards discovered to contain young worms, which
attain their perfect development within such creatures as eat the flesh
containing the cysts. Under circumstances so unprecedented, it required
no small skill and patience to determine the life-history of these
singular creatures. The cysts differ in size and form according to the
genera, and are embedded singly or in groups in the flesh of their
victim, on whose ready prepared juices they live.

The greater number of the Tænia genus begin their lives as sexless
cysted larvæ, and on entering their final abode, segments are
successively added till the worm has arrived at its adult state. The
tape-worm of the cat has its origin in the encysted larvæ found in the
livers of the mouse and rat. One species of Entozoa, while in its
primary state, inhabits the stomach of the stickleback, and only comes
to perfection within the aquatic birds that feed on this fish. Another
species infests the livers of the salmon tribe, and gets its perfect
form in the pike and perch.

Sheep and the hog are more tormented with cysted worms than any other
domestic animals used for food. If introduced into the human intestines
by eating raw ham or sausages, the larvæ soon acquire the perfect form.
The eggs of the Tænia may be introduced into the human or animal
stomach; for dogs and other carnivora which eat raw unwholesome meat are
infested by full grown tænia, which fix themselves to their entrails by
their hooks and suckers, while at the same time egg-bearing segments
separate successively from their posterior extremity, and being voided
scatter the eggs far and wide on land and in water.

The young of some Entozoa undergo various transformations, as those of
the Distoma of the Lymnæa. When full grown that entozoön is like a sole,
flat, broad, and long, with a kind of head at the broad end, and two
suckers on its under-surface, in one of which there is a pore serving as
a mouth, whence an alimentary canal extends, which spreads in branches
almost throughout the whole body. This animal has a filamentary nerve
round its gullet, from which minute fibres pass to the mouth, and two
filaments extend backward on each side as far as the second sucker. The
eggs which occupy the whole margin of the body are developed into worms,
each of which seems to be merely a mass of structureless cells enclosed
in a contractile case. By a second change each of these cells is
transformed into a freely swimming ciliated zooid endowed with eyes.
Having escaped from their contractile case, they remain for a time in
that state, and then imbed themselves in the mucus on the foot of the
fresh-water mollusk Lymnæa, or pond snail, where they are transformed
into true Distomata, and ultimately enter into the body of the Lymnæa
itself, where they lose their eyes and cilia, which are no longer of use
in their dark and permanent abode. The Fluke found in the livers of
sheep that have the rot is a Distoma.

The Nematoid order, or thread-worms, that live in the muscles of men and
animals, are long, smooth, and cylindrical, with a structureless skin
covering layers of longitudinal and circular fibres, by means of which
they can stretch and contract themselves. They are generally pointed at
both ends with a mouth at one extremity and an orifice at the other. The
Filariæ are slender, sometimes of great length, as the Guinea worm,
which varies in length from six inches to two, eight, or even twelve
feet. In Persia they are believed to be introduced into the system by
drinking water in which their eggs have been deposited. This worm may
grow in the muscles of a man to the size of five or six feet without
giving much annoyance, but when its head bores through the skin it
produces a painful sore unless extracted. In Persia, where the worm is
common, the natives seize it by the head, draw it carefully out, and
wind it round a bit of wood, an operation which may require several days
to accomplish. It has a numerous viviparous progeny, which come out
through the mouth. There are certain very small slender species of
Filaria which attack the eyes both of men and horses; some bury
themselves close to the eye, and a very minute kind enters the ball
itself.

The Ascaris lumbricoïdes, a common intestinal thread-worm of the hog,
ox, and the human race, is sometimes of great length. The sexes are
distinct, and their fertility enormous. The ovaries are two tubes
sometimes several feet long, in each of which the eggs are arranged in
whorls round a central stem, like the flowers of a plantago. By counting
the number of microscopic eggs in a whorl, and the number of whorls, Dr.
Eschricht ascertained that in a full grown female the average number of
eggs amounted to sixty-four millions. In this species of worm the embryo
is not developed from the egg while within the victim, so that most of
the eggs perish.

Different species of Anguillulæ, which are minute eel-like worms slender
as a hair, inhabit the alimentary canal of fresh-water snails, frogs,
and fishes, but many species are not parasitic. These are often united
in swarming masses that nestle in mud, wet moss, wet earth, and aquatic
plants. One species causes the cockle in wheat, appearing like a living
tuft of white wool in the blackened grains. They appear in sour paste
and in other decomposing substances, and are so tenacious of life that,
after being completely dried for months, and apparently dead, they
revive on being moistened.


                             _Turbellariæ._

The Turbellariæ are fresh- and salt-water animals, distinguished by
having the whole surface of their bodies covered by cilia, under which
in some species there are thread-cells containing six, eight, or a
greater number of darts. Most of the members of this tribe have
elongated flattened bodies, and move by a sort of crawling or gliding
motion over the surface of aquatic plants and animals. Some of the
smaller kinds are sufficiently transparent to allow their internal
structure to be seen by transmitted light. The mouth, which is situated
at a considerable distance from the rounded end of the body, opens into
a sort of gullet leading into the stomach, which has no other orifice,
but a great number of branching canals are prolonged from it, which
carry its contents into every part of the body. A pair of oval
nerve-centres are placed near the rounded end of the animal, whence
nerves extend to various parts of the body; and near to these there are
from two to forty rudimentary eyes according to the species, each of
which has its crystalline lens, its pigment layer, nerve bulb, and its
cornea. The power of the Planaria to reproduce portions which have been
removed is but little inferior to that of the Hydra.


                              _Annelids._

The Annelids are the most highly organized of all the worm tribe. They
are exceedingly numerous and varied; some are inhabitants of fresh
water, others are terrestrial, but by far the greater number and most
highly endowed are marine. They generally have a long, soft, and smooth
body, divided or marked by transverse rings into a succession of similar
segments. In many the first and last segments are alike; in others the
first segment can scarcely be called a head, though it exercises several
functions, while in the highest two orders the head is the seat of
several senses. On each side of the bodies of the Annelida there are one
or two long rows of tufted bristles or feet, which may be regarded as
the earliest form of symmetrical locomotive organs. Most of the Annelids
have ocelli or eye-specks, and in many of them the head supports soft
cylindrical tentacles, which are obviously organs of touch. These worms
are divided into four orders, the Suctorial, Terrestrial, Tubercular,
and Errantia, or Wandering Worms.[32]

The first order consists of Leeches of different kinds: their body is
long, slightly segmented, with a suctorial disc at each end. Their skin
is smooth, whitish, and translucent; beneath it are cells filled with
brown or greenish matter, and three layers of muscular fibres follow;
the first are transverse, the second cross one another diagonally so as
to form a network, and the third are longitudinal. The mouth, which
occupies the centre of the principal sucking disk, varies in form with
the genera. In the common leech it has an enlarged lip, and opens into a
short gullet leading into a capacious and singularly complicated
stomach, divided by deep constrictions into eleven compartments, the
last of which is connected with an intestinal canal, which ends in a
vent in the middle of the terminal sucker.

Within the mouth there are three crescent-shaped jaws, presenting their
convex edges towards the cavity of the mouth, beset with from seventy to
eighty teeth, formed of a highly refractive crystalline substance
resembling glass. The leech makes a vacuum with its sucker, which forces
the part to which it is applied into contact with the three-toothed
jaws, which are moved sidewise by strong muscles, and saw through the
skin and small bloodvessels below it.

The leech, like the other Annelids, has two distinct systems of
circulating liquids, one red, the other colourless. The red liquid or
blood is kept in circulation by the pulsations of a heart, or rather a
contractile vessel behind the head. It is carried away from the heart by
a pulsating canal passing along the back of the leech, and is brought
back to the heart by a similar canal extending along its ventral side.
During this course, portions of the liquid are sent off through veins to
different parts of the body. The respiratory organs of the leech are
pores arranged at regular distances on each side of the body which open
into little sacs having capillary bloodvessels distributed under the
skin through which the blood is aërated.

The colourless liquid which contains many organic molecules, occupies
the space between the alimentary canal and the inner wall of the body,
from whence it passes into canals which ramify extensively, but are not
furnished with returning passages. This liquid forms a support to the
muscles of the skin, and is kept in circulation by the motions of the
leech.

Fig. 131 shows the highly developed nervous system of the leech. From
the double lobe of the brain ten optic nerves go to the bases of ten
black eye-specks, which mark at equal distances the upper margin of the
expanded lip. A nerve-centre below the gullet supplies the lip and jaws
with strong nerves. A double longitudinal cord, united at equal
distances by twenty-one double nerve-centres, extends from a ring round
the gullet throughout the whole length of the body, supplies the
different organs with nerves, and ends near the vent in a nerve-centre,
from whence nerves radiate through the terminal sucker.

[Illustration: Fig. 131. Nervous System of Leech.]

The circulation of the blood and of the colourless liquid, as well as
the nerve system, prevail generally in the Annelids, modified by the
structure of the individual.

The leech, though greedy of blood, lives in fresh-water ponds, wet
grass, and damp places, where it never can meet with warm-blooded
animals. It probably lives on minute aquatic insects.

The common Earth-worm, which is a principal member of the second order
of Annelids, has a more important part assigned to it in the economy of
nature than its humble appearance leads us to suspect. It has a long,
soft, cylindrical body tapering to a point at both ends, divided into
numerous rings. The mouth is furnished with a short proboscis, or snout,
without teeth. A long salivary glandular mass surrounds a short wide
gullet, which leads to a digestive organ similar to a gizzard, whence a
canal is continued to the vent. The circulation of the two fluids, and
the nervous system modified at head and tail, are like those of the
leech. Four rows of minute bristles extend longitudinally along the
ventral surface of the worm, two on each side. With a low
magnifying-power they appear to be minute points regularly pushed out
and drawn in; but when more highly magnified each point is seen to
consist of two transparent glassy rods having their points bent
backwards: on these feet the worm crawls very rapidly.

While making its cylindrical burrow a slimy mucus exudes from the body
of the worm, which cements the particles of earth together and renders
the walls of the burrow perfectly smooth and slippery. When the worm
pierces the earth it stretches its snout into a fine point that it may
penetrate more easily, and when it is fixed, it draws its ringed body
towards its head by a muscular effort; and to prevent it from slipping
back again, it fixes the hooks of its posterior feet firmly into the
ground. Having thus secured a point of support it penetrates deeper into
the earth, draws up its body, fixes the hooks of the posterior feet into
the smooth surface of the burrow, and continues the same process till
the burrow is deep enough. Thus the feet are employed as points of
resistance for the exertion of muscular force. This worm swallows earth
mixed with decaying animal and vegetable matter, assimilates the
nutritive part, and casts out the refuse in the form of fine mould,
which may be seen in little heaps at the edges of their burrows. In
fact, nearly all the fine vegetable mould so precious to gardeners and
farmers has passed through the intestines of the common earth-worm.

[Illustration: Fig. 132. Foot of Naïs.]

There is a colourless little fresh-water species of the genus Naïs,
remarkable for the beauty of its bristled feet. There are two pairs on
each ring of the worm, consisting of wart-like perforated protuberances,
through which a number of microscopic bristles protrude, arranged in a
radiating pencil like a fan. They are very slender, bent at the tip, and
so transparent that they look like threads of spun glass; the worm
thrusts them out and draws them in with extreme rapidity.

A blood-red Naïs lives in burrows in the mud at the bottom of springs
and pools in immense multitudes; large tracts of the mud of the Thames
are red with a species of them; half of their bodies stretched out of
their burrows maintain a constant oscillating motion on its surface,
but, like the earth-worm, they instantly shrink into their burrows on
the least alarm. They have no respiratory organs; but their blood is
aërated through their skin, which is so transparent that, with a
microscope, the whole of the internal structure, the motions of the
liquids, and the particles they contain are distinctly visible. The
blood acts the part of internal gills, by aërating the colourless liquid
contained in a set of vascular coils surrounding the organs of
digestion.

The Tubicola are marine worms, forming the third order of Annelida,
according to the system of M. Milne-Edwards. They live in tubes, either
of a shelly calcareous substance, which forms naturally on the tenacious
mucus of their skins, or in tubes artificially constructed by themselves
of sand and particles of shell glued together. All the Tubicola can
protrude their gills and the anterior part of their bodies, and some can
leave their tube and return to it again. These worms, which form
beautiful objects for the microscope, have ringed bodies with tubular
bristled feet, and respiratory organs or gills fixed either on the head
or near it. They have an alimentary canal loosely attached to the
ventral wall of the body, and two systems of circulating liquids, one
red, the other colourless. In the Tubular Annelids the principal organs
of respiration are the contractile plumes on the head.

In the Terebella there are distinct organs for the aëration of both
liquids, which form a beautiful plume when expanded, as in fig. 133,
which shows the animal when out of its tube. What may be called the head
is fixed upon the first ring of the body. The mouth has a lip like a
funnel-shaped cup with numerous long slender tubular tentacles; and two
delicate arborescent branches or gills are fixed immediately behind the
head. The colourless liquid which occupies the space between the
alimentary canal and the ventral wall of the worm, is sent by the
contractions of the body into the slender tubular filaments round the
mouth, which are covered by cilia, whose action continually renews the
stratum of water in contact with them. The blood in its usual course
enters the capillary tubes of the arborescent gills, where it is
oxygenized, and, after being distributed to the different parts of the
body, returns to the heart and gills again.

[Illustration: Fig. 133. Terebella conchilega.—_a_, lip, surrounded by
tentacles, _b b_, all placed upon the first segment of the body, _c_;
the skin of the back, _d_, is laid open, exposing the circulatory
system; _e_, pharynx; _f_, intestine; _g_, muscles of the belly; _h_,
gland, supposed to be the liver; _i_, generative organs; _j_, feet; _k
k_, gills; _l_, heart; _m_, dorso-intestinal vessel; _n_, intestinal
vessel; _n_, venous sinus; _o o_, ventral trunk, branching into smaller
veins, _p_.]

The slender filaments which radiate from the head of the tubicular worms
are flattened, sometimes tortuous, always ciliated, and are often barred
and variegated by bright purple, green, and yellow tints, forming a rich
and gorgeous crown.

The mucus, which cements together the particles of sand and shell for
the artificial tubes of this kind of worms, is believed to be secreted
from glands in the first segment of the body; but the long slender
filaments of the head are the active agents in the structure. The
tentacles are hollow bands with strong muscular edges, which the worm
can bring together so as to form a cylinder, at any point of which it
can take up a particle of sand, or a whole row of particles, and apply
them to its glutinous body. The fibres at the free ends of the tentacles
act both as muscular and suctorial organs; for when the worm is going to
seize a particle of sand or food, the extremity of the tentacle is drawn
in by the reflux of the colourless liquid in its interior, so that a
cup-shaped cavity is formed in which the particle is secured by
atmospheric pressure, aided by the power of the circular muscular fibres
at the extremity of the tentacle.

[Illustration: Fig. 134. Pushing poles of Serpula.]

The Serpula and its allies are richly-coloured worms, living in
contorted tubes with lids, frequently seen encrusting rocks, the shells
of oysters, and other mollusca. By a peculiar mechanism of their bristly
feet they can open the lid of their tube, push out their fan of gorgeous
tentacles, pull it in again, and shut up the tube. As the protrusion of
the worm from its tube is slow, cautious, and gradual, the retreat swift
and sudden as lightning, there are two distinct sets of organs in the
feet by which these motions are performed.[33]

On the back of the worm there is a sort of shield, the sides of which
bear seven pairs of wart-like feet, which are perforated for the working
of protrusile microscopic bristles (fig. 134). Their upper parts are
double-edged, with a groove between them, and serrated with close-set
teeth. The organs of retreat are much more complicated and numerous. Mr.
Gosse has computed that there are about 1,900 blades on the seven pairs
of feet, each movable at the will of the worm, and that there are nearly
10,000 teeth hooked into the lining of the tube when it wishes to
retreat. The manner in which it comes out of its tube and retires into
it again is the same as that employed by the earth-worm.

There are twenty-four genera of the order Errantia, or wandering
sea-worms. Multitudes swarm on every coast; they have considerable
muscular strength, and are highly irritable; some are called
sea-centipedes, from the number of their feet and length of their
segmented bodies, which are slender, and vary from a few inches or less
to thirty-five or forty feet. They are generally coiled up under stones,
or wander by the slipperiness of their smooth skins through masses of
sea-weeds or shells at low tide. In most of them the rings are decidedly
marked; the first and last segments are unlike, while the rest are mere
repetitions one of another. Their locomotive organs are a pair of
perforated fleshy warts on each of their numerous segments, through
which groups of rigid, simple or barbed bristles are protruded and
retracted.

The Errant Worms have a distinct small head with a mouth, or rather an
orifice, on the upper side of it, through which a cylindrical gullet is
from time to time turned inside out, forming a kind of pear-shaped bag,
whose surface is studded with secreting glands; and its extremity, which
is perforated, is surrounded by a muscle that contracts strongly on
whatever it is applied to, and holds it firmly while the re-inversion of
the sac draws it into the body to be digested. This apparatus is unarmed
in the genera Arenicola, Phyllodoce, and others, but in the Nereis it
has one pair of strong curved horny jaws. In the Eunice there are three
toothed jaws on one side and four jaws on the other side of the gullet,
each pair having a different form, and the tiny Lombrinereis has eight
little black hooks which are seen through its pellucid tissues, snapping
like so many pairs of hooked scissors. The Errant Worms are voraciously
carnivorous, and when the gullet is turned inside out the toothed jaws
project, seize the prey, and drag it into a ciliated alimentary canal,
for there is no proper stomach in these worms. The canal is generally
straight, and terminates in a vent at the posterior end of the body.

The respiratory organs of the Errantia are external gills of great
variety of forms: they are chiefly like branching trees, or filamentary
bushes, traversed by capillary bloodvessels. They are sometimes small,
and arranged on every segment along both sides of the back; sometimes
they are large and fixed only at intervals. Like the lower Annelids,
they have two liquid systems, one red and the other colourless, and the
circulation of the blood is the same; but as the pulsations of the
vessel behind the head are too feeble to send the blood through the
labyrinth of capillary vessels in these long worms, there is a
supplementary heart, or pulsating vessel, in each segment of the worm,
which partakes in and facilitates the general circulation.

The Eunice and other very long worms may have hundreds of these centres
of propulsion, which make the circulation rapid; and it is increased by
the restlessness and activity of the worms themselves, which bring their
gills perpetually into new strata of water.

The nervous system of the Errantia consists of a double cord extending
along the ventral side of the body, and united at equal intervals by
double nerve-centres, as in fig. 131; but in the Annelids the two cords
diverge below the gullet, surround it, unite again above that tube, and
form a principal bilobed nerve-centre or brain. Each segment of the worm
is occupied by a small double nerve-centre. In some of these marine
worms there are hundreds of segments and as many nerve-centres. There
are more than a thousand of these pairs of nerve-centres on the ventral
cord of the Nemertes gigas, or Great Band Worm, which is sometimes forty
feet long and an inch broad. The head is like a snake, and the bristled
feet are jointed to enable it to move over hard surfaces.

The movements of the bristly feet of the Errantia are reflex, depending
on the nerve-centres in their segments; but they are controlled and
connected by the double cord which passes through them.

Every hair, cirrus, and tentacle on the bodies of the Errant Worms is a
living organ of feeling, shrinking at the smallest touch, but enabling
them to select their food, to move towards and retreat from objects, and
to thread their way through the most intricate labyrinths with unerring
certainty, which seems to render them independent of eyes; yet many of
them have multitudes of eyes, or rather eye-specks, according to the
genera. Some have but one eye-speck placed in the forehead; one genus
has a double row throughout their whole length, two in each segment,
while the Amphicora has two in its tail. All these eye-specks have their
crystalline lens, pigment-layer, and nerve-bulb, so that the Errant
Worms must see objects, and their motions show that they do; but we can
form no idea of the kind of vision.

Besides the variety of organs on the skin of the Errantia, some of these
worms have two rows of flat plates on their backs overlapping each other
at their edges like the scales of a fish. They are well developed in the
Aphrodita hystrix, or the Sea Mouse of fishermen, and its congeners.
That Annelid, which is an inhabitant of European coasts, is thicker and
broader than other sea-worms. The two rows of overlapping shields on its
back, and the quantity of iridescent hairs, cirri, and other appendages
covering the body, is so great as to form a kind of felt or fur like the
skin of a mouse. The members of this genus of sea-worms have no gills
properly so called; the only external sign of respiration is a
periodical elevation and depression of the shields on their backs by the
action of a complex system of muscles. The thick covering of felt on the
body of the worm below the shields becomes filled with water during
their elevation, which is ejected forcibly at the posterior end of the
body during their depression. Although the water does not penetrate the
thin skin on the back of the worm, its oxygen does, and is accumulated
in the colourless liquid in which the stomach floats; and from it the
blood, which is of a pale yellow colour, receives its oxygen. The feet
of the worm are fan-shaped groups of sharp glassy bristles enclosed
between two plates, which prevent them from hurting the animal when it
puts them out or draws them in. The Aphrodita is male and female: the
eggs escape through pores in the female, and are received in a kind of
pouch beneath the dorsal shields till hatched. The embryo is an oval
locomotive mass, with groups of cilia, and indications of an eye-speck:
after swimming about for twenty-four hours, the segments begin to be
developed.

Worms of the genus Polynoë have also two rows of shields on their backs,
but they are studded with transparent oval bodies on short stems,
supposed to be organs of touch. The filiform tentacles and antennæ that
are developed between the shields, as well as the cirri or curly
bristles of the feet, are likewise covered with similar sensitive
organs. Fig. 135 shows the foot, cirri, and bristles of a Polynoë, which
are enclosed in plates which preserve them from hurting the worm. These
glassy bristles are beautiful objects under the microscope; still more
so are the jointed feet, transparent as the purest flint glass, of the
Phyllodoce viridis, one of the most beautiful Annelids on our coasts,
where it threads its way among young mollusca like a slender green cord,
exhibiting foliaceous gills in the highest perfection.

[Illustration: Fig. 135. Foot of a Polynoë.]

In the marine Annelids the embryo, on leaving the egg, is a gelatinous
globular mass of cells furnished with strong cilia. In a few hours the
mass elongates and divides into four parts, a head, a large ciliated
segment, a smaller one without cilia, and a ciliated tail. After a time
a succession of new segments are interposed, one by one, next to the
tail segment, and the corresponding internal organs of each are
developed till the worm arrives at its adult state. In many Annelids the
embryo is highly developed within the parent; that of the Eunice has
from 100 to 120 segments before it leaves her; and in the Nereis
diversicolor the young, covered with cilia, come out by hundreds at an
orifice in the side of the mother.

Many of the marine Annelids are luminous; electric scintillations are
given out during the act of nervous contraction, which are increased in
brilliancy and rapidity by irritation.

According to Professeur Quatrefages, the Annelida Errantia and Tubicola
have no zoological regions characterized by one or more special types
like the other classes of animals; they have representatives in all
seas. But it is exactly the contrary with regard to species. The number
of species common to any two seas, or the shores of two continents, is
very small; there is not a single species common to the Atlantic coasts
of France and the Mediterranean. The sea-worms are not affected by
climate, but they are said to be more abundant on granitic and schistose
coasts than on the calcareous.[34]

With regard to fossil remains, worm-tracks are seen in the Forest
marble, long calcareous tubes occur in the Upper Silurian and
Carboniferous strata, and in all the later formations tubercular
Annelids abound, especially of the genera Serpula, Spirorbis, and
Vermilia.[35]


                             _Tardigrada._

The Tardigrades are slow creeping animalcules, which seem to form a link
between the Worms and the Rotifers, though they are more nearly allied
to the former in having a vermiform body divided transversely into five
segments, the first of which is the head, and each of the others has a
pair of little fleshy protuberances furnished with four curled hooks.
They resemble the Rotifers in their jaws, in their general grade of
organization, and in the extreme length of time they can remain dried up
without loss of life. When in the dried state they can be heated to a
temperature of 250° Fahr. without the destruction of life, although when
in full activity they cannot endure a temperature of more than from 112°
to 115° Fahr. When alive the transparency of their skin is such as to
show a complicated muscular system, the fibre of which is smooth; and as
no respiratory organs have yet been found, their respiration must be
cutaneous. These animalcules have no nerve-centre in the head, but they
have one in each segment of the body; and they are furnished with a
suctorial mouth at the end of a retractile proboscis, on each side of
which are two tooth-like styles, the rudiments of lateral jaws. The
structure of these creatures is microscopic.


                              _Rotifera._

Although the Rotifera are microscopic objects, their organization is
higher than that of the Annelida in some respects. They are minute
animalcules, which appear in vegetable infusions and in sea-water, but
by far the greater number are found in fresh-water pools long exposed to
the air: occasionally they appear in enormous numbers in cisterns which
have neither shelter nor cover; a few can live in moist earth, and
sometimes individuals are seen in the large cells of the Sphagnum or
Bog-Moss.

The bodies of the Rotifers have no cilia; they are perfectly
transparent, elongated, or vermiform, but not segmented; they have two
coats, both of which in some genera are so soft and flexible that the
animal can assume a variety of forms; while in others the external coat
is a gelatinous horny cylindrical shell or tunic enclosing the whole
body except the two extremities, which the animal can protrude or draw
in. The soft kind can crawl over solid surfaces by the alternate
contraction and extension of their bodies like a worm, and the stiff
Rotifers are capable of doing the same by the contractility of their
head and tail. All can swim by means of cilia or lobes at their head.
The greater number possess means of attaching themselves to objects by
the posterior end of their bodies and of removing to another place.

The wheel-like organs from which the class has its name, are most
characteristic in the common Rotifer (fig. 137), where they consist of
two disk-like lobes projecting from the body whose margins are fringed
with long cilia. It is the uninterrupted succession of strokes given by
these cilia, passing consecutively like waves along the lobes, and
apparently returning into themselves, which gives the impression of two
wheels in rapid rotation round their axes.

The Brachionus pala (fig. 136) affords another instance of the
two-wheeled Rotifers. Though of unusually large dimensions in its class,
it is just visible to the naked eye as a brilliant particle of diamond
when moving in a glass of water. Its transparent horny tunic, when
viewed in front with a microscope, is a cup of elegant form, bulging at
the sides. One side of the rim is furnished with four spines, of which
the middle pair are slender and sharp as needles, with a deep cleft
between them; the other side of the rim is undulated but not toothed,
and the bottom of the cup ends in two broad blunt points.

Between the terminal blunt points there is a round opening for the
protrusion of the foot of the animal. The tunic is of glassy
transparency, so that every organ and function of the animal can be
traced with perfect distinctness.

[Illustration: Fig. 136. Brachionus pala, with three eggs attached to
its foot.]

The foot of the animal is long, rough and wrinkled, not unlike the
flexible trunk of an elephant. It can be lengthened, shortened, drawn
within, or pushed out of the tunic in an instant. It terminates in two
short conical fingers or toes, which can be widely separated or brought
into contact. By means of these, the Brachion has the power of mooring
itself even to the smooth surface of glass so firmly, that it can
stretch itself in all directions, shaking itself to and fro with sudden
violence without letting go its hold. The Rotifers usually fix
themselves before they set their wheels in motion in search of food.

From the anterior rim of the shelly cup, the Brachion protrudes a waved
outline of limpid flesh which, as soon as it rises above the level of
the sharp-pointed spines, spreads out into three broad flattish muscular
lobes. On the edges of the middle one there are very strong cilia like
stiff bristles, which do not vibrate, but are either erect or converge
to a point, whereas the edges of the other two lobes are thickly fringed
with long stout cilia, which, by striking the water in perpetual rapid
succession, each cilium bending and rising again, produce the appearance
of two circles of dark spots in rapid horizontal rotation, like wheels
on their axis. It is merely an optical deception, for both the animal
and its lobes may be at rest. The vibrations of the cilia can be
instantaneously arrested, and the whole apparatus drawn out of sight,
and as instantaneously protruded and set in motion.

In the flesh, on the ventral side of the Brachion, there is a deep
cleft, the edges of which as well as the whole interior of a tube of
which it is the orifice, are thickly covered with vibratile cilia. This
tube leads to a mouth with powerful jaws of unwonted structure, which is
so deeply sunk in the tissues of the body, that it never comes into
contact with the water. It opens into a gullet leading to a stomach,
intestine, and vent, at the posterior end of the body.

The vibrations of the cilia on the lobes of the animal’s head form two
circular currents in the water, like whirlpools, which draw all floating
particles into their vortices, and the streams from the two whirlpools
uniting into one current, flow off horizontally and pass immediately
over the slit on the ventral side of the animal. Some of the floating
particles are arrested by the cilia on the edges of the slit, and are
drawn into the sunken mouth by the vibrations of the cilia in the tube.
The edges of the slit act like lips, and seem to possess the sense of
taste, or of some modification of touch, which enables them to select
from the particles presented to them, such as are fit for food; these
are admitted into the mouth, where they are bruised by the powerful
jaws. The mouth or masticating apparatus is the most extraordinary and
complex part of this animal. It consists of two horny toothed jaws,
acting like hammers upon an anvil. The two hammers, which approach each
other from the dorsal sides of the body, are each formed of two parts
united by a hinge; the first parts correspond to the handles; the second
parts, which are bent at right angles to the first, resemble hands with
five or six finger-shaped teeth united by a thin membrane. The teeth are
parallel to one another when they meet on the anvil, and are seen
through the transparent mass tearing the food into fragments. Some of
the Rotifers resemble the Errant Annelids in being able to turn this
complicated machine inside-out through the ciliated tube and slit, so as
to bring it into contact with the water. When the food has been
masticated it is sent into the stomach, where it is digested. The whole
of this process is seen through the transparent and colourless body of
the Brachion, because its favourite food is the Syncryn velox, a minute
bright green plant, which from its active motions was at one time
believed to be an animal.

The Brachion has four longitudinal muscular bands transversely striated,
which move the ciliated lobes of the head, push them out and draw them
in. From these muscular threads are sent to the different parts of the
body, to the mouth especially, two strong bands, which bend and unbend
the joints of the hammer-like jaws. The vigorous motions of the long
serpentine foot and the firm hold of its anchors are owing to muscular
bands fixed high up on the interior wall of the body, which extend
throughout the whole length of the flexible organ. As long as the
Brachion is fixed, the vibrations of the cilia on its lobes only produce
whirlpools in the water, but the moment that it lets go its hold, these
vibrations, in consequence of the reaction of the water, give the animal
both a smooth progressive motion and a rotation round its axis.

Minute as the Brachionus pala is, it has several organs of sense. A
sparkling, ruby-coloured, square eye-speck with a crystalline lens and
crimson pigment layer is placed on a wart-like prominence on its back,
and this prominence Mr. Gosse believes to be the brain of the animal. In
the cleft between the spines and close to the eye-speck are two tubes,
one within the other. The innermost tube, which can be protruded and
withdrawn, has a bunch of bristles at its extremity that have the
sensibility of antennæ. Nerves from the brain pass into these, to the
various organs of the body, and to the lobes on the head.

The Brachion has no propelling vessel or heart to maintain the
circulation of its liquids, but, like the Annelids, a colourless liquid
occupies the general cavity between the alimentary canal and the
internal wall of the body. It is believed to be connected with
nutrition, and is furnished with oxygen by a complicated organism, and
is kept in motion by the vibrations of long cilia. The determination of
the whole structure and motions of a creature barely visible to the
naked eye, is a wonderful instance of microscopic research, and of the
perfection of the mechanism exhibited in the most minute objects of
creation.

[Illustration: Fig. 137. Common Rotifer:—_a_, mouth; _b_, eye-spots;
_c_, wheels; _d_, probably antenna; _e_, jaws and teeth; _f_, alimentary
canal; _g_, glandular mass enclosing it; _h_, longitudinal muscles; _i_,
tubes of water-vascular system; _k_, young animal; _l_, cloaca.]

Fig. 137 represents the common Rotifer when its wheels are expanded and
when they are retracted. The body is slender and flexible, it is
stretched out by longitudinal muscles, and its girth is diminished by
circular ones. The internal structure is similar to that of the
Brachion, but there is a prominence or head between the wheels on which
there are two crimson eye-specks, and the foot terminates in three
concentric movable tubes that can be protruded and drawn in like the
tubes of a telescope; each has a pair of claspers to enable the Rotifer
to fix itself to any object.

The Rotifers are male and female, but, like the greater number of
Infusoria, the males are only produced at intervals. The female Rotifers
have their perfect form when they leave the egg: they even come out of
the egg while it is attached to the tail of the mother, as in the
Brachionus pala (fig. 136). The males, when hatched, have neither spines
nor mouth, yet, during their short lives, their motions are very fleet
on account of the vibrations of long cilia round their front.

Some Rotifers are remarkably fertile. Professor Ehrenberg estimated
that, in the course of twenty-four days, the offspring of a single
individual of the genus Hydatina might amount to seventeen millions.
Female eggs laid in autumn are collected in heaps and covered with a
gelatinous substance, which protects them from the cold in winter,
though the Rotifers themselves are sufficiently protected by their great
tenacity of life. They revive after being frozen; they may be dried for
an unlimited time, but, as soon as they meet with warmth, moisture, and
food, they resume their vitality.




                              SECTION VI.

                             ECHINODERMATA.


THIS class consists of five orders, all of which are marine. They are,
with one exception, creeping animals, and the whole class is remarkable
for having most of their members and general structure either in fives
or multiples of five. Their skin is hardened by calcareous deposits,
sometimes of beautiful microscopic structure: they have a digestive
cavity, a vascular fluid system, and some distinct respiratory organs,
so that they are comparatively of a high grade.


                      _Echinodermata Asteroïdea._

The Asteroïdea, or Star-Fishes, which are the highest order, form two
natural families, the Stelleridæ and Ophiuridæ, which comprise
twenty-two genera.

The simplest form of the Stelleridæ is the common star-fish, with its
flat regularly five-sided disk. A tough membrane, strengthened by
reticulated calcareous matter, covers the back, and bends down along the
sides, while the under-side of the body or disk, on which the animal
creeps, is soft and leathery, with the mouth in its centre. In the other
genera, although the body is still a flat, five, equal-sided disk, the
angles are extended into long arms, broad whence they diverge from the
disk, but decreasing rapidly in width to their extremities, so that the
animal is exactly like a star with five long, equal, and flexible rays.

The backs of all the star-fishes are covered with most minute movable
spines, and with microscopic organs like minute pincers, called
pedicellariæ, which are diffused generally over the surface, and form
dense groups round the spines. They have a slender, contractile,
calcareous stem, and a head formed of two blades, which they continually
open and shut, the whole being coated with a soft external tissue. They
grasp anything very firmly, and are supposed to be used to free the
star-fish from parasites. In some species of Goniaster the pedicellariæ
resemble the vane of an arrow, and are so numerous as to give a villous
appearance to the skin of the back.

On the under-side of each ray of a star-fish, a central groove or furrow
extends throughout its whole length, and the semi-calcareous flexible
membrane which covers the back and rays not only bends down round the
sides of the rays, but borders both edges of the grooves. Upon these
edges ridges of small calcareous plates beset with spines are placed
transversely: they are larger near the mouth, and gradually decrease in
size as they approach the point of the ray.

Interior to the spines, these ridges are pierced by alternate rows of
minute holes for the long rows of feet, which diminish in size to the
end of the ray. The feet are contractile muscular tubes communicating
through the holes with internal muscular sacs, which are regarded as
their bases. The sacs are full of a liquid, and when the animal
compresses them the liquid is forced through the holes into the tubular
feet, and stretches them out; and when the muscular walls of the hollow
feet are contracted, the liquid is forced back again into the sacs, and
the feet are drawn in. The liquid is furnished by a circle of small
vascular tentacles, or sacs, surrounding the mouth, which are both
locomotive and prehensile. From these a canal extends through the centre
of each ray, which in its course sends off lateral branches to the bases
of the feet to supply them with liquid. The whole of this system of
vessels and feet are lined with vibratile cilia, which maintain a
perpetual circulation in the liquid.

The toothless mouth on the under-side of the disk dilates so as to admit
large mollusca with their shells. The short gullet and stomach are
everted, protruded through the mouth, and applied round the object to be
swallowed, which is then drawn in, digested, and the shell is discharged
by the mouth. However, in three orders of this family there is a short
intestine and vent. From the large stomach, which occupies the central
part of the disk of the star-fish, a couple of tubes extend to the
extremity of each ray, where they secrete a substance essential for
digestion: the stomach is in fact a radiating organ, partaking the form
of the animal it sustains.

A pulsatory vessel near the gullet propels the yellow blood into a
system of fine tubes, that are spread over the walls of the stomach and
its rays. Through these walls the blood receives a nutritious liquid,
which it carries with it into a network of capillary vessels, widely
extended throughout the body, being propelled by the contractile powers
of the vessels themselves, and after having supplied the tissues with
nourishment, it is carried by tubes to the point from whence it started,
to begin a new course. The capillary network passes immediately under a
portion of the skin of the star-fish, through which an exchange of the
respiratory gases takes place. Besides, the star-fishes breathe the
sea-water through numerous conical tubes, that project in patches from
the back. Through these tubes, which can be opened and shut, the water
is readily admitted into the cavity containing the digestive organs,
with which they are in communication. The star-fish slowly distends
itself with water, and then gives out a portion of it, but at no regular
time. The cavity is never empty of water, and as its lining is densely
bristled with cilia, their vibrations keep the vascular surface of the
digestive organs perpetually bathed with the respiratory medium.

The star-fishes have a radiating system of nerves suited to their form.
A ring of slender nerve-cords surrounds the mouth, from whence three
nerves are sent off at the commencement of each ray: two of these, which
are filaments, go to the organs in the central disk, while the middle
one, which is a great trunk, passes through the centre of the rays, and
terminates in a nerve-centre, or ganglion, placed under a coloured
eye-speck at their extremity. The structure of the rays, the eye-specks,
and the nerve-centres below them, are so similar, that they are merely
repetitions of one another; hence no nerve-centre can control the
others, but they are all connected by the ring encircling the mouth,
which is a common bond of communication. How far the movements of these
animals indicate sensation we have not the power to determine, but they
feel acutely, for the mouth, the feet, and especially the pedicellariæ,
are highly sensitive, and shrink on the least touch. The eye-specks are
probably sensitive to light, and as the star-fishes often feed on putrid
matter, they are supposed to be endowed with the sense of smell.

The family of the Ophiuridæ, or Snake Stars, are widely distributed in
the ocean. The genus Euryales with branching rays, and that of Ophiura
with simple rays, comprising the Brittle and Sand Stars, are abundant in
the British seas. In the sand stars there are cavities full of sand at
the points from whence the rays diverge, which appear like warts on the
surface of the disk. Their rays are exceedingly long, thin, and
flexible; they have no central groove nor feet, but they are employed as
organs of locomotion and prehension, for by their alternate strokes the
sand stars can elevate or depress themselves in the water, creep on the
bottom, and by twisting them round objects they can fix themselves,
firmly aided by spines or bristles on their edges. The Ophionyx has the
addition of movable hooks beneath bristled spines. The rays are bent by
the contraction of internal muscles, and extended again by the
elasticity of the external leathery coat. The Ophiuridæ, like the Luidia
fragilissima belonging to the preceding order, cast off a ray if
touched, and even all the five if rudely handled; but they can replace
them with as much ease. If only a fragment of a disk remains attached to
a ray the whole animal may be reproduced.

The Ophiuridæ have an internal calcareous skeleton or framework, in the
form of spicules, scattered in their tissues. They have a capacious
mouth with tentacles and ten small chisel-shaped teeth, five on each
side, which meet and close the mouth. The mouth is separated from the
stomach by a circular muscle that opens and shuts the passage, but no
canal diverges from the stomach through the rays. The nervous system and
the circulation of the blood are similar to those in the Stelleridæ; and
respiratory organs, in the form of from two to four plates, or lamellæ,
project from each of the spaces between the bases of the rays into the
central cavity, by which sea-water has free access to bathe the
digestive organs and aërate the blood.

The colour of the star-fishes, as well as of other marine invertebrate
animals, seems to be independent of light. The Ophiuridæ that had been
living at a depth of 1,260 fathoms in the North Atlantic were coloured,
though not a ray of light could reach their dark home, and those dredged
up from 100 to 300 fathoms on the coast of Norway were of brilliant
hues—red, vermilion, white, and yellow. In general, both plants and
animals of the lower kinds become of a sickly white when kept in
darkness.

The Stelleridæ are male and female, and form fertilized eggs of an
orange or red colour. These eggs are first converted into a mass of
cells and then into larvæ, not radiating symmetrically like their
parents, but of a bilateral form, the two sides being perfectly alike
and bordered by a ciliated fringe nearly throughout their whole length.
These two fringes are united by a superior and inferior transverse
ciliated band, and between the two the mouth is placed. A stomach,
intestine, and vent are formed; the creatures can provide for
themselves, and swim about as independent zooids. A young star-fish is
gradually developed by a succession of internal growths, part of the
original zooid is retained, and the rest is either thrown off or
absorbed; then the star-fishes having lost the power of swimming, crawl
slowly away and acquire their full size. There is great diversity in the
external form of the zooids of the different genera, as well as in the
portion of them retained in the adult star-fish.

Fossil star-fishes have a very wide range. They are found among the
earliest Silurian organic forms, but they scarcely bear any resemblance
to existing genera. The Ophiuridæ, fished up from the bottom of the
North Atlantic, come nearest to them. Five genera are found in the
Oolitic formation, all extinct; three genera range from the Lias to the
present seas; and five genera belonging to the Cretaceous period are
represented by living species.


                       _Echinodermata Crinoïdea._

The Crinoid Echinoderms, or Stone-Lilies, are like a tulip or lily on an
upright stem, which is firmly fixed to a substance at the bottom of the
sea. During the Jurassic period, miniature forests of these beautiful
animals flourished on the surface of the Oolite strata, then under the
ocean. Myriads of their fossil remains are entombed in the seas, and
extensive strata of marble are chiefly composed of them. Their hollow
joints are known in several parts of England as wheel stones, and as St.
Cuthbert’s beads on the Northumbrian coast, in honour of the patron
saint of Holy Island, where they abound. The Crinoïdea are of two kinds:
the Encrinites, which chiefly flourished in the Palæozoic period and are
now represented by a minute species (Rhizocrinus Lofotensis) lately
discovered on the coast of Norway by Professor Sars, have a smooth,
cylindrical, jointed stem; and the Pentacrinites, which began at the
Lias, and have a five-sided jointed stem, the present representative of
which is the Pentacrinus caput-Medusæ, found in the West Indian seas.

The hollow, five-sided, calcareous, jointed stem of the living
Pentacrinite is filled with a spongy substance, and supports a cup on
its summit, containing the digestive organs, mouth, and tentacles of the
animal. The cup is formed of a series of calcareous plates, and from its
margin five long many-jointed rays diverge, each of which is divided
into two-jointed branches. Groups of curled filaments, called cirri, are
placed at regular distances from the bottom of the stem to the extremity
of the rays, while, on the opposite side of the rays, there are groups
of feathery objects called pinnæ at each joint. Food is caught by the
tentacles and digested by the stomach and viscera at the bottom of the
cup, from whence vessels diverge through a system of canals in the axes
of the rays, pinnæ, and down the stem, all of which convey sea-water
mixed with nutritious liquid, for the nourishment of the animal.

The genus Comatula are star-fishes, believed to have alternately a fixed
and a free state. Mr. J. V. Thomson discovered that the Pentacrinus
Europæus is merely the fixed state of a Comatula. These star-fishes have
pairs of pinnæ placed at regular distances along their long-jointed
rays, and in the pinnæ sacs containing eggs are placed as far as the
fifteenth or twentieth pair. The eggs yield active ciliated larvæ, which
attach themselves in the form of flat oval disks to corallines and
sea-weeds. By degrees they develop a stem, about three-fourths of an
inch high, with twenty-four distinct joints. Its expanded top bears five
sulphur-coloured bifurcating rays with their pinnæ and dorsal cirri. A
mouth is formed in the centre with its tentacles, and a lateral
prominent vent. The actual change of a Pentacrine into a Comatula has
not been seen, but as the small Pentacrinites disappear in September, at
which season the Comatulæ appear, it is believed that when full grown
the top of the fixed Pentacrinite falls off and becomes a Comatula,
which swims backwards with great activity by striking the water
alternately with its long rays. The Pentacrinus caput-Medusæ, which is
fixed by its stem to sea-weeds and zoophytes, forms a most beautiful
object for the lower magnifying powers when viewed in a fluid by a
strong refracting light.


                      _Echinodermata Echinoïdea._

The family of Echinidæ, commonly known as Sea-Eggs or Sea-Urchins, have
a beautiful but complicated structure. The calcareous shell of an
Echinus is a hollow spheroid with large circular openings at each pole.
In the larger of the two, called the corona, the mouth of the animal is
situated; in the lesser circle the vent is placed. The spheroid itself
is formed of ten bands extending in a meridional direction from the
corona to the lower ring; that is, from one polar circle to the other.
Each band consists of a double row of pentagonal plates increasing in
size from the poles to the equator, nicely dovetailed into one another,
and the bands are neatly joined by a zigzag seam. Every alternate band
is perforated by a double series of minute double holes for the passage
of the tubular feet of the animal. The five perforated or ambulacral
bands have rows of tubercules parallel to the series of feet holes,
supporting spines movable in every direction. The five imperforated
bands are characterized by a greater number of spines, but there are
none within the polar circles. The spines may be long rods, or merely
prickles, or stout, club-shaped bodies, according to the genera.

[Illustration: Fig. 138. Section of Shell of Echinus. _a_, portions of a
deeper layer.]

The microscopic structure of the shell of the Echinus is everywhere the
same; it is composed of a network of carbonate of lime, with a very
small quantity of animal matter as a basis. In general, the network
extends in layers united by perpendicular pillars, but so arranged that
the open spaces, or meshes, in one layer correspond to the solid
structure in the next.

The spheroid of the Echinus is covered with spines, and both outside and
inside by a contractile and extensile transparent membrane, which
supports the shelly plates at the poles, and dips between the bands but
does not penetrate them. Its extensile nature admits of the addition of
calcareous matter to the edges of the plates when the animal is
increasing in size. The membrane lining the interior of the shelly globe
is tough; it encloses the digestive organs, and forms a muscular lip to
the mouth, which is armed with five triangular, sharp-pointed, white
teeth, and surrounded by five pairs of pinnate tubular tentacles. The
outer margin of the lip is fringed with a circle of snake-headed
pedicellariæ visible to the naked eye.

The five teeth, whose sharp tips meet in a point when closed, are
triangular prisms, the inner edge is sharp and fit for cutting. Each
tooth is planted upon a larger triangular socket, two sides of which are
transversely grooved like a file, and as these two sides are in close
contact with the sides of the opposite socket, the food when cut by the
small teeth is ground down by the sockets, and a salivary secretion
finishes the mastication. The sockets of the teeth are connected by ten
additional solid pieces, placed two and two between them, which
completes the pyramidal apparatus called Aristotle’s lantern; it
consists of forty solid calcareous pieces arranged in fives, and moved
by forty muscles attached to five calcareous ridges, and five arches
near the internal edge of the corona.

Five pairs of these muscles when acting together protrude and retract
the teeth; when acting separately they draw them to one side or to the
other; five pairs separate the five teeth, five pairs shut them, and the
remaining five pairs work the bruising machine. The masticated food
passes through a short gullet into the stomach, where it is digested,
and the indigestible part is carried by an intestine to the vent in the
smaller polar circle.

The smaller polar circle is formed of ten triangular plates, five are
attached to the bands containing the feet holes, and five to the
intermediate bands. The last five are perforated, and are the
reproductive plates: the other five are also perforated for the
discharge of the liquid that moves the tubular feet, and which, after
having circulated in the body, is no longer of use. In five of these
polar plates there are red specks, the rudiments of eyes, the only
organs of sense these creatures seem to possess except that of touch and
probably smell. The nervous system is a slender, equal-sided pentagon
round the gullet, from the sides of which five nerves are sent to the
muscles of the mouth, and others, extending along the ambulacral or feet
bands, end in nerve-centres under the eye-specks.

[Illustration: Fig. 139. Sucker-plate of Sea-Egg.]

[Illustration: Fig. 140. Section of a sucker-plate.]

The mechanism for extending and retracting the feet by a liquid, is the
same with that in the star-fishes, but the pores which admit the liquid
into the feet are double. The tubular feet swell at their extremity into
a fleshy sucker, within which there is a thin glassy reticulated rosette
(fig. 139), of which fig. 140 is a highly magnified segment. It is
perforated in the centre by a large round opening. The sea-urchins can
stretch their feet beyond the spines, and by means of the suckers they
can attach themselves even to smooth objects, or aided and directed by
their spines they roll themselves along with a rotatory motion head
downwards.

The circulation of the bright yellow blood is like that of the
star-fishes. It is aërated both internally and externally. The external
respiratory organs are short, branched, and highly vibratile bodies
attached in pairs to the oval extremities of the fine imperforated
bands.

There are pedicellariæ scattered among the spines of the sea-urchins
which are in constant motion, protruding themselves beyond the spines
and withdrawing again, snapping their pincers, and grasping firmly
anything that comes within their reach, or that is presented to them.
The pedicellariæ vary much in form and position in the different genera
of the Echinidæ; but they invariably consist of a long, slender,
calcareous stem, and generally tripartite head, the whole coated with a
gelatinous fibrous transparent substance. The head of the Pedicellaria
globosa is a formidable weapon; at the apex of each of its three
serrated and toothed blades there is a strong sharp spine directed
horizontally inwards, so that the three spines cross each other when the
blades close, which they do so energetically that nothing could escape
from such a grasp. The pedicellariæ are curious microscopic objects;
they are extremely irritable, and although their use is unknown, they
must be essential to the well-being of the animals, since hundreds are
scattered over their shells.

The spines of the Echinidæ vary in shape and structure in the different
genera and species. Those of the Scutella form merely a velvety pile. On
the common sea urchin the spines are simple, and shed twice in the year;
those on the Amphidetus are both club and spoon-shaped; and, on the
Cidaris, they are large formidable clubs moved by a ball and socket. All
the spines, whatever their form may be, are moved in that manner; for
there are little tubercules on the surface of the shell on which a cup
at the bottom of the spines is pressed down by the muscular skin which
covers the shell and spines, and by its contractile power it enables the
animal to move the spines in any direction.

The microscopic structure of the calcareous spines is often beautifully
symmetrical. Those of the Acrocladia mamillata consist of concentric
alternate layers of network and sheaths of pillars; so that a section of
the spine perpendicular to its axis exhibits a succession of concentric
rings like those of an exogenous tree. The cup at the bottom of the
spine is very dense network, and the last of a sheath of encircling
pillars form the ribs, sometimes seen on the exterior of the spines.

[Illustration: Fig. 141. Spine of Echinus miliaris.]

The spines of the Echinus miliaris, of which fig. 141 represents the
segment of a section highly magnified, are fluted columns of calcareous
glass, the grooves of which are filled with solid glassy matter curved
on the exterior. The innumerable hair-like objects attached to the
shells of some of the Echinidæ, the almost filamental spines of others,
and the pedicellariæ themselves, are formed of a regularly reticulated
substance. When the Echinidæ are stripped of their spines and all their
appendages, their shells show 2,400 plates united with the symmetry of a
tesselated pavement.

[Illustration: Fig. 142. Pluteus of the Echinus:—_a_, mouth; _b_,
stomach; _c_, echinoid disk; _d d d d_, four arms of the pluteus-body;
_e_, calcareous framework; _f_, ciliated lobes; _g g g g_, ciliated
processes of the proboscis.]

The Echinidæ are male and female, and the eggs are excluded through the
five perforated productive plates at the posterior end of the shell.
According to the observations of Prof. Fritz Müller the embryo, soon
after issuing from the egg, takes a form represented (magnified) in fig.
142.

All parts of this creature, which is called a Pluteus, are strengthened
by a framework of calcareous rods tipped with orange colour, all the
rest being transparent and colourless. It swims freely, back foremost,
by means of its cilia.

[Illustration: Fig. 143. Larvæ of Echinus in various stages of
development within the Pluteus, which is not represented:—B, disk with
the first indication of the cirrhi; C, disk with the origin of the
spines between the cirrhi; D, more advanced disk with the cirrhi, _g_,
and spines, _x_, projecting from the surface.]

While in this active state a circular disk (_c_, fig. 142), covering the
stomach (_b_, fig. 142), appears within it, which gradually expands, and
sends through the skin of the Pluteus spines, pedicellariæ, and
tubercules, ultimately developed into hollow feet. Then the feet are
pushed out and drawn in, the pedicellariæ (D, fig. 143) snap their
pincers; and while the half-formed Echinus is making these motions
within the Pluteus, the mouth and gullet of the Pluteus itself are in
constant activity; and, while it swims about, the unformed Echinus
within it gets a globular shape, the shell is formed, and when the
Echinus is complete, the rest of the Pluteus is thrown off, and the
young animal rolls away.

The free swimming larval zooids of the Echinodermata are generally
hyaline, and some are phosphorescent. The Pluteus is also the larval
zooid of the ophiurid star-fishes; they may be seen in great numbers on
the surface of the sea in August and September. The young star-fish is
formed in them by a process analogous to that described. The motions of
the Echinidæ are reflex; nothing indicates volition.

The fossil Echinidæ first appeared in the lower Ludlow limestone, and
attained their maximum in the Cretaceous strata. A species of Diadema,
with annulated hollow spines, common in the Chalk, still exists.
Numerous species of the genus Clypeaster, remarkable for their flattened
form, and known as lake urchins, are peculiar to the Tertiary strata and
existing seas; and, lastly, five species of Spatangidæ, heart-shaped
urchins, which lived in the Tertiary periods, still exist. In
consequence of the porous texture of the solid calcareous parts of the
Echinidæ, their fossil remains are commonly impregnated with pyrites or
silex, without altering their organic structure, so that they exhibit a
fracture like that of calcareous spar.


                     _Echinodermata Holothuroïdea._

The Holothuridæ, or Sea-Cucumbers, are of a higher organization than the
preceding Echinoderms. They are soft, worm-shaped, five-sided animals,
covered by a flexible, leathery integument or skin, in which are
imbedded a vast multitude of microscopic calcareous plates of
reticulated structure. The mouth, which is placed at one end of the
animal, is surrounded by ten bony plates forming a lantern, analogous to
that of the Echinus; they support branching, tubular, and retractile
tentacles, which encompass the mouth like a star. The tentacles are
connected with sacs at their bases, and are extended and retracted by
the injection of a watery liquid contained in them. Innumerable tubular,
suctorial feet, precisely similar to those of the Echinus, are protruded
and retracted through corresponding pores in the skin of the animal by a
watery liquid, in sacs, at their bases. The water is supplied by a
system of canals connected with an annular reservoir round the top of
the gullet, which is supplied with water by a bottle-shaped bag at the
mouth.

Besides transverse muscles, five pairs of muscles attached to the
lantern at the mouth, extend throughout the whole length of the animal.
Nerve-chords from the ring at the gullet accompany these, and such is
the irritability of this muscular system, that the Holothuriæ eject
their viscera when alarmed or caught; but they have the power of
reproducing them: sometimes they divide their whole body into parts.

The respiratory organs are two very long and beautifully arborescent
tubes veined with capillary bloodvessels. The circulation of the blood
is similar to that of the star-fishes, but more complicated.

The minute calcareous particles scattered independently in the tough
leathery skin of the Holothuridæ remain as fine dust when the flesh is
dissolved and washed away; but, upon microscopic observation, Mr. Gosse
found that the forms of these particles are remarkable for elegance,
regularity, and variety of structure, but that the normal form is an
ellipse of open work built up of five pieces of a highly refractive,
transparent, glassy material, having the shape of dumb-bells.

The Holothuriæ found under stones at low spring tides, on the British
coasts, are small; those dredged up from deep water are five or six
inches long, and not unlike a well-grown warty cucumber; they do not
form an article of food in Europe, but they are highly esteemed by the
inhabitants of the Indian Archipelago and in China, where many shiploads
of the trepang are imported annually. It is a species that swarms in the
lagoons of the coral islands, the reefs of the coral seas, and at
Madagascar. Some species are two feet long, and six or eight inches in
circumference.

The order of the Holothuridæ form eggs like all the other Echinoderms;
the larval zooid has the same form as that of the star-fishes, and
changes its form twice, while the members of the Holothuria are forming
within it; at last they combine with those of the zooid, and no part is
cast off.


                       _Echinodermata Synaptidæ._

The Synaptidæ are five-sided creatures, similar in structure to the
Holothuriæ, though more worm-like. The whole order, which consists of
the two genera of Synapta and Chirodota, have twelve calcareous plates
round the mouth, five of which are perforated for the passage of the
vascular water canals, which convey the liquid for the protrusion of the
feet.

[Illustration: Fig. 144. Skeleton of Synapta.]

The calcareous particles imbedded in the skin of the genus Synapta are
anchor-shaped spicules fixed to elliptical or oval plates, (fig. 144).
The plates are reticulated and sometimes leaf-shaped, and the flukes of
the anchors are either plain or barbed. All the anchors are fixed
transversely to the length of the animal, lying with great regularity in
the interspaces of the longitudinal muscular bands. Sometimes a thousand
anchors are crowded into a square inch, each elegant in form, perfectly
finished, and articulated to an anchor-plate, whose pattern as well as
that of the anchor itself is characteristic of the species to which it
belongs. In the Synapta digitata, which has four fingers and a small
thumb on each of its twelve oval tentacles, the anchors are but just
visible to the naked eye;[36] in all the other species they are
microscopic. Besides the anchors, the skin of the genus Synapta contains
innumerable smaller particles, ‘miliary plates,’ which are crowded over
the muscular bands. The muscular system of the Synapta digitata is so
irritable that, on being touched, it divides itself into a number of
independent fragments, each of which keeps moving for a time, and
ultimately becomes a perfect animal like its parent. Specimens of this
Synapta have been found on the southern coasts of England and in the
West of Scotland, but the genus is rare, although containing several
species in the British seas; it is more common in the Adriatic; but they
cannot be compared, as to size, with the great Synapta of Celebes, which
is sometimes a yard in length, and is known among the natives as the Sea
Serpent.

[Illustration: Fig. 145. Wheel-like Plates of Chirodota violacea.]

The calcareous particles imbedded in the skin of the allied genus
Chirodota are wheel-shaped when viewed with a microscope (fig. 145). One
species is British, but they are mostly inhabitants of warm seas. In
Chirodota violacea, a Mediterranean species, the skin is full of groups
of broad thin hyaline wheels lying upon one another and connected by a
fine thread. The wheels have five or six flat radiating spokes.[37] The
wheels are exceedingly small in the Chirodota lævis, and are arranged in
groups; in the C. myriotrochus they are imbedded in myriads, as the name
implies.


                      _Echinodermata Sipunculidæ._

The Sipunculidæ, which form the last order of the Echinoderms, consist
of several genera of marine worm-shaped animals which burrow in the
sand, and form a link between the Holothuridæ and the true sea-worms.
They have no calcareous particles in their flexible skins, nor have they
any tubular feet, or special respiratory organs, but a vascular liquid
is kept in motion in the internal cavity by the cilia with which it is
lined. The mouth of the Sipunculus is a kind of proboscis with a
circular fringed lip and two contractile vessels, supposed to serve for
raising the fringes. An alimentary canal extends to the end of the
animal, turns back again, and the intestine ends in a vent near the
mouth, so that the creature need not leave its burrow and expose itself
to enemies in order to eject the refuse of its food. The locomotive
larval zooids from the rose-coloured eggs undergo two metamorphoses; at
last the young Sipunculus unites with the zooid, and no part is thrown
off.




                              SECTION VII.

                             THE CRUSTACEA.


THE Crustacea are free, locomotive, articulated animals, covered with a
crust or external skeleton, and distinguished by having jointed limbs,
and gills that fit them for aquatic respiration. They are male and
female, and, though extremely diversified, they have a similarity in
their general structure. Many are microscopic.

The Crustacea constitute ten orders, many genera, and innumerable
species. The Decapods, or the ten-footed order, are by far the most
complicated in organization. They have prominent eyes, movable on
jointed stalks, antennæ, gills in a cavity on each side of the throat, a
mouth opening into a digesting apparatus, a heart, liver, circulation of
the blood, and a nervous system, and are therefore animals of a higher
grade than any that have come under consideration.

The Decapods are divided into three tribes:—the Macrura, or long-tailed
Crustacea, of which the Lobster and Astacus fluviatilis, or fresh-water
Crawfish, are types; the Anomura, or tailless tribe, of which the Hermit
crab is the type; and the Brachyura, or short-tailed crustaceans, which
are represented by the common Crab. The greater number of these animals
are marine; some inhabit fresh water; and some are amphibious, living in
holes in the ground; others climb reeds and bushes with their long
claw-feet; the last two kinds come to water to spawn.


                               _Macrura._

The body of the Macrura, or long-tailed crustaceans, consists of a
number of segments or rings joined end to end, having jointed members on
each side. Every individual joint is covered with a hard crust to afford
support to the muscles. A certain number of the rings, which form the
tail, are always distinct, similar, and movable on one another, whilst
the remainder, which form the carapace or shell, are confluent so as
entirely to obliterate the divisions. But generally the arrangement of
these twenty-one rings is such that seven of them are confluent and form
the head, seven confluent rings form the thorax or throat, and the seven
non-confluent rings form the tail. In the Decapods the three last head
rings greatly expanded are cemented to those of the thorax, so as to
form the carapace or shell, which covers all the body of the animal
except the tail. This structure may be traced on the under-surface of
the crab.

A ring consists of an upper and an under arch, with a space between
them, so as to let the feet and other appendages pass through. In the
long-tailed tribe the tail is bent and unbent by muscles attached to the
under and upper surfaces of each ring, which give the tail a powerful
motive force, for, by bending it suddenly under the body, and then as
suddenly stretching it out, the animal darts backwards through the
water.

The Decapods have five pairs of walking feet; the front pair are claws
employed to seize their prey, and occasionally for walking; the other
four pairs are cylindrical, and end in sharp hooked points.


                              _Brachyura._

The Brachyura surpass all the other Decapods in compactness and
concentration, and are without exception the highest of the Crustacea.
Though apparently without a tail, they really have one, as their name
implies; but it is short, rudimentary, and folded under the posterior
end of the carapace. The genera and species are exceedingly numerous,
many swim and inhabit the deep oceans, others live on the coasts but
never leave the water; a numerous tribe live as much in the air as in
the water, hiding themselves under stones and sea-weeds on the rocky
coasts, while some dig holes for themselves in the sand, and the land
crabs only come to the sea or to fresh-water lakes to spawn. The
Brachyura have two claws, and are divided into the two chief families of
walking and swimming crabs, according as their posterior pairs of legs
end in a sharp horny nail, or a ciliated lamellar joint.

The great shell or carapace which covers the body varies in form with
the genera; it may be square, oval, or circular, longer than it is
broad, or broader than it is long; it may be straight or beaked between
the eyes; but its lateral edges always extend over the haunches of the
feet. In the Cancri, or walking crabs, of which there are eighteen
genera and many species, the carapace is generally much broader than it
is long, and broader before than behind.

The carapace, or shell, of the common crab is too well known to require
a particular description. The deep lines which indent it correspond with
the limits of the internal organs; the parts between the lines often
bulge very much above the parts occupied by the stomach, heart, gill
chamber, &c., but in the flat crabs these divisions are not so evident.

The compound eyes, which in all the crabs have hexagonal facettes, are
on short jointed stems placed in deep and nearly circular orbits like
cups, so that the stems are scarcely visible. These orbits, whose edges
are sometimes smooth and sometimes notched, are so constructed that the
crab can bend the eye-stems horizontally to the right and left, and the
front of the carapace either conceals the orbit, or forms the eyebrow.

In all crabs the antennæ appear in front between the eyes. The first or
interior pair are short, jointed, and capable of being bent into
cavities, which contain their basal joints; these cavities are near the
eye orbits, with which they are connected in certain species.
Well-developed ears are placed in their basal joints. Fig. 146
represents a magnified ear seen from behind, and Mr. Gosse mentions that
the large eatable crab, whether at rest or feeding, carries these
antennæ erect and elevated, always on the watch, and either vibrating
them, or incessantly striking the water with them in a peculiar jerking
manner.

[Illustration: Fig. 146. Ear of Crab.]

The exterior or lower pair of antennæ are always longer than the
interior pair; sometimes they are simple and similar to them, as in the
flat crabs; and sometimes they have jointed filaments at their
extremities. In all the species they are attached to the under-side of
the crab, and the organs of smell are openings at the point of junction
between their second and third joints. These openings, which lead into
the mouth, are covered by a membrane, and closed by a calcareous lid.
Each lid is fastened by a little hinge to the side of its cavity, and is
opened and shut by muscles fixed at the extremity of a long tendon. Thus
the lower antennæ are the organs of smell, while the upper pair are the
organs of hearing, and both are probably the organs of touch.

The mouth of the crab is on the under part of the head, its lips are
horny plates, and it has a pair of mandibles to cut the food; their
action is from side to side. On each side of the mouth there are two
pairs of jaws, followed by three pairs of foot-jaws; so called because
they are legs modified to serve as jaws, but in some crustaceans they
are also instruments of locomotion or prehension, and sometimes of both.
The two last pairs have palpi, or feelers, at their base. All the jaws
and foot-jaws, when not in use, are folded over the mouth; the joints of
the two last are so broad that they completely conceal this complicated
apparatus.

Posterior to the mouth and its organs there is a flat broad plate, which
forms the ventral side of the body, with a groove in its surface, into
which the rudimentary tail is folded back, as in the Carcinus mœnas (D,
fig. 148), and the feet are fixed by movable joints on each side of this
sternal plate. The first pair, which are a little in advance of the
others, and bend forwards in a curve towards each other, may be called
hand-feet, as they occasionally serve for both. They have very thick
short arms and swollen hands, having a curved finger and a thumb with a
movable hinge, armed throughout their internal edge with a row of blunt
teeth, and terminated by sharp points. The other four pairs, which are
the real walking feet, spread out on each side of the animal, and often
bend a little backwards; they are rather thin, compressed, and end
either in a horny nail, or flattened blade for swimming.

The gills, which are the breathing organs of the crabs and other
Decapods, are spindle-shaped bundles of long, slender, four-sided
pyramids, fixed by their points on each side of the mid line of the
throat, so that they extend in opposite directions, and their spreading
bases fit and rest upon the vaulted sides of the carapace, or rather
gill chambers, to the right and left. Each of the pyramids is formed of
a multitude of parallel membranous cylinders fixed to the axis of the
pyramid, and an infinity of capillary bloodvessels form a network in
their surfaces.

The crab has nine of these bundles of gills in each gill chamber; a few
of them are shown in fig. 147. Each gill chamber has two openings; the
water is admitted by a slit in the base of the claw feet, and ejected by
another into the mouth. But the act of breathing is regulated by a plate
on the second pair of jaws, so connected with the exterior pair of
foot-jaws that, when the crab applies the latter to its mouth, the plate
shuts the slit, the water in the gill chamber is ejected by the mouth,
and in order to admit a fresh supply, the crab must open the foot-jaws
again, so that they are in constant motion. There are plates called
whips on all the appendages of the crab, from the last pair of foot-jaws
to the fourth pair of walking feet inclusive, which ascend and descend
vertically between the bunches of gills to sweep particles of sand or
other foreign matter out of them.

[Illustration: Fig. 147. Section of a Crab.]

The heart of the crab, as in all the Decapods, is placed under the skin
of the back next to the throat; and the blood, which is white or bluish,
flows from the heart through a complicated system of vessels, and,
having nourished the different organs, it is collected in reservoirs at
the base of the gills, is aërated while passing through them, and
returns to the heart again.

The mouth opens through a short gullet into a large globular stomach,
from the walls of which calcareous toothed organs meet in the centre.
One serves as an anvil, while the others bruise the food on it. Some of
the long-tailed crustaceans can evert this apparatus and push it out of
their mouth. The bruised food is liquefied by solvent juices from the
liver and stomach, and the nutritious part enters the bloodvessels by
imbibition.

The nervous system is condensed to suit the form of the crab. An oval
nervous mass with a hole in its centre surrounds the gullet, from each
side of which a nerve extends to a nerve-centre in the head. The organs
of sense are as usual supplied with nerves from the latter, and, from
the circumference of the massy ring, nerves radiate to every part of the
animal, voluntary or reflex, as may be required.

[Illustration: Fig. 148. Young of Carcinus mœnas in different stages of
development:—A, first stage; B, second stage; C, third stage; D, perfect
form.]

Dr. Carpenter has proved, by microscopic observations, that the shell of
the Decapod, in its most complete form, consists of three strata: the
first is a horny structureless layer covering the exterior; the second,
a cellular stratum; and the third is a laminated tubular substance. In
the large, thick-walled crabs, as the Cancer pagurus, the three strata
are most distinctly marked. The tubuli of the lowest layer rise up
through the pigment stratum in little papillary elevations, which give
the coloured parts of the shell a minutely speckled appearance. There
are various deviations from this general plan. In many of the small
crabs belonging to the genus Portunus, the whole substance of the shell
below the structureless horny investment is made up of hexagonal,
thick-walled cells; and in the prawns there are large stellate coloured
cells.

The eggs of the Brachyura are attached by gluten to the false ciliated
feet of the tail of the female, which being bent up under the body forms
a temporary protection till they are hatched. On leaving the egg the
young have not the smallest resemblance to the parent; it is only after
the fourth moult that they even acquire the crab form. When the young of
our common shore crab, the Carcinus mœnas, leaves the egg, it is
scarcely half a line in length. The body is ovoid, the dorsal shield
large and swelled (fig. 148, A). On the middle of its upper edge there
is a long, hollow spine bending backwards, in which the white blood may
be seen to circulate with a sufficient microscopic power. In front there
is a pair of large sessile eyes, and the circumference of the pupils is
marked by radiating lines: behind, there is a long, six-jointed tail,
the last segment of which is forked and spined. On each side of the
shield there is a pair of swimming feet attached to its waved margin.
Fixed also to the margin, but in advance of these, there are three pairs
of jointed feet ending in slender hairs. Immediately in front, between
the eyes, there is a very long compressed appendage, which is bent
backwards between the claws when the animal moves. Under each eye there
is another appendage, shorter and rather more compressed. There are
three pairs of claws, each composed of three joints, and ending in four
long slender hairs: the claws stand at right angles to the body. The
young, when it escapes from the egg, is quite soft, but it rapidly
hardens by the deposition of calcareous matter on its surface. The
progress of the consolidation is shown by the circulation of the white
blood in the hollow dorsal spine. When the creature is yet soft, the
blood globules may be seen ascending to its apex; but, as the
consolidation advances, the circulation becomes more and more limited
till at length it is confined to the base. This creature, whose shield
is sap green and the rest transparent, swims with great activity,
beating the water with his claws and tail. Such is the first stage in
the life of the common shore crab. At this period the young of the
Decapods bear a strong resemblance to one another, whether they are
afterwards to become long or short tailed crustaceans.

After a time this creature loses its activity, moults, and is no longer
to be recognised as the same, so great is the change (fig. 148, B). The
dorsal spine has vanished, the shield has become flatter, its anterior
part pointed, the eyes raised on stalks, and certain rudimentary organs
that were below the eyes now form long antennæ. The first pair of feet
have got hands, the others are jointed and simple, except the last pair,
which are still natatory: with these and with the tail, which is now
much smaller, these creatures swim and congregate round sea-weeds and
floating objects. After the third moult they have the form of a crab,
though neither that of the genus nor species of the parent (fig. 148,
C). The tail is folded under a square carapace, the four pairs of
walking feet spread widely and laterally, while the great hand-feet
attached to the anterior sides of the carapace stretch straightforwards,
the antennæ are short, and the eye-stalks bent to the right and left. It
requires several moults to bring this creature to its final size and
form.[38]

Crabs sometimes die while moulting, and occasionally are unable to
extricate a limb from its shell, and consequently lose it. But if a limb
be fractured they can cast it off at the second joint, and soon after a
diminutive limb is formed, which attains its full size at the next
moult; but if the crab has not strength enough to cast it off, it bleeds
to death.


                               _Anomura._

The Anomura is a family of Decapods intermediate between the long and
short-tailed Crustacea. There are nine or ten genera and many species,
chiefly distinguished by the development of the head and thorax, and the
softness of a non-locomotive tail: of these the Pagurus, or Hermit crab,
is assumed as the type or representative.

The carapace is long and convex, scarcely extending over the basal
joints of the feet. The claw feet are short, with a very broad hand and
sharp pincers; but the Hermit crab and some of its congeners are
irregularly formed; for the last pair of walking feet, instead of being
attached to the thorax, like the others, are fixed to the first part of
the tail, are generally folded over the back, and are employed to sweep
foreign matter out of the gills. The mouth and its masticating organs
are similar to those in the crab, except the exterior pair of foot-jaws,
which are longer and move like feet. But that which distinguishes the
Pagurus and its fellows from every other Decapod is the softness of its
unsymmetrical tail, all the appendages of which are abortive, and the
extremity, instead of ending in a swimming fin, terminates in a pair of
grasping organs. In order to protect this soft-skinned tail, the Hermit
crab folds it up and thrusts it into some old empty shell, clasps the
column of the shell with its grasping organs, draws in the rest of its
body, and covers it with the broad hands folded in such a manner as to
close the mouth of the shell, and to defend itself if attacked. It holds
so fast that it cannot be drawn out; but, when in search of food, it
stretches out its mailed head and legs, and walks off with its house on
its back. However, it sometimes comes out of its shell to feed, and,
like some other crustaceans, it holds its prey with one claw, and tears
it to pieces with the other. They are very pugnacious, and come out of
their shells to die. The larvæ of the Paguridæ undergo transformation,
and they moult when full grown.


                              _Stomapoda._

The Stomapods are all swimmers; they have long bodies with a carapace;
but it is so varied in form and size, that no general description of it
can be given. They have external, instead of internal, organs of
respiration; gills in the form of tufts are in some cases attached to a
few of the foot-jaws, but they are much more frequently fixed to the
basal joints of their swimming feet, so that the blood in their
capillary veins is aërated through their thin skin as they float in the
water. In the Squilla mantis, or S. Desmarestii, members of a genus of
this family, the gills, which are fixed to the basal joint of their last
pair of feet, consist of a long conical tube, on each side of which
there are numerous parallel tubes, like the pipes of an organ, and each
of these has a row of many long cylindrical filaments that drag in the
water. The mouth and its appendages are similar to those of the common
Decapods, with the exception of the anterior jaw-feet, which are of a
singular and formidable structure. They are bent outwards, and their
basal joint is exceedingly large, broad, and compressed; the next joint
is less, with a groove in its side; the third joint is a blade like a
scythe, whose cutting edge is furnished with long pointed teeth. The
Squillæ are carnivorous, and, if any unfortunate animal comes within
their grasp, they bend back the toothed edge of the first joint into the
groove of the second joint like a clasp-knife, and cut it in two. These
prehensile foot-jaws, or ‘pattes ravisseurs,’ are like the fore-feet of
the praying Mantis, and like them weapons of defence.

The genus Mysis, or Opossum Shrimps, have a long straight carapace,
which covers most of the thorax, and folds down on each side so as to
conceal the base of the feet: in front it is narrow, and ends in a
flattened beak; at the posterior end it is deeply scooped out. The two
last rings of the thorax are more or less exposed; the tail is long,
almost cylindrical, tapering to the end, and terminating in a swimming
fin composed of five plates spread like a fan. Both pairs of antennæ
have jointed stems ending, the outer in one, the inner in two very long
many-jointed filaments. On the top of the basal joint of the outer pair
there is a very long lamellar appendage, ciliated on the side next the
joint. Between the second and third joints of the exterior antennæ, Mr.
Spence Bate found the organ of taste: the aperture is simply covered by
a membrane, as in the lobster. The ears are in the last appendage of the
tail.

The Mysis has two pairs of jaw-feet differing little from feet; five
pairs of thoracic feet, all thin and divided into two branches, which
increase in length as they are nearer the tail, and are all provided
with a ciliated appendage to adapt them for swimming. In the female,
broad horny plates, attached to the two last pairs of legs, are bent
under the body so as to form a kind of pouch, destined to lodge the eggs
and the young during the first period of their lives, whence their name,
‘Opossum Shrimps’: the young are crowded in this pouch, and acquire
their adult form before they come into the water. The circulation of the
white blood of the Mysis was discovered by Mr. Thompson: the pulsations
of the heart are so rapid that they resemble vibrations. There are many
species of these small shrimps.

[Illustration: Fig. 149. Lucifer, a stomapod crustacean.]

The genus Lucifer is one of the most singular of the crustaceans from
its almost linear form (fig. 149), the excessive length of the anterior
part of the head, the extreme shortness of the thorax, the smallness of
the carapace or shell, and the great development of the tail, which is
more than three times as long as the thorax. The thin eye-stalks, which
are of exaggerated length, extend at right angles from the top of the
long cylindrical part of the head, and terminate in large, staring,
dark-coloured eyeballs covered with a multitude of facettes. The two
pairs of antennæ are placed between and below the eye-stalks. The
undermost pair, which are the shortest, have a little lamellar appendage
at their base: in some Lucifers, when viewed in front, it looks like a
cross. The salient mouth is placed at the base of the long organ that
carries the eye-stems. It has strong toothed mandibles, two pairs of
jaws with plates attached to each jaw, and three pairs of foot-jaws. The
tail is very narrow, consisting as usual of seven rings movable on one
another; but they are quite abnormal, for each of the rings is at least
as long as the thorax; the last has five plates spreading like a fan.
All the bristly feet, which seem to hang loosely down from the animal,
are fitted for swimming; those of the tail have long ciliated plates in
their basal joints. These creatures are small, and inhabitants of warm
seas.


                              _Amphipoda._

The Amphipods are very numerous, and abound in the British seas. They
have long, slender, and many-jointed bodies which have no carapace: the
tail in some genera is more fitted for swimming, in others for leaping.
The Talitrus, or Sandhopper, common on every sandy shore in Europe, is a
well-known example of the leaping genus. It is very small and
exceedingly active. The upper antennæ are very short, the inferior pair
are large, and longer than the whole body. The anterior feet are thin
and not prehensile. The first pair end in an immovable claw; the second
pair have a kind of hand, and are folded beneath the body; the following
feet end in a crooked nail. The appendages of the last three rings of
the tail are thick and spiny, and the tail serves as a leaping organ.

The sandhoppers hide themselves between tidemarks in large communities
under masses of wet sea-weeds, on which they feed. When disturbed they
leap away with great agility, and bury themselves in the sand by digging
with their fore-feet, and kicking the sand away with their tail-feet.
They have a strong sense of smell, for if a dead fish be buried in the
sand, it is devoured by these little voracious animals in a few days.

In the fin-tailed genera the gills are suspended between the bases of
the thoracic legs: they swim lying on their side, and their feet are
very varied in form, but always more or less furnished with spines and
hairs.

There are several genera of Amphipods that are nest-building animals;
all have hooks at the end of their tails, The Amphithoæ enclose
themselves in a cylindrical tube open at both ends. The animal is very
active, running along the branches of the sea-weeds by means of its
antennæ instead of its feet, which remain within the tube. In general
only the first pair of antennæ are put out to catch prey. If the animal
be prevented from advancing, it immediately turns its body within the
tube, and protrudes its head from the other extremity.


                               _Isopoda._

The order of Isopoda are so called because of the sharp and equal claws
of their walking feet, which are often prehensile. Their body is short
and flattened, and their small head is almost always distinct from the
throat. They are very numerous, and are divided into walking, swimming,
and sedentary animals; the females have horny plates on some of their
feet, which fold under the throat and form a pouch, in which the eggs
are hatched.

The Oniscus, common Wood-louse, or Slater, is a terrestrial Isopod. It
is an oval jointed creature, which rolls itself into a ball when
touched. The second of its six pairs of posterior limbs perform the part
of lungs: they contain hollow organs in their interior, into which the
atmospheric air penetrates directly through openings in their exterior
covering: so the Oniscus and its congeners, which live on land, are
drowned when put into water.

In the swimming Isopods, the five first pairs of tail-limbs are false
feet, and are suspended under the tail. The gills, consisting of two
great oval leaves, are fixed to them by a stalk; and are dragged through
the water. This group is very numerous; many live among the sea-weeds on
the coasts, others perforate submerged wood in all directions, and live
in the winding galleries they have formed. The Limnoria lignorum is
particularly destructive in the harbours on the British coasts, and in
the locks of the canals. The tortuous holes it bores are from the
fifteenth to the twentieth of an inch in diameter, and about two inches
deep. The female Isopod is not more than a line or two in length, the
male is a third less, and of a grey or greenish brown. These minute
creatures bore their holes with their mandibles, which are so sharp and
strong that they can penetrate the hardest wood, and appear to feed on
it, from the quantity found in their stomachs. Their bodies are covered
with pinnated hairs, their antennæ are short, and their posterior end or
tail is rounded.

Most of the genus Cymothea are parasitical; they can bend the sharp nail
of the three first pairs of feet upon the preceding joint, so as to form
hooks with which they fix themselves to the fishes on whose juices they
feed.

The Isopods bear a strong resemblance, an almost identity of structure,
with the Trilobites, a jointed race of Crustaceans long extinct. Some of
the Isopods roll themselves into a ball, as these most ancient
inhabitants of the ocean were wont to do; whose large compound eyes are
exactly like those of the Isopods; whence it was inferred by Dr.
Buckland, that neither the constitution of the sea nor the light of the
sun had changed for innumerable ages. The discovery of the Eozoön has
proved that Nature has not varied during a period immeasurably prior
even to that.


                            _Entomostraca._

The Entomostraca form an immense group of the lower Crustacea,
consisting of five orders. A vast number are just visible to the naked
eye, and many are microscopic; they teem in every climate along the
coasts, and in the deep blue oceans. The horny coat, enclosing the
minute bodies of these animals, is often so transparent that their
internal structure, and occasionally the process of the assimilation of
the food, is distinctly seen by the aid of a microscope. Small as they
are, their beauty is often very great; when transparent they sometimes
radiate all the prismatic colours; when opaque, they are frequently of
the most brilliant and varied hues, others shine with vivid
phosphorescent light. The segments of their bodies are often very
numerous, and similar to one another; but their appendages are very
different. They form two distinct natural groups of the bristly-footed
and gill-footed Crustacea.


                              _Copepoda._

The first order, Copepoda, or oar-footed tribe, have a distinctly
articulated body formed of movable rings, bristly swimming limbs; and
the females carry their eggs in huge pouches suspended on each side of
the posterior part of their bodies.

The Sapphirina fulgens is a beautiful example of the two-eyed tribe; its
body is nearly oval, divided into nine distinct joints, and so flat that
it is almost foliacious. The head has two brilliantly coloured eyes,
with large cornea so connected with the shell that they look like
spectacles. The two pairs of antennæ are silky, and the last pair of
foot-jaws that cover the mouth are garnished with silky plumes. It has
five pairs of swimming feet, and the tail ends in two little plates.

The Sapphirina is about a line and a half long, of a rich sapphire blue,
and floats on the surface of the Mediterranean and tropical oceans. It
shines with the most brilliant phosphorescent colours, passing from deep
blue to a golden green, or splendid purple. The brilliant colouring is
seated in the layer of cells that secrete the firm substance of the
body. With a microscope the cells are seen to pass alternately from one
colour to another. There is a little three-lobed body between the eyes
connected with the central nervous system by a small nerve; it contains
several corpuscules, which Professor Gegenbaur regards as the remains of
the single eye of the larva which undergoes many transformations before
it arrives at its adult form.

According to Professor Gegenbaur, the Sapphirina fulgens is a true
Copepod and the Mediterranean Phyllosoma is a Decapod, although it has a
lacunar blood system.

Some genera of the order Copepoda inhabit salt water, others fresh, as
the Cyclops quadricornis (fig. 150), which abounds in the water with
which London is supplied.

[Illustration: Fig. 150. Female Cyclops:—_a_, body; _b_, tail; _c_,
antenna; _d_, antennule; _e_, feet; _f_, plumose setæ of tail; B, tail,
with external egg-sacs; C, D, E, F, G, successive stages of development
of young.]

The genus Cyclops is a type of the bristly-footed group, distinguished
by a single compound eye placed in the middle of the forehead. The head
and thorax are almost entirely covered with an oval jointed buckler,
which has an opening below to let the bristly limbs pass through (fig.
150); and the tail, which is five-jointed, ends in two plates furnished
with bristly plumes. It is traversed by the intestine, which ends near
its extremity. The brilliant little eye in front consists of a number of
simple eyes placed under one glassy cornea. It rests upon the base of a
cone of muscular fibres, which give it a movement of rotation upon its
centre. Its upper pair of antennæ, situated below the eye, spread to the
right and left. In the female they have numerous joints with a bristle
at each joint; the lower pair of antennæ are short-jointed and bristled.
The mouth of the Cyclops has a pair of jaws, and two pairs of foot-jaws
covered with bristles. The five pairs of branching legs, which are
fitted for swimming, are thickly beset with plumose tufts. In the female
the egg-sacs are hung on each side of the tail (B, fig. 150) by a
slender tube, through which the eggs pass from the ovary within the
mother into the sacs where they are deposited in rows, and there they
remain till hatched. When the larvæ come into the water the sacs drop
off, and the young undergo various changes before coming to maturity, as
shown in fig. 150. The Cyclops swims with great activity, striking the
water with its antennæ, feet, and tail; and the rapid movement of its
foot-jaws makes a whirlpool in the water which brings minute animalcules
to its mouth, and even its own larvæ, to be devoured.

Some species of the Calanus, a marine genus of the one-eyed group, are
eminently social. Professor Dana found that the colour of those vast
areas of what the sailors call bloody water, met with off the coast of
Chili, was owing to shoals of the Calanus pontilla; and another immense
area of bloody water he met with in the North Pacific was owing to a
vast multitude of the Calanus sanguineus. Although this genus abounds
more in individuals in the temperate seas, the species are more varied
in the tropical. Those figured and described in Captain Maury’s works
were mostly microscopic and very beautiful; one fished up was grey with
a bunch of yellow feathers at the end of its tail. The egg-bags were
purple, another was green marked with scarlet tufted antennæ longer than
itself spread out at right angles from its head. This creature shone
with a bright phosphorescent light, visible even when a candle was
burning. These and many more were taken in tropical seas. They were
remarkable for the length of their antennæ; and it was observed that no
eyes were perceptible in such Crustacea as had these exaggerated
antennæ; these organs of intelligence and warning were probably
sufficient for their wants. When animals live without eyes on the
surface of a tropical sea, it is quite conceivable that similar
instruments of touch may suffice for those who live in the dark abyss
below.

The Ostrapods, which form the second order of the bristly-footed
Crustacea, are defended by a bivalve carapace; they have swimming limbs
and a confluent eye; that is, a number of simple eyes placed under a
glassy cornea.

[Illustration: Fig. 151. Cypris.]

The genus Cypris belongs to this group. Several species may be seen
swimming in our streams and fresh-water pools. The body of the common
Cypris (fig. 151) is enclosed between two flat oval shells, united by a
hinge on the back. The little animal can open and shut the valves by
means of two slender muscles, extending from its back to the shells,
which are much curved above and rather flat below. There are two pairs
of antennæ beneath the eye, they are perfectly transparent,
many-jointed, and end in tufts of filaments. One pair projects forward
and then bends gracefully backwards; the other pair are bent downwards.
The mouth has no foot-jaws, and there are only two pairs of feet. Only
one pair is seen in the female, for the other pair is bent upwards to
support the egg sacs. The Cypris attaches her eggs to the leaves of
aquatic plants by a greenish fibre. Not more than twenty or thirty eggs
are deposited by one individual, while the heaps contain several
hundreds; so many females contribute to form one heap. The young are
hatched in the form of their parent in about four days and a half. As
the pools dry up, the Cyprides bury themselves in the sand or mud at the
bottom; if that remain moist they survive, if it becomes dry they
perish; but the eggs remain dormant till the return of rain, when they
are hatched, and the surface of the water is soon crowded with a swarm
of young Cyprides.

[Illustration: Fig. 152. Section of Daphnia pulex.]

The Cladocera is the first order of the gill-footed Crustacea: their
body is defended by a bivalve carapace; they have from four to six
gill-footed limbs, one compound eye, and two pairs of antennæ, one pair
of which is large and adapted for swimming. The Daphnia pulex, or
Arborescent Water-flea, of which fig. 152 is a section, is a common form
of this tribe. It is very abundant in pools and ditches, coming in
groups to the surface in the mornings and evenings in cloudy weather.
The bivalve shell is transparent, flexible, and open below; it ends
behind in sharp toothed peaks. The eye placed in front is moved by four
muscles, and on each side of it are the great antennæ, which are
jointed, branched, and garnished with feathery filaments, and are the
chief organs of locomotion. This animal has no foot-jaws, but it has a
nervous system and a heart, whose pulsations are repeated two or three
hundred times in a minute, and the blood is aërated by gills at the
extremities of six pairs of bristly feet situated behind the mouth, and
only used for respiration and prehension.

The eggs, when laid, are deposited in a receptacle between the back and
the shell of the female Daphnia, and after the young come into the water
they undergo no transformations. Between each brood the Daphnia moults,
and the egg receptacle is thrown off with the exuvia. After several
changes of skin the young Daphniæ come to maturity and lay eggs, which
produce successive generations of females throughout the spring and
summer; but in the autumn males appear, and then the eggs are retained
in the receptacle of the female and are not hatched till spring. If the
female should moult after this, the case with the eggs in it is cast off
with her outer skin, which then becomes a protection to the eggs during
the winter, and they are hatched in spring, producing females.


                             _Phyllopoda._

The second order of gill-footed Crustacea are called Phyllopoda, because
they have gills like the leaves of a book attached to their lamelliform
swimming feet. Their bodies are divided into many segments, and they
form two groups, one of which has a carapace, the other has not. The
Apus cancriformis is an example of the first. It is about two inches and
a half long, and is a large animal compared with the others of its
class. Its head and thorax are covered by an oval carapace, and its
cylindrical body is composed of thirty articulations. It has a compound
movable eye in the middle of its forehead, and a sessile eye on each
side of it. All the members that follow the apparatus of the mouth have
a foliaceous form, and are in constant motion even when the animal is at
rest. The Apus has sixty pairs of jointed legs; the number of joints in
these and in the other appendages is estimated to be not less than two
millions. However, the instruments chiefly used for locomotion are the
first pair of feet, which are very long and serve for oars; with these
the animal can swim freely in any position, but when they are at rest it
floats on the surface of the stagnant water in which it lives, and the
fin feet maintain a constant whirlpool in the water, which brings the
small animals on which it feeds to its mouth.

The Branchipes stagnalis, which may be taken as a type of the second
order, has a perfectly transparent segmented body nearly an inch long,
eleven pairs of pale red gill-feet, antennæ of bluish green, and a long
tail ending in red bristles. The head has two large eyes on movable
stems, and a sessile black oculus between them. Filiform antennæ spring
from the upper part of the head; the other pair, like two large horns,
are turned downwards. The last ring of the swimming tail has two plates
with ciliated appendages.

The Artemia salina differs very little from the Branchipes. It abounds
so much in the brine pans at Lymington and other salt works, as to give
a red tinge to the nearly concentrated brine, the temperature of which
is so high that no other animal could live for a moment in it.


                    _Pycnogonoïdea or Spider Crabs._

Some of the Spider crabs hook themselves to fishes, while others live
under stones, or sprawl with their long hairy legs over sea-weeds, and
feed on the gelatinous matter these weeds afford. The throat with its
members, and the head soldered to its first ring, forms nearly the whole
animal. It has a pair of antennæ and four rudimentary eyes, set on a
tubercule. A proboscis-like projection extends from the front; the mouth
is furnished with cilia and one pair of foot-jaws. Four pairs of long
hairy legs proceed from the throat, spread widely on each side, and end
in a hooked claw. The stomach, which occupies the centre of the animal,
sends off five pairs of long closed tubes like rays; one pair enters the
foot-jaws, the others penetrate the legs. This digesting system is in a
state of perpetual vermicular motion, which, as well as the movements of
the animal itself, aërate its transparent blood through the skin, by
keeping it in circulation. So this insignificant-looking creature has a
very curious and complicated mechanism.[39]


                          _Fossil Crustacea._

Analogues to the Anomura are found in the Chalk formation, but the
Macrura are the prevailing forms. Extinct species of lobster, crawfish,
and shrimps are met with in the secondary strata, from the Chalk to the
Coal measures. In the Coal formation all these higher forms disappear,
but then the gigantic King Crab, or Limulus, is found accompanied by the
minute Entomostracan forms in infinite variety of species.


                   _Epizoa, or Suctorial Crustacea._

The Epizoa infest the skin, eyes, and gills of fishes. Many of them in
their adult state bear a strong resemblance to the lowest of the
Crustacea; but, in general, the resemblance between these two classes of
animals can only be traced during the extraordinary changes which the
Epizoa undergo in their early life, and they differ so much in their
perfect state that it is wonderful any connection should ever have been
discovered between them. The Epizoa are extremely varied in their
perfect forms, and the class generally is supposed to be more numerous
than the whole race of fishes. In the lower orders of the Epizoa the
mouth is suctorial; the higher orders adhere to their victim by jointed
mandibles ending in hooks. The Epizoa are male and female: the male is
small and free, the female is fixed, and generally has a pair of long
egg-sacs hanging from her body.




                             SECTION VIII.

                              CIRRIPEDIA.


THE metamorphoses of the Cirripeds, and their resemblance to the lower
Crustacea at each moult, are still more remarkable than those of the
Epizoa. They form two primary groups, the Balanidæ, or Acorn shells, and
the Barnacles or Lepadidæ, which have peduncles or stalks. Both are
parasites, but they do not draw their sustenance from the substances
they adhere to.

[Illustration: Fig. 153. Balanus culcatus.]

The Balanidæ (fig. 153) are grouped in innumerable multitudes, crowded
together on the rocks of the southern and western coasts of England,
like brown acorns. They have an obscurely articulated body, enclosed in
a membrane, and defended by a multivalve conical shell. The base of the
shell is a broad disk fixed to a foreign substance by a cement secreted
by the animal. The walls consist of twelve triangular compartments. Six
rise upright from the edge of the disk, and end in a point at the open
margin of the shell; the other six are inverted and wedged into the
interstices. The whole cone thus constructed is divided into from four
to eight pieces by expansive seams. The mouth of the cone is closed by a
lid formed of four triangular valves, which meet in a point in the
centre, and shut in the creature.

Six pairs of long, slender, curly feet rise from the throat of the
animal, and bend over the prominent mouth, which is placed at the bottom
of a kind of funnel, formed by the divergence of these six pairs of
thoracic feet. It is furnished with a broad upper lip, two palpi, and
three pairs of jaws, of which the outermost are horny and toothed, the
innermost soft and fleshy. Each foot is divided into two similar
many-jointed branches: the shortest pair is nearest to the mouth, the
others increase gradually in length and number of joints to the most
distant (fig. 154). Mr. Gosse estimated that, in a specimen he
possessed, the whole apparatus included nearly five hundred distinct
articulations. Since each joint is moved by its own system of muscles,
the perfection of the mechanism may be conceived. But it is as sensitive
as flexible, for every separate joint is furnished with a system of
spinous hairs, which are no doubt organs of touch, since the whole of
the branches are supplied with nerves. These hairs, which extend at
somewhat wide angles from the axis of the curling filaments, are barbed,
for they have numerous projections, or shoulders, surrounded by whorls
of microscopic hairs.[40]

[Illustration: Fig. 154. Tentacles or feet of the Balanus.]

This beautiful and complicated structure is the fishing apparatus of the
animal, which it is continually pushing out and drawing in through the
valved lid of the shell. When the whole is thrown out it is widely
spread, and the filaments uncurled; then, as they close again, the
innumerable hairs meet and form a sieve through which the water escapes,
but whatever minute particles it may contain are inextricably entangled,
and when the small animals fit for food have been selected, the
filaments curl inwards, and carry them to the mouth; there they are
seized by the jaws and sent through a short gullet to be digested.

The feet and cirri are moved by very strong muscles, the valves of the
lid are opened and shut by muscles attached to the mouth of the shell;
and when the animal wishes to protrude its cirrhated feet, the
longitudinal muscles attached to the lid come into action, and it draws
itself in again by short muscles attached to the base. All the organs of
the animal are supplied with nerves by a double nerve-centre in the
head, and a circle of nerve-centres round the gullet. The ears are
situated at the base of the first pair of cirrhated feet, and consist of
a cavity enclosing a vesicle closed by a nerve, and containing a liquid,
but no otolites.

[Illustration: Fig. 155. Section of Lepas anatifera.]

The common Lepas anatifera, of which fig. 155 is a section, as well as
its allies, have a thick stem and a conical shell closed on the back,
but gaping in front. Their internal structure does not differ
essentially from that of the Balanidæ, and it has been proved by Mr. J.
V. Thompson and others, that there is no material difference between
their transformations during the early stages of their lives.

Each individual cirriped is both male and female, and the eggs are
hatched before they come into the water. Mr. Gosse mentions that he had
seen the Balanus porcatus throw out a dense column of atoms from the
mouth of its shell for several successive days; each column was composed
of thousands of active microscopic creatures, bearing a strong
similarity to the young of the Cyclops Crustacea. A, fig. 156,
represents one of these creatures. Its body is enclosed in a carapace
with a pair of flexible organs like horns, six swimming feet, and a very
black eye deeply set in front. The creature swims and sometimes rests,
but never alights on anything. After some changes this creature takes a
form whose front is represented at B, and its side by C, fig. 156. It is
larger, more developed, and swims with its back downwards.

[Illustration: Fig. 156. Development of Balanus balanoïdes:—A, earliest
form; B, larva after second moult; C, side view of the same; D, stage
preceding the loss of activity; _a_, stomach; _b_, nucleus of future
attachment.]

A new series of transformations changes this embryo into the form
represented by D, fig. 156, which is closely allied to the Daphnia
pulex, or Water Flea. The body is enclosed between two flat oval shells,
united by a hinge on the back, and is capable of being opened in front
for the protrusion of a pair of prehensile limbs; and six pairs of
swimming feet cause the animal to swim by a succession of bounds.
Instead of the single eye, it has now two raised on pedestals, attached
to the anterior part of the body.

[Illustration: Fig. 157. Lepas.]

This animal having selected a piece of floating wood for its permanent
abode, attaches itself to it by the head, which is immovably fixed by a
tenacious glue exuded from glands at the base of the antennæ. The
bivalve shell is subsequently thrown off, a portion of the head becomes
excessively elongated to form the peduncle of the Barnacle or Lepas, and
in that state it is exactly like the Lucifer Stomapod. In the Balanus,
on the contrary, the head expands into a broad disk of adhesion, and the
animal resembles the Mysis or Opossum Shrimp.

From the first segment of the throat a prolongation is sent backwards
which covers the whole body, and the outer layer is converted into the
multivalve shell; and the three pairs of cirrhated feet, which were
formed in the larval state, now bend backwards from the other three
rings of the throat.

Though the Cirripeds lose their eyes in their mature state, they are
sensitive to light. They draw in their cirrhated feet, and the Balanus
even closes the lid of its shell under the shadow of a passing cloud.




                              SECTION IX.

                          BRYOZOA, OR POLYZOA.


A BRYOZOON is a microscopic polype, inclosed in an open horny or
calcareous sheath, out of which it can protrude and draw in the anterior
part of its body. It is seldom or never seen alone, on account of its
tendency to propagate by budding. When the buds spring from the sides of
the sheath or cell, it is known as the Sea Mat, or Flustra. The Flustra,
which is common on our coasts, spreads its hexagonal cells like a
delicate network over sea-weeds, shells and other marine substances.
Sometimes the polypes are so closely arranged on both sides of a leaf
that a square inch may contain 1,800. In the calcareous genera, Eschara
and Cellipora, the cells have a lid movable by two muscles, so that the
polypes can close the orifice, and shut themselves in.

In the greater number of the Polyzoa the polype has a cylindrical form,
a mouth at its anterior extremity surrounded by an annular disk, which
forms the roof of the internal cavity containing the stomach and the
other digestive organs. The disk is furnished with eight, ten or a
greater number of tubular tentacles, which surround the mouth, their
tubes being continuations of the internal cavity below. The mouth leads
into a funnel-shaped space, separated by a valve from the gullet; and
the gullet ends in a capacious stomach. Short vibratile cilia are
arranged like a fringe on the opposite sides of each tentacle, which
form two currents in the surrounding water—an ascending stream on the
outside, and a descending one on the inside. When any particles of food
that may be carried down the inner surface of the tentacles arrive at
the mouth, a selection is made, the rejected particles being carried off
by the stream, while those that are chosen are received by the
funnel-shaped mouth, and pass through a valve in the gullet into the
stomach, where they are kept in continual motion by cilia, and the
refuse is ejected by an orifice near the mouth.

[Illustration: Fig. 158. Cells of Lepraliæ.—A, L. Hyndmanni; B, L.
figularis; C, L. verrucosa.]

[Illustration: Fig. 159. A, Cellularia ciliata; B, ‘bird’s head’ process
of Bugula avicularia, highly magnified, seizing another.]

Fig. 158 represents the cells of different species of the genus
Lepralia, which form crusts upon marine objects. Other genera grow as
independent plant-like structures, and some take an arborescent form,
and creep over rocks and stones. The Cellularia ciliata, of which fig.
159 is a magnified portion, rises in upright branching groups like
little shrubs; and as many are commonly assembled together, they form
miniature groves, fringing the sides of dark rocky sea pools on our
coasts. Most of the Polyzoa have pedicellariæ attached to their stems,
either sessile or stalked. Their forms are various: a jointed spine, a
pair of pincers, &c. But on the Cellularia they are like a bird’s head
with a crooked beak, opening very wide, and attached by a stem. B, fig.
159, represents a highly magnified pedicellaria in the act of seizing
another. These bird’s head appendages are numerous on the Cellularia
ciliata, and in constant motion; the jaws are perpetually snapping
little worms, or anything that comes in their way, while the whole head
nods rhythmically on its stalk. Two sets of muscles move the jaws; when
open, a pencil of bristles projects beyond them, which is drawn in again
when they are closed; they are supposed to be organs of feeling. The
Polyzoa have organs also called vibracula, which are bristle-shaped, as
on A, fig. 158; these sweep over the surface of the Polyzoon to remove
anything that might injure the polypes.

It is believed that the polypes of the Polyzoa are male and female, and
that the ciliated locomotive larvæ which appear in spring are developed
from eggs. The fresh-water Polyzoa are as worthy of microscopic
examination as the marine.




                               SECTION X.

                        TUNICATA, OR ASCIDIANS.


THE form of the Tunicata is irregular. They have two orifices—one at the
top, for the entrance of a current of water, and another at one side for
its egress. They have two tunics only adhering to one another at the
edges of these orifices, which are furnished with a circle of cilia. The
irregular or scattered condition of the nerve-centres, as well as the
alternation in the circulation, are eminently characteristic of the
whole class. They consist of three distinct groups: the compound or
social gelatinous Ascidians; the solitary Tunicata, with leathery coats;
and the Salpæ, which are gelatinous. The two first, though mobile when
young, become fixed when they arrive at maturity; the third floats free
on the surface of the ocean.

[Illustration: Fig. 160. Magnified group of Perophora.]

The fixed gelatinous Ascidians resemble the Polyzoa in structure and
tendency to gemmation; nevertheless, they differ in their circulating
and respiratory systems. The Perophora Listeri is an example which is
found on the south coast of England and Ireland (fig. 160.) It consists
of minute globes of clear jelly, not larger than a pin’s head, spotted
with orange and brown, and attached by a foot-stalk to a silvery stem
like a thread which stretches over the surface of stones, or twines
round the stalks of sea-weeds; and as the stem increases in length, buds
spring from it, which in time come to maturity, so that the silvery
thread connects a large community; but, though thus connected, every
member has its own individuality. Fig. 161 represents one of these
transparent individuals very highly magnified.

[Illustration: Fig. 161. Highly magnified Perophora.]

The respiratory sac occupies the upper part of the body. It is
perforated by four rows of narrow slits, edged with cilia, whose
vibrations are distinctly seen through the transparent tunic of the
little animal. A portion of the water which is drawn by the cilia into
the upper orifice or mouth, passes into the respiratory sac, escapes
through the narrow slits into the space between the sac and the tunic,
and from thence into the stomach, where any particles of food it may
bring are digested, and the refuse is carried by the current through the
intestinal canal, and ejected at the lateral orifice.

The heart is a long multiform muscle, attached to the respiratory sac,
from whence capillary vessels spread over that sac and throughout the
body. The pulsations of the heart drive the blood through the general
system, and bring it back to the heart again. After a time the pulses of
the heart become faint, and the blood ceases to flow. A short pause
takes place, the heart gives an opposite impulse, and the blood makes
its circuit in a direction exactly contrary to what it did before. The
circulation in all these little globes is brought into connection by a
simultaneous circulation through two tubes in the silvery thread to
which they are attached.

The average duration of the ebb and flow of the blood is probably the
same, but the period between the changes varies from thirty seconds to
two minutes. As the blood is colourless and transparent, it probably
would have been impossible to determine its motion had it not been for
solid particles floating in it.

The larva of the compound sessile Ascidians is like the tadpole of a
frog, which swims about for a time; it then fixes itself by the head to
some object, the tail falls off, and in a few days it becomes a solitary
Ascidian, with its two orifices and currents of water. This solitary
animal gives origin by budding to a connected group, which in its turn
lays fertilized eggs, so that there is an alternation of generations.

The Botryllidæ or Star-like Ascidians, appear as masses of highly
coloured gelatinous matter, spread over stones or fuci in which from ten
to twenty minute oblong Ascidians are arranged in a circle round a
common open centre which is the discharging orifice of the whole group,
for the mouth of each individual is at the opposite extremity. The only
indication of life given by this compound creature is the expansion and
contraction of an elastic band surrounding the discharging orifice. The
organization of each of these individuals is similar to that of the
Perophora.

Although many Tunicata form composite societies, the most numerous and
largest in size are always solitary, as the Ascidia virginea (fig. 162).
Its outer tunic contains cellulose, it is pale and semitransparent, the
inner tunic is orange-coloured or crimson. These creatures vary in
length from one to six inches: therefore they are not microscopic, yet
their internal structure, which is similar to that described, cannot be
determined without the aid of that instrument. The organ of hearing is a
capsule containing an otolite and coloured spots placed between the
orifices; the uppermost orifice or mouth is surrounded by eight
eye-specks, and six of a deep orange colour surround the lateral one, a
nerve-centre between the two supplies the animal with nerves. These
Tunicata live on diatoms and morsels of sea-weeds, and, like all the
fixed Ascidians, they show no external sign of vitality except that of
opening and shutting the two orifices. More than fifty species of these
solitary Ascidians inhabit the British coasts from low-water mark to a
depth of more than one hundred fathoms.

[Illustration: Fig. 162. Ascidia virginea.]


                             _Pyrosomidæ._

The Pyrosomidæ are floating compound Ascidians, composed of innumerable
individual animals united side by side, and grouped in whorls so as to
form a hollow tube or cylinder open at one end only, and from two to
fourteen inches long, with a circumference varying from half an inch to
three inches. The inhalent orifices of the component animals are all on
the exterior of the cylinder, while the exhalent orifices are all on its
inside, and the result of so many little currents of water discharged
into the cavity is to produce one general outflow which impels the
cylinder to float with its closed end foremost. The side of each animal
in which the nerve-centre is placed is turned towards the open end of
the cylinder, the whole of which is cartilaginous and non-contractile.
Each of the Ascidians forming this compound creature has its outer and
inner tunic united and lined with a vascular blood system, a respiratory
cavity of large size completely enclosed by a quadrangular network, and
digesting organs. The sexes are combined, and they are propagated by
buds and single eggs. The Pyrosomidæ are gregarious and highly luminous;
vast shoals of them extend for miles in the warm latitudes of the
Atlantic and Pacific Oceans, and as soon as the shade of night comes on
they illuminate ships with bright electric flashes as they cleave the
gelatinous mass; half a dozen of these animals give sufficient light to
render the adjacent objects visible. The intensity depends upon muscular
excitement, for Professor Fritz Müller observed that the greenish blue
light of the Pyrosoma Atlantica is given out in a spark by each of the
separate individuals; it first appears at the point touched, and then
spreads over the whole compound animal. This species appears in such
aggregations in the Mediterranean as to clog the nets of the fishermen.


                               _Salpidæ._

The Salpidæ are another family of free-swimming Ascidians. The tunic is
perfectly hyaline, the body is somewhat cylindrical, but compressed and
open at both ends (fig. 163). The mouth is a slit, the discharging
orifice is tubular and can be opened and shut. The breathing apparatus
is in the form of a ribbon extending obliquely across the cavity of the
tunic, the ear with four otolites is in the ventral fold, and the flux
of the pale blood is alternate as in other Tunicata.

[Illustration: Fig. 163. Salpa maxima.]

[Illustration: Fig. 164. Young of Salpa zonaria.]

The Salpidæ are produced by alternate generation. A solitary floating
Salpa is always found to contain a chain of embryos joined end to end
winding spirally within her. They are all of one size, and portions are
liberated in succession through an aperture in the tunic. In a little
time these connected larvæ are developed into a chain of adult Salpæ.
The individuals are from half an inch to several inches long, according
to the species, and when joined end to end the chain may extend many
feet, but the attachment is so slight that they often break up into
shorter portions. The chains swim with an undulating serpentine motion
either end foremost by the simultaneous expulsion of water from the
muscular tunic of each individual. A single egg is formed by each of
these creatures, which remains within the parent till a solitary Salpa
is hatched, and then it comes into the water, and after a time produces
a chain of larvæ.

The aggregate young of the Salpa zonaria, instead of being united end to
end, are applied side to side, and as the individuals are broad at one
extremity and narrow at the other, they constitute groups continually
diminishing in size, which take a spiral form.

The reproduction of the whole genus of Salpidæ is rapid and enormous.
Dr. Wallich mentions that while sailing between the Cape of Good Hope
and St. Helena, the ship passed for many miles through water so crowded
with the Salpa mucronata that it had the appearance of jelly to
apparently a great depth. The Salpæ, which were from one to two inches
long, had yellow digestive cavities, about the size of a millet seed,
which contained diatoms, Foraminifera, Polycystinæ, small shrimps, and
other microscopic creatures.




                              SECTION XI.

                               MOLLUSCA.


ALTHOUGH the Mollusca do not come within the limits of this work they
nevertheless afford objects worthy of microscopic investigation. The
gills of a bivalve mollusk are like crescent-shaped leaves fixed by
their stalks to the transverse extremities of the mantle, so that the
greater part floats freely in the water.

To the naked eye the gills appear to be formed of radiating fibres of
admirable structure; but the microscope shows that each leaf consists of
a vast number of straight transparent and tubular filaments, arranged
side by side so close that 1,500 of them might be contained in the
length of an inch. These filaments, however, apparently so numerous, in
fact consist of only one exceedingly long filament in each gill, bent
upon itself again and again throughout its whole length, both at the
fixed and free ends of the leaf. These long filaments are fringed on
both sides by lines of cilia continually vibrating in contrary
directions. By this action a current of water is perpetually made to
flow up one side of the filaments and down the other, so that the blood
which circulates in their interior is exposed throughout their long
winding course to the action of oxygen in the water. The duration of
these vibrations in the mollusca is marvellous. The cilia on a fragment
of a gill put into water by Mr. Gosse fifteen hours after the death of
the mollusk caused a wave to flow uniformly up one side of the filaments
and down the other. Even twenty-hours after the death of the animal the
ciliary motion was continued on such parts as were not corrupted, a
remarkable instance of the inherent contractility of the animal tissues.

The refined mechanism of the gills of the common Mussel enables it to
live when attached to rocks above high-water mark, so as only to be
immersed at spring tides. By the movements of cilia, water is retained
in the gill-chamber, which derives oxygen from the atmosphere, and
animalcula supply the Mussel with food.

[Illustration: Fig. 165. Cardium or Cockle.]

The mollusks that burrow in sand or mud have two tubes fringed with
cilia, which they protrude into the water above them. The water which is
drawn into one of these tubes by the action of the cilia passes in a
strong current over the gills, aërates the blood, brings infusorial food
for the animal, and is expelled in a jet from the other tube. The foot
at the other extremity of the shell is the organ with which the mollusk
makes its burrow in sand, clay, chalk, stone or wood.[41]

The common Cockle digs into the sand, and uses its foot both for digging
and leaping; it is cylindrical, and when the Cockle is going to leap, it
puts out its foot and bends it into an elbow; then having fixed the
hooked point firmly in the sand, by a sudden contraction of the muscles
it springs to a considerable height and distance, and leaps actively
along the surface of the sand. The lowest part of fig. 166 is a
magnified section of the foot, showing the muscular system which gives
the animal that power. It consists of many rows of longitudinal muscles,
interlaced at regular distances by transverse fibres. When the foot is
extended, the Cockle has the power of distending it by filling a network
of capillary tubes with water till it is almost transparent. The water
is also distributed through the body and into the gill-chamber, which
opens and shuts every ten minutes or oftener, in order to maintain the
supply; and it has egress through the pores in the mantle and foot, for
some burrowing mollusks squirt it out through the foot when disturbed.
This water-system is unconnected with the circulation of the blood.

[Illustration: Fig. 166. Foot of Cockle.]

Each bivalve mollusk is both male and female; and the fertilized eggs
pass into the gills of the parent, where they undergo a kind of
incubation. At a certain time the yellow yolk of the egg is divided into
a granular mass, which separates from the liquid albumen and produces
cilia. The cilia cause the albumen to revolve round the interior of the
egg; at last the granular mass revolves with it, while at the same time
it rotates about its axis in a contrary direction at the rate of six or
eight times in a minute. When still in the egg, all the organs of the
little embryos are formed in succession, even the little valves of the
shells are seen to open and shut, but the embryos are hatched before
they leave the parent, and swim about in the cavity of the external
gill.


                       _Shells of the Mollusca._

When these mollusks come into the water, they soon find their
transparent white shell too small, and begin to increase its size by
means of the mantle, which is an exquisitely sensible fleshy envelope
applied to the back of the animal, extending round its sides like a
cloak, only meeting in front, and it is for the most part in close
contact with the whole interior of the shell. Its edges are fringed with
rows of slender tentacles, and studded with glands, which secrete the
colours afterwards seen in the shell; the glands in the rest of the
mantle secrete only colourless matter.

When the animal begins to enlarge its shell, it attaches the borders of
the mantle to the margin of the valves, secretes a film of animal
matter, and lines it with a layer of mucus containing carbonate of lime
and colour in a soft state, which soon becomes hard, and is then coated
internally by the other glands of the mantle with colourless carbonate
of lime.

The two strata thus formed, one richly coloured, the other white, often
nacreous and brilliantly iridescent, are highly organized substances.
Examined with the microscope, they present remarkable varieties in some
of the natural groups of bivalve Mollusca; the structure of the
Monomyarian Oyster is characteristic of the division which has but one
muscle; the Dimyaria, having two muscles, are represented by the Cockle.

[Illustration: Fig. 167. Section of shell of Pinna transversely to the
direction of its prisms.]

[Illustration: Fig. 168. Membranous basis of the shell of Pinna.]

The exterior laminæ at the edge of the fragile valves of a Pinna are
often so thin and transparent that the organization of the shells may be
seen with a low magnifying power. A fragment has the appearance of a
honeycomb on both surfaces (fig. 167), whereas its broken edge resembles
an assemblage of basaltic columns. The exterior layer of the shell is
thus composed of a vast number of nearly uniform prisms, usually
approaching to the hexagonal structure, whose lengths form the thickness
of the lamina, their extremities its surfaces. When the calcareous part
of the lamina is dissolved by dilute acid, a firm membrane is left,
which exhibits a hexagonal structure (fig. 168), as in the original
shell; but it is only in the shells of a few families of bivalves nearly
allied to the Pinna that this combination of the organic and mineral
elements is seen in this distinct form; it is beautifully displayed in
the nacreous shells.

[Illustration: Fig. 169. Section of nacreous lining of the shell of
Avicula margaritacea (pearl oyster).]

In many shells the internal layer has a nacreous or iridescent lustre,
shown by Sir David Brewster to depend upon the striation of its surface,
by a series of nearly parallel grooved lines. When Dr. Carpenter had
dissolved the calcareous matter from a thin piece of nacreous substance,
taken from the shell of the Haliotis splendens, or Ear Shell, there
remained an iridescent membrane, which presented to the microscope a
series of folds or plaits somewhat regular, and splendidly iridescent,
but when the plaits were unfolded and the membrane stretched, the
iridescence vanished. So the varied hues of mother-of-pearl are owing to
the folds of an organic membrane.

The shells of the Gastropoda, or crawling mollusks, have a structure
peculiar to themselves, but by no means so much varied as that of the
bivalve class. The Strombus gigas, or Queen Conch, the Cassis, or Helmet
Shell, and the beautiful porcellanous Cyprææ or Cowries, are much valued
by the artists who cut cameos, on account of the structure of their
shells, which consists of three strata, the same in composition, but
differing in arrangement, and sometimes in colour. Each stratum of the
shell is formed of many thin laminæ, placed side by side, perpendicular
to the plane of the stratum, and each lamina consists of a series of
prismatic spicules with their long sides in close approximation; the
laminæ of the inner and outer strata have their spicules parallel to one
another, while the spicules of the intermediate lamina are perpendicular
to those on each side. According to Dr. Bowerbank, who discovered this
complicated structure, the spicules are microscopic tubes filled with
carbonate of lime.

The Spondylus gædaropus has sixty ocelli constructed for accurate
vision. One can form no idea of the effect of so many eyes, unless they
combine to form one image as our eyes do. The common Pecten, or Scallop,
pretty both in form and colour, has a number of minute brilliant eyes
arranged along the inner edge of the mantle, like two rows of diamond
sparks. Some families of mollusks are destitute of eyes, even of the
simplest kind; and it has been observed that those mollusks most
liberally provided with eyes are also endowed with the most active and
vigorous motions. The bivalves do not appear to have either taste or
organs of hearing, but they are exceedingly sensitive to touch. It is
singular that animals which have neither head nor brain should have any
senses at all. A nerve-collar round the gullet with a trilobed
nerve-centre on each side supplies the place of a brain; nerves extend
from these; besides there are nerve-centres in various parts of the
unsymmetrical bodies of the acephalous mollusks.

The Gastropoda, or crawling mollusks, have a head, and are consequently
animals of a higher organization than the Conchifera or bivalve class.
Their mantle forms a vaulted chamber over the head and neck, and
envelopes the foot or crawling-disk; all these the animal can protrude
or draw in at pleasure. The head is of a globular form, with two or four
exceedingly sensitive tentacles, arranged in pairs on each side of it,
as in the garden snail, which has four, two long and two short. These
tentacles, which the snail can push out and draw in at pleasure, are
hollow tubes, the walls of which are composed of circular bands of
muscle. The tentacles are pushed out by the alternate contractions of
these circular bands, but they are drawn in again like the inverted
finger of a glove by muscular cords proceeding to the internal extremity
of the tentacle from the muscle that withdraws the foot. The structure
of the tentacles is the same in all the crawling mollusks; they are most
sensitive in the Helix or Snail family, but they are believed to be
delicate organs of touch in all.

The Gastropod mollusks never have more than two eyes, either placed on
the tips, or at the base of one pair of tentacles; in the snail they may
be seen as black points on the tips of the longest pair. In some of the
higher Gastropods they are of great beauty, and appear to be perfectly
adapted for distinct vision. Organs of hearing were discovered by Dr.
Siebold at the base of one of the pairs of tentacles, consisting of
vesicles containing a liquid and calcareous otolites, which perform
remarkable oscillations due to the action of vibratile cilia. In the
Snail and Slug group the number of otolites varies from eighty to one
hundred.

The mouth of a Gastropod is a proboscis, with fleshy lips, generally
armed with horny plates or spines on the jaws. The Snail has a
crescent-shaped cutting plate on its jaw, and a soft bifid lip below,
but the tongue is the most remarkable microscopic object in this group
of Mollusca. In the terrestrial Gastropods, it is short and entirely
contained within the nearly globular head. It is tubular behind, but in
front it is spread into a nearly flat narrow plate, traversed by
numerous rows of minute recurved teeth, or spines set upon flattened
plates; in the Garden Snail or Slug each principal tooth has its own
plate. Fig. 170 represents a magnified portion of a Snail’s tongue by
Dr. Carpenter; the rows at the edge are separated to show the structure.
The teeth are set close one to another, and are often very numerous. In
the Helix pomatia, a snail found in the middle and southern counties of
England, they amount to 21,000, and in the great slug (Arion ater),
there are 26,800. This kind of tongue only serves for rasping vegetable
food. All the Trochidæ, which are marine mollusks that are supposed by
some naturalists to live on fuci, are remarkable for the length and
beauty of their narrow spiny tongues. Fig. 171 is a small portion of the
tongue or palate of the Trochus zizyphinus, highly magnified; the large
teeth of the lateral bands, as well as the small teeth in the centre,
have minutely serrated edges. Fig. 172 shows the Trochus granulatus in
the act of crawling.

[Illustration: Fig. 170. The tongue of Helix aspersa.]

[Illustration: Fig. 171. Palate of Trochus zizyphinus.]

[Illustration: Fig. 172. Granulated Trochus.]

The Limpet lives on sea-weeds. The animal is large in proportion to the
size of its conical shell; it has a long leaf-shaped gill under the edge
of the mantle. The head has a short proboscis and pointed tentacles,
with eyes at their base. The mouth has a horny jaw and a very long
tongue, moved by muscles rising from firm objects on each side of it.
Fig. 173 represents the tongue beset with recurved hooks, and A shows a
portion highly magnified. These recurved teeth are transparent,
amber-coloured, and in the Limpet, as in most of the other Gastropods,
they are chitinous. The teeth towards the point of the tongue are
sufficiently hard to rasp the food; and it is said that when they are
worn down, the part of the tongue supporting them falls off, and that
the waste is supplied by a progressive growth of the tongue from behind,
and a hardening of the teeth in front. The soft reserved portion is
coiled in a spiral when not in use.

[Illustration: Fig. 173. Tongue of Limpet:—A, portion of surface
magnified.]

All the species of Patellidæ, or Limpets, have the power of making
cavities with their foot in the surface of the rocks to which they
adhere. The cavity exactly corresponds in shape and size with the mouth
of the shell, which is sunk and very strongly glued into it, yet the
Limpet dissolves the glue with a liquid secretion, roams in quest of
food, and returns again to its home: both fluids are secreted by a
multitude of glands in the foot, which is the instrument of adhesion.

The tongue of the carnivorous Gastropods is a very formidable weapon,
used for boring holes in the hardest shells. The round holes in dead
shells frequently met with on our coasts show that their inhabitants had
fallen a prey to some of these zoophagous Mollusks. The tongue of these
predatory Gastropods is a narrow mechanical file, sometimes twice or
even three times the length of the whole animal, and when not in use it
is curled up near the foot. It is spined in various microscopic patterns
according to the genus, and is supported by two firm parts from whence
the muscles spring that work the rasp.

The Periwinkles have a ribbon-shaped tongue, rough with hooked teeth;
the Scalariæ have also predatory tongues, but of all the Gastropod
mollusks, the Whelk and its numerous allies are the most predacious. The
Purpura or Dog Whelk especially is the most ravenous of mollusks. Its
long tongue is armed with hooked and spined teeth, placed three in a
row; with this weapon and a proboscis capable of boring, they have been
known to exterminate a whole bank of Mussels.

The Common Whelk is represented in fig. 174. When in the act of
crawling, its head, with two tentacles, is at one extremity, its foot at
the other, sometimes used as an organ of prehension; and it has a siphon
for carrying water to the gills at the end of the shell.

All the families of the naked mollusks or Sea Slugs, furnish beautiful
objects for the microscope. The two sexes are united in the same
individual, and in their embryonic state they have a shell, which is
cast off long before they come to maturity. The gills placed on the
naked body are capable of being withdrawn into a cavity in the medial
line of the back, and are either plumose, or like the leaf of a plant
pinnated again and again, but they vary in form and position in the
different genera.

[Illustration: Fig. 174. Whelk.]

[Illustration: Fig. 175. The Crowned Eolis.]

In the group of the Eolidæ, the gills are like leafless trees in most
genera, but in the principal genus Eolis, they are long, spindle-shaped,
sharp-pointed papillæ, arranged in transverse rows or clusters along the
sides of the back, leaving a space between them, as in fig. 175. They
are covered with long cilia, whose vibrations send a perpetual current
of sea-water along each of them, the respiration is aided by vibrating
cilia, scattered almost over the whole body, and the circulation of the
blood is very simple.

[Illustration: Fig. 176. Tongue-teeth of Eolis coronata.]

The Eolis has a head prolonged into a pair of tentacles which are active
and as sensitive as antennæ. Another pair on the back have ten or twelve
narrow plates twisted in a spiral round them; the eyes are at the base
of these horns. The mouth contains horny jaws and a spiny tongue like a
mere strap covered by numerous transverse plates armed with recurved
spines not more than a sixth part the thickness of a human hair. Fig.
176 represents the tongue and some of the spines greatly magnified. The
mouth leads into a short and large membranous stomach, from each side of
which branches are sent off, from whence long canals traverse the
papillæ longitudinally, and perform the part of a liver. In many species
these tubes are brilliantly coloured, but none are more beautiful than
those in the Eolis coronata, which is found under stones, like a mass of
jelly, not larger than a pea, at low spring tides, on our own coasts.
When put into sea-water it expands till it is about an inch long (fig.
175). It is then pellucid, tinged with pink, and the central tubes in
its numerous papillæ are of a rich crimson hue, their surface reflects a
metallic blue, and their tips are opaque white; as the animal keeps its
papillæ in constant motion the effect is very pretty.

The Eolis coronata, like all its congeners, has a stinging apparatus,
consisting of an oblong bag, full of thread cells, placed at the
extremity of each papilla, from whence darts can be ejected through an
aperture in the tip. The whole of the Eolididæ are carnivorous, fierce,
and voracious, setting up their papillæ like the quills of a porcupine
when they seize their prey; they tear off the papillæ of their weaker
brethren, and even devour their own spawn, though their chief food
consists of zoophytes.

The Pteropoda, or wing-footed mollusca, are very small; they are
incapable of crawling or fixing themselves to solid objects, but they
are furnished with two fins like the wings of a butterfly, with which
they float or row themselves about in the ocean, far from land in vast
multitudes. The shell of the typical species Hyalæa (A, fig. 177), which
resembles the thinnest transparent glass, consists of two valves, one,
which is placed on the front of the animal, is long, flat, and ends in
three points; the other valve, which is applied to the back, is short
and convex, and in the lateral fissure between the two, the mantle is
protruded. The head and fins project from an opening at the top of the
shell. The fins, which are formed of muscular fibre, are fixed on a
short thick neck, with the mouth lying between them, containing a tongue
crossed by rows of long reversed teeth. The head has no tentacles, and
the animal appears to be blind, but it has an auditory vesicle lined
with cilia, which keeps a few otolites in motion. This little animal is
highly organized; it has a gullet, a kind of crop and gizzard, a liver,
a respiratory tube, a heart, a circulating and nervous system, which
enables it to swim with a flapping motion of its fins.

[Illustration: Fig. 177. A, Hyalæa; B, Clio.]

The Clio pyramidata (B, fig. 177) is an elegant animal belonging to the
same class. Its fragile transparent shell has the form of a triangular
pyramid; and from its base proceeds a slender spine, and a similar spine
extends from each side of the middle of the shell. The posterior part of
the body is globular and pellucid, and in the dark it is vividly
luminous, shining through the glassy shell. The fins of the Hyalæa and
Clio or Cleodora are of a bright yellow, with a deep purple spot near
the base. Both are inhabitants of the ocean.

[Illustration: Fig. 178. Clione borealis.]

The Clione borealis (fig. 178), which exists in millions in the Arctic
Seas, is the most remarkable instance of the Naked Pteropods. It has
neither shell nor mantle; its membranous body is not more than half an
inch long, its head is formed of two round lobes, on each side of the
neck there is a large muscular wing or fin; in swimming the animal
brings the tips of the fins almost in contact, first on one side of the
neck and then on the other. In calm weather, they come to the surface in
myriads, and quickly descend again. There is a pair of slender tentacles
close to the head, which are organs of feeling, a pair of eyes are
placed on the back of the neck, and acoustic vesicles lined with cilia
keep otolites in motion. Besides these organs of sense, the Clione has
respiratory, digestive, and nervous systems. The latter consists of a
nerve-collar round the gullet, with two nerve masses in its upper part,
so the Clione is well supplied with nerves.

Upon each of the two round lobes of the head, there are three tentacles,
totally different from those of feeling. They are, in fact, organs of
prehension, which can be protruded or withdrawn at pleasure into a fold
of the skin. When protruded, these six tentacles form a radiating crown
round the mouth, which is terminal, and furnished with fleshy lips. Each
of these tentacles is perforated by numerous cavities, appearing like
red spots to the naked eye; however, Professor Eschricht discovered that
each spot consists of a transparent sheath, enclosing a central body
composed of a stem terminated by a tuft of about twenty microscopic
suckers, capable of being thrust out to seize prey. The whole number of
these prehensile suckers in the head of one Clione was estimated by
Eschricht to amount to 330,000. Notwithstanding the vast prehensile
power and multitude of these animals, they find abundance of food in the
Arctic Ocean, for although the water is generally of the purest
ultramarine blue, one fourth of the Greenland Sea, extending over 10° of
latitude and some hundred feet deep, is green and turbid, with a
profusion of minute animal life. The indefinite increase of the Clione
borealis is checked by the whales, who feed upon them, and other minute
inhabitants of the Arctic Seas. The Pteropods first appear in a fossil
state in the Lower Silurian strata.


                          _Naked Cephalopods._

The Naked Cephalopods have an internal skeleton instead of a shell, in
the shape of a transparent horny pen in the Calamary, or the well-known
internal shell of the Cuttle Fish; they are divided into Octopods and
Decapods, according to the number of their tentacles: the Poulpe, or
Octopus vulgaris, is a type of the first, the Sepia or Cuttle Fish, fig.
179, and the Loligo vulgare or Squid, are types of the last. These
creatures may be seen on rocky coasts, or in the ocean hundreds of miles
distant from land. They are nocturnal, gregarious, carnivorous, and
fierce,—their structure enables them to be tyrants of the ocean. They
are strange-looking, repulsive creatures, with staring bright-coloured
eyes, while crawling awkwardly on their fleshy arms head downmost; yet
they are the most highly organized of all mollusks.

[Illustration: Fig. 179. Cuttle Fish.]

They have a distinct brain, enclosed in a cartilaginous skull, and all
their muscles are attached to cartilages. The lower part of their body
is surrounded by a mantle, which extends in front to form a gill
chamber, in which there is a pair of plume-like gills; a funnel or
siphon projects from the gill chamber immediately below the tentacles.
All the naked Cephalopods propel themselves back foremost in the sea, by
the forcible expulsion of water from the gill chamber through this
siphon.

The head protrudes from the top of the mantle; it has a pair of large
eyes on sockets, and some species of these animals have eyelids. The
ears are cavities under the cartilage of the skull, containing a small
sac and an otolite. The mouth, which is terminal, and surrounded by the
tentacles, has powerful jaws like a parrot’s beak reversed, acting
vertically. The tongue is large, the posterior part is covered with
recurved spines, and the organ of smell is a cavity near the eyes. The
Naked Cephalopods are remarkable for having three hearts, or propelling
vessels—one for the circulation of arterial blood through the body, the
others for projecting venous blood through the gills, at whose base they
are situated.

The arms or tentacles of all the Naked Cephalopoda are formidable organs
of defence and prehension, but are most powerful in the Loligo vulgaris,
the Poulpe, and the Cuttle, on account of one pair of the tentacles
being long slender arms, dilated at their extremities into flat clubs.
On the inner surface of each of the tentacles, and upon the lower
surface of the dilated extremities of the long ones, there are
multitudes of sucking disks, which, once fixed to an object, adhere so
firmly that it is easier to tear off a portion of the animal’s tentacle
than to make it release its hold. These sucking disks, which are placed
in parallel rows, are represented magnified in fig. 180. Each sucker
consists of a firm cartilaginous or fleshy ring (_e_), across which a
disk of muscular membrane (_f_) is stretched, having a circular opening
(_g_) in its centre. A cone-shaped mass of flesh fills the opening like
a piston, capable of being drawn backwards; the membranous disk can also
be drawn in. When one of these sucking-disks touches a fish, the fleshy
piston is instantaneously retracted, a vacuum is formed, and the edges
of the disk are pressed against the victim with a force equal to the
pressure of the superincumbent water and that of the atmosphere. The
fish is powerless when embraced by the eight tentacles and their
hundreds of suckers; but, if large enough still to struggle, the force
is increased by drawing in the membranous disk. The Poulpe, the most
powerful of the group which swims far from land, and has to contend with
large slippery fishes, has a hooked claw in the centre of each
sucking-disk, which is clasped into the fish the instant the vacuum is
formed. The expansions at the extremities of their two long arms, which
are thickly and irregularly beset with hooked sucking-disks, not only
drag the fish into the embrace of the eight short tentacles, but they
clasp round it and interlock, so that the fish can be torn to pieces by
the parrot-like jaws, and eaten at leisure. The tentacles, long and
short, have strong nerves, and a little nerve-mass occupies the centre
of each sucking-disk, which gives the tentacles great power.

[Illustration: Fig. 180. Arm of Octopus.]

The sepia, or inky liquid, which all the Naked Cephalopods possess as a
means of defence, is secreted in a pyriform bag, which has an outlet
near the respiratory siphon. If the animal be alarmed when devouring its
prey, it instantly lets go its hold, discharges the inky liquid into the
water, and escapes unseen.

The skin of this class of animals is thin and semi-transparent; the
surface immediately below it consists of numerous cells, of a flattened
oval or circular form, containing coloured particles suspended in a
liquid. The colour is seldom the same in all these cells; the most
constant kind corresponds with the tint of the inky secretion. In the
Sepia there is a second series of cells, containing a deep yellow or
brownish colour; in the common Calamary, or Squid, there are three kinds
of coloured cells—yellow, rose-coloured, and brown; and in the Poulpe
there are red, yellow, blue, and black cells. The cells possess the
power of rapid contraction and expansion, by which the coloured liquid
is drawn into deeper parts of the surface, and is again brought into
contact with the semi-transparent skin—thus constantly varying. In
consequence of the high development of the nervous system, the skin of
the Naked Cephalopods is of extreme sensibility; a mere touch brings a
blush on that of the Poulpe, and they all assume the colour of the
surface on which they rest as readily as the chameleon. Many of these
nocturnal animals are luminous, and are easily attracted by bright
metallic objects.




                            RECAPITULATION.


NUMEROUS instances of microscopic structure may be found in the
vertebrate series of marine animals, but the field is too extensive for
the Author to venture upon.

In the first section of this book, an attempt has been made to give some
idea of the present state of molecular science—far short, indeed, of so
extensive a subject; yet it may be sufficient, perhaps, to show the
views now entertained with regard to the powers of nature, the atoms of
matter, and the general laws resulting from the phenomena of their
reciprocal action. By spectrum analysis it has been shown that not only
many terrestrial substances, in a highly attenuated state, are
constituents of the luminous atmospheres of the sun and stars, but that
the nebulæ in the more distant regions of space contain some of the
elementary gases of the air we breathe.

In the succeeding sections it has been proved that the atmosphere teems
with the microscopic germs of animal and vegetable beings, waiting till
suitable conditions enable them to spring into life, and perform their
part in the economy of the world. The life history of the lower classes
of both kingdoms has been a triumph of microscopic science.

The molecular structure of vegetables and animals has been investigated
by men of science in their minutest details; the fragment of a tooth,
bone, or shell, recent or fossil, is sufficient to determine the nature
of the animal to which it belonged; and, if fossil, to assign the
geological period at which it had lived, whether on the earth, in the
waters, or the air. By the microscopic examination of a minute
Foraminifer or shell-like organism, it has been proved beyond a doubt
that the Eozoön, an animal which existed at a geological period whose
remoteness in time carries us far beyond the reach of imagination, only
differs in size from a kind living in the present seas. Simplicity of
structure has preserved the race through all the geological changes
which, during millions of centuries, have swept from existence myriads
of more highly organized beings. The Eozoön is the most ancient form of
life known, and was probably an inhabitant of the primeval ocean.
Patches of carbonaceous matter imbedded in the same strata show that
vegetation had already begun; so at that most remote period of the
earth’s existence, the vivifying influence of the sun, the constitution
and motions of the atmosphere and ocean, and the vicissitudes of day and
night, of life and death, were the same as at the present time.


                               Footnotes

Footnote 1:

  The nervous system is ably explained in Dr. Carpenter’s ‘Manual of
  Physiology.’

Footnote 2:

  A pointer and greyhound, belonging to a friend of the author’s,
  repeatedly brought home hares. Upon watching the dogs, the pointer was
  seen to find the hare, which was coursed and killed by the greyhound.
  Singular as this may seem, it is by no means unprecedented.

Footnote 3:

  From _rhizon_, a root, and _pous_, _podos_, a foot.

Footnote 4:

  ‘On the Amœba princeps and its reproductive cells,’ by Mr J. H.
  Carter: _Annals of Natural History_, July 1863.

Footnote 5:

  ‘On Difflugian Rhizopods,’ by G. C. Wallich M.D. _Annals of Natural
  History_, March, 1864.

Footnote 6:

  Dr. Wallich.

Footnote 7:

  ‘Introduction to the Study of the Foraminifera,’ by W. B. Carpenter.

Footnote 8:

  A complete description of this complex type is given by Dr. Carpenter
  in the Phil. Trans. 1856.

Footnote 9:

  Dr. Carpenter.

Footnote 10:

  Structure of the Organic Remains in the Laurentian Rocks of Canada: by
  J. W. Dawson. Esq., Principal of M‘Gill University, Montreal.

Footnote 11:

  The discovery of Eozoön and the minute details of its structure are
  published in the _Intellectual Observer_ for May 1865. Also the
  ‘Laurentian Rocks of Canada,’ a small work, contains articles by
  various authors on the occurrence, structure, and mineralogy of
  certain organic remains of these rocks.

Footnote 12:

  ‘Histoire Naturelle des Animaux sans Vertèbres,’ par MM. Deshayes et
  H. Milne-Edwards.

Footnote 13:

  Memoir by Dr. Bowerbank in the Transactions of the Microscopic
  Society.

Footnote 14:

  Professor Huxley’s Lectures.

Footnote 15:

  M. Milne-Edwards.

Footnote 16:

  Professor Huxley’s Lectures.

Footnote 17:

  ‘Palæontology,’ by Professor Owen.

Footnote 18:

  Prof. Owen.

Footnote 19:

  Mr. Gosse.

Footnote 20:

  ‘Lectures on Comparative Anatomy,’ by Professor Owen.

Footnote 21:

  Described in ‘The Microscope,’ by Dr. Carpenter.

Footnote 22:

  ‘Lectures on Comparative Anatomy,’ by Professor Owen.

Footnote 23:

  ‘Lectures on Comparative Anatomy,’ by Professor Owen.

Footnote 24:

  Dr. F. Müller, of Santa Caterina.

Footnote 25:

  ‘Recherches sur quelques Animaux inférieurs de la Méditerranée,’ par
  C. Vogt: _Mémoires de l’Institut National Génevois_, tom. i.

Footnote 26:

  Published in 1858, by the Ray Society.

Footnote 27:

  ‘Lectures on Comparative Anatomy,’ by Professor Owen.

Footnote 28:

  ‘Evenings at the Microscope,’ by P. H. Gosse, Esq.

Footnote 29:

  ‘Observations on the Caryophyllia Smithii,’ by Mrs. Thynne, in the
  _Annals and Magazine of Natural History_.

Footnote 30:

  ‘Histoire des Corallines,’ par Professeur Milne-Edwards.

Footnote 31:

  ‘Histoire des Corallines,’ par Professeur Milne-Edwards.

Footnote 32:

  According to the system of M. Milne-Edwards, who made the Annulosa a
  particular object of investigation.

Footnote 33:

  Dr. Thomas Williams on ‘British Annelides,’ British Association, 1852.

Footnote 34:

  ‘Comptes rendus,’ July 1864.

Footnote 35:

  ‘Palæontology,’ by Professor Owen.

Footnote 36:

  Messrs. Woodward and Barrett on the Synapta. Trans. of Zoological
  Society, London.

Footnote 37:

  ‘The Microscope,’ by Dr. Carpenter.

Footnote 38:

  Mr. C. Spence Bate.

Footnote 39:

  ‘Histoire naturelle des Crustacés,’ par M. Milne-Edwards.

Footnote 40:

  ‘Evenings at the Microscope,’ by Mr. Gosse.

Footnote 41:

  Jeffrey’s ‘British Conchology.’




                                 INDEX.


                                   A

 Abrothallus, organs of reproduction of, i. 307

 Absorption of light and heat, i. 34
   effects of, i. 35, 36
   generally independent of colour, i. 36
   Melloni’s investigations into the laws of, i. 38
   Prof. Tyndall’s experiments, i. 38
   amount of radiant heat absorbed by the perfumes of flowers and
      plants, i. 43-45
   experiments showing radiation to be equal to absorption, i. 46
   great absorption of olefiant gas, i. 47
   absorptive power of aqueous vapour, i. 53
   dynamic absorption, i. 49
   absorption a phenomenon irrespective of aggregation, i. 53
   absorption of invisible rays by solids, liquids, and gases, i. 65

 Absorption bands in the red and green parts of the solar spectrum, i.
    131

 Acalepha, the simple hydra a phase in the life of the, ii. 91

 Acalephæ, Campanograde, characters of, ii. 103

 Acalephæ, Ciliograde, characters of the, ii. 101
   mode of reproduction of, ii. 103

 Acalephæ, Physograde, characters of, ii. 107

 Acanthometræ, structure and habitat of, ii. 19

 Acetic acid, chemical combination forming, i. 97

 Acetic fermentation, fungus producing, i. 288

 Acetylene, formation of, i. 97

 Acetylene, chemical composition of, i. 128

 Achlya prolifera, structure and reproduction of, i. 220

 Acids, their affinity for alkalies, i. 96

 Acineta, structure and mode of reproduction of, ii. 77, 78
   food of, and mode of seizing it, ii. 78

 Acorn-shell, structure of the, ii. 213

 Acrocarpi, characters of the group, i. 328

 Acrocladia mamillata, spines of the, ii. 180, 181

 Acrosticheæ, characters of the, i. 359, 360

 Actinia sulcata, structure of, ii. 132

 Actinian polype, structure and mode of reproduction of, ii. 130, 131

 Actinian zoophytes, characters of, ii. 130, 131
   thread-cells, ii. 132
   structure of, ii. 134-136

 Actinism of star-light, i. 55
   of moon-light, i. 55
   of sun-light, i. 56

 Actinocyclus undulatus, structure of, i. 199, 200

 Actinomma drymodes, structure of, ii. 21

 Actinophrys sol, structure and reproduction of, ii. 16-18
   certain degree of instinct exhibited by, ii. 18
   its enemy, the Amœba, ii. 18

 Adder’s tongue fern, i. 365

 Adiantieæ, characters of the groups, i. 358

 Adiantum Capillus-Veneris, or maiden’s-hair fern, structure and habitat
    of, i. 359

 Adriatic sea, Foraminifera in the ooze of the bed of the, ii. 51

 Adularia, or moonstone, fluorescent property of, i. 66

 Æcidium, characters of, i. 280

 Ægean sea, zones of Algæ in the, i. 259

 Æthalium septicum, structure and habitat of, i. 270-272
   Amœba-like motions of, i. 270
   mode of reproduction of, i. 271

 Affinity, chemical, of kind and of degree, i. 95
   relation between chemical affinity and mechanical force, i. 98

 Agaricini, the order, i. 261

 Agaricus, the genus, i. 261, 263
   structure of the, i. 263

 Agaricus arvensis, spawn of, i. 262

 Agaricus campestris, or common mushroom, i. 261
   structure and mode of reproduction of, i. 261

 Agaricus gardneri, luminosity of, i. 264

 Agaricus olearius, luminosity of, i. 264

 Air, a non-conductor of electricity, i. 32
   amount of force exerted by the sun’s light within the limits of the
      terrestrial atmosphere, i. 34
   absorption of radiant heat by, i. 41
   effect of the rays of the sun falling on the earth through pure dry
      air, i. 53
   spectrum analysis of the component parts of the atmosphere, i. 139

 Air-vessel of Nereocystis Lutkeana, i. 249, 250

 Alaria esculenta, fruit of, i. 249

 Alaria Pylaii, fruit of, i. 249

 Albumen, formation and structure of, in vegetable organisms, i. 423

 Alcohol, chemical combination forming, i. 97

 Alcyon zoophytes, characters of, ii. 119, 120
   structure and mode of reproduction of, ii. 120-123

 Alcyonidium elegans, structure of, ii. 120

 Alcyonium digitatum, or dead man’s fingers, structure of, ii. 121, 122

 Aldebaran, spectrum of, i. 155

 Algæ, i. 179
   classification of the group, i. 179-181
   green Algæ, i. 181-220
   zones or degrees of depth at which different Algæ exist, ii. 221,
      258, 259
   brackish water often a cause of change in the form of Algæ, i. 245
   colossal Algæ of the North Pacific, i. 245
   rings of growth of some of the larger sea-weeds, i. 252

 Alizarine blue, from what obtained, i. 124, 125

 Alkalies, their affinity for acids, i. 96

 Alkaloids, chemical structure of, i. 427

 Allosorus crispus, or parsley fern, structure and habitat of, i. 358

 Allotropism, i. 8
   of carbon, i. 15

 Alsophileæ, structure of, i. 360

 Alum, a free radiator but a bad conductor of radiant heat, i. 37
   partial decomposition of, by diffusion, i. 109
   manufacture of, i. 120

 Alumina, effect of electricity on, i. 32

 Aluminum, i. 4
   spectrum of, i. 65

 Amalgams of metals, illustrating the relation between chemical affinity
    and mechanical force, i. 98

 Amansia, structure of, i. 242

 Amaryllidaceæ, structure and mode of reproduction of, i. 388

 America, North, enormous quantities of petroleum in, i. 126

 Amidogen combines with other substances and simple atoms as if it were
    a simple element, i. 106

 Ammonia, amount of absorption of radiant heat by, i. 41
   characteristics and constituents of, i. 119
   chemical composition of, i. 128
   combination forming, i. 107
   carbonate of, i. 119
   extent of the manufacture of, i. 120
   muriate of, i. 120
   manufacture of, i. 120
   sulphate of, i. 120
   how manufactured, i. 120

 Ammonium, sulphide of, i. 119

 Amœba princeps, structure, motions, food, and reproduction of, ii.
    13-16
   its capture of the Actinophrys, ii. 18
   a new species found in dust from Egypt, ii. 65

 Amorphous substances, conduct of, under magnetic influence, i. 76

 Amphicora, two eye-specks in the tail of, ii. 158

 Amphidetus, spines of the, ii. 180

 Amphipoda, characters of the, ii. 201

 Amphithoæ, structure of, ii. 203

 Analysis, chemical, electricity and light instruments of, i. 95
   the most powerful means of, i. 96

 Andræa subulata, leaves of, i. 330

 Andræaceæ, characters of the tribe, i. 328

 Angiocarpei, structure, mode of reproduction, and habitat of, i. 309,
    310

 Angiopteris evecta, perfume from, i. 364, 365

 Angræcum sesquipedale, structure and mode of reproduction of, i. 402

 Anguillulæ, eel-like entozoa, structure and habitat of, ii. 147, 148

 Anemone, sea, structure and mode of reproduction of, ii. 130, 131

 Aniline, formation of the fulminate of, i. 92
   combination forming, i. 107
   how produced, i. 121, 122
   constituents and characteristics of, i. 122
   formation of the aniline colours, i. 122, 123
     mauve, i. 122
     magenta, i. 122
     purple, i. 123
     yellow, i. 124
   chemical composition of, i. 128

 Aniline, acetate of, i. 123

 Animals, functions of the frame of, ii. 1
   sarcode and muscle, ii. 2
   blood, ii. 2
   waste and repair, ii. 3
     food, ii. 3
   animal heat, ii. 3
   the heart, digestion, heat, and respiration, ii. 4
   nerve-force of animals, ii. 4
   nervous system, ii. 5
     structure and functions of the brain and spinal cord, ii. 8
   intelligence, or the mental principle, in animals, ii. 11, 12

 Anise seed, amount of radiant heat absorbed by the perfume of, i. 44

 Annelida, characters of the, ii. 149
   mode of reproduction of, ii. 160
   luminosity of, ii. 160
   fossil remains of, ii. 161

 Annulosa, or worms, characters of, ii. 144

 Anomura, structure and mode of reproduction of, ii. 197

 Antennaria Robinsonii, mode of reproduction of, i. 296

 Antennariei, structure of, i. 296

 Anthoxanthum odoratum, or sweet-scented vernal grass, mode of
    reproduction of, i. 386

 Anthozoa zoophytes, or living flowers, characters of, ii. 119

 Antiaris toxicaria, causes of the virulence of, i. 426
   fruit of, innocuous, i. 426

 Antozone, i. 8

 Aphrodita hystrix, structure and mode of reproduction of, ii. 159

 Apolemia contorta, structure and mode of reproduction of, ii. 108, 109

 Apple, fructification of the, i. 381

 Apus cancriformis, structure of the, ii. 210

 Aqueous vapour, i. 53
   absorptive power of, i. 53-55

 Arachnoidiscus Ehrenbergii, structure of, i. 199

 Arcella, structure and mode of formation of the shell of, ii. 21, 22,
    23

 Aregma speciosum, spores of, i. 276

 Arenicola, structure of, ii. 157

 Arragonite, i. 108
   external and internal alteration of, by heat, i. 108

 Artemia salina, structure of, ii. 210

 Artocarpus, or bread-fruit tree, i. 426

 Arum, structure and mode of reproduction of, i. 388
   water secreted by the, at night, i. 429

 Arundo Donax, size of the, i. 386

 Ascaris lumbricoïdes, structure and mode of reproduction of, ii. 147

 Asci, or sporidia sacs, of Ascomycetes, i. 290

 Ascidia virginea, structure, food, and habitat of, ii. 224, 225

 Ascidians, or Tunicata, character of, ii. 222
   compound or social gelatinous, ii. 222
   star-like, ii. 224

 Ascomycetes, structure of, i. 290

 Ascophora elegans, fructification of, i. 226

 Ashes, i. 14

 Aspergillus dubius, spores of, i. 286

 Aspergillus glaucus, spores of, i. 286

 Asphalt, sources of, in various parts of the world, i. 126

 Aspidieæ, characters and habitat of the group, i. 346, 348

 Asplenieæ, characters and habitat of the group, i. 351, 352, 353

 Asplenium lanceolatum, structure, fructification, and habitat of, i.
    354

 Asplenium marinum, structure and habitat, i. 355, 356

 Asplenium Ruta-muraria, or wall rue, structure, habitat, and
    reproductive organs of, i. 353, 354

 Asteroïdea, or star-fishes, characters of the, ii. 169
   fossil star-fishes, ii. 174

 Athyrium Filix-fœmina, or lady fern, herbaceous caudex of, i. 340
   structure and fructification of, i. 352, 353

 Atlantic Ocean, profound depths of, north and south, ii. 49
   foraminifera at great depths in the, ii. 49, 50

 Atmolysis, or method of analysing gases, i. 111

 Atmosphere, effect of the heat radiated by the moon on the higher
    regions of the, i. 55
   opalescence of the, and its effect on the chemical power of the sun’s
      light, i. 55
   causes of this opalescence, i. 55
   its permeability to every kind of chemical rays, i. 58

 Atmosphere of the sun, thirteen terrestrial substances in the, i. 59

 Atolls, or coral lagoons, ii. 140

 Atoms or molecules of matter, power of atoms of matter on the rays of
    the solar beam, i. 58
   aggregation of matter by electricity, i. 74
   effect of the physical forces on molecular arrangement, i. 90, 91
   proof of the individuality and polarity of the atoms of matter, i. 91
   effect of motion, i. 91

 Atoms: effect of catalysis, i. 91
   of matter, i. 2
   cohesion of, i. 25
   unit of mechanical force, i. 26
   the atomic theory, i. 93
   the law of definite proportions, in combination and resolution of the
      component parts of substances, i. 94
   affinity of kind and of degree, i. 95
   force of chemical combination, i. 97, 98
   relative atomic weights, i. 99
     table of atomic weights compared with that of hydrogen, i. 100
     relation between the atomic weights and the specific gravities of
        substances, i. 100
     capacities of atoms for heat and electricity, i. 100, 101
   law of equivalency in weight and volume, i. 102, 103
     substitution the basis of this law, i. 104
     sequence of atomic numbers, i. 105
   groups of substances whose atomic weights are in regular arithmetical
      series, i. 106
   groups of combined atoms called compound radicles, i. 106
   the polyatomic theory, i. 107
   the force of molecular attraction more powerful than gravitation, i.
      109
   internal movement of molecules of matter in a gaseous state, i. 113

 Aulocantha scolymantha, structure of, ii. 21

 Avicula margaritacea, or pearl oyster, nacreous lining of shell of, ii.
    234

 Azores, the black mildew of the, i. 297

 Azote in vegetable organisms, i. 423

 Azuline dye, production of, i. 123


                                   B

 Babylon, the asphaltic mortar used in building, i. 126

 Bacillaria cursoria, motion of, i. 203

 Bacillaria paradoxa, motion of, i. 203

 Back, Sir George, his arctic journey, i. 308

 Bacteria, their structure, size and habitat, ii. 64

 Bæomyces, structure of, i. 306

 Balanidæ, structure of, ii. 213

 Balanus balanoïdes, development of, ii. 216

 Balanus culcatus, structure of, ii. 213-215

 Baldness caused by fungi, i. 275

 Balms, chemical combination forming the principle of the, i. 97

 Bamboo, size and structure of, i. 386

 Bangia, characters of the genus, i. 222

 Bangia ceramicola, structure and mode of reproduction of, i. 223

 Bangia ciliaris, structure and reproduction of, i. 223

 Bangia fuscopurpurea, structure and reproduction of, i. 223

 Barium, i. 3
   one of an isomeric triad with calcium and strontium, i. 105
   spectrum analysis of the rarefied vapour of, i. 142
   effect of high temperature, i. 143
   chloride of, spectrum of, i. 146

 Bark of trees, structure of, i. 406, 407

 Baryta, green coloured light obtained by the combustion of, i. 133

 Batarrea, structure of, i. 267

 Batrachospermeæ, habitat and structure of, i. 210

 Bean, common, fungus of the, i. 280

 Beans, caseine obtained from, i. 125

 Beer, yeast of, i. 287

 Beetles, fungi in the stomachs of, i. 274

 Begonia, structure and mode of reproduction, of, i. 376

 Bengal, Bay of, causes of the scarlet colour of the, ii. 72

 Benzol, combination forming, i. 118, 127
   constituents and characteristics of, i. 121
   chemical composition of, i. 128

 Bergamot, amount of radiant heat absorbed by the perfume of, i. 44

 Beroë Forskalia, structure of the, ii. 102, 103

 Bhang, whence obtained, i. 427
   intoxicating effect of, i. 427

 Biddulphia pulchella, structure of, i. 198

 Bignonia, fructification of, i. 381, 382

 Bilin, polishing slate of, of what it consists, i. 206

 Biscay, Bay of, zones of Algæ in the, i. 259

 Bisection, propagation of diatoms by, i. 200, 204

 Bismuth, diamagnetism of, i. 75, 76
   chloride of, spectrum of, i. 146

 Bittern, i. 18

 Bleaching properties of:—
   chlorine, i. 19
   bromine, i. 20
   iodine, i. 21

 Blechnum Spicant, structure, habitat, and mode of reproduction of, i.
    356, 357

 Blood, formation and functions of, ii. 2, 3

 Blue dyes, i. 123, 124

 Bog moss, structure, fructification, and habitat of, i. 332

 Boleti, habitat and structure of the, i. 265

 Boletus æneus, large amount of heat evolved by, i. 265

 Bombyx rubi, fungi destructive of, i. 282

 Boron, i. 18
   whence obtained, i. 18
   insolubility of, i. 18
   heat at which it burns, i. 18

 Botrychium, habitat of the genus, i. 366

 Botrychium Lunaria, or moonwort, structure and habitat of, i. 366

 Botryllidæ, or star-like Ascidians, ii. 224

 Botrytis Bassiana, a fungus parasite of the silk-worm, i. 274

 Botrytis curta, development of, i. 294

 Botrytis, the cause of the murrain in potatoes, i. 284
   structure of, i. 284

 Brachionus pala, structure of, ii. 162-166

 Brachyura, structure and mode of reproduction of, ii. 189-197

 Brackens, i. 358

 Brain, structure and functions of the, ii. 8

 Branchipes stagnalis, structure of, ii. 210

 Bread, fungus which attacks, hot from the oven, i. 296

 Bread-fruit tree, food obtained from the, i. 426

 Bristle fern, structure and habitat of, i. 361

 Bromine, i. 18
   whence obtained, i. 18
   properties of, i. 20, 22
   Professor Schönbein’s views as to, i. 20, 21
   spectrum of, i. 21
   amount of absorption of radiant heat by, i. 41, 42, 43
   atomic weight of, compared with that of hydrogen, i. 100
   spectrum analysis of, i. 140

 Bryei, characters and structure of the, i. 329

 Bryopsis, structure and habitat of, i. 219, 224

 Bryozoa, or Polyzoa, characters of, ii. 218

 Buckthorn, green dye obtained from, i. 124

 Buds of trees, structure and functions of, i. 411

 Bugula avicularia, structure of, ii. 220

 Bulbous plants, beauty of, i. 387

 Bulbous plants, structure and reproduction of, i. 388

 Bunt, germination of the spores of, i. 281


                                   C

 Cadmium, i. 4
   spectrum of volatilized cadmium, i. 64
   chloride of, spectrum of, i. 146

 Cæomacei, structure and mode of reproduction of, i. 278

 Cæsium, i. 3, 4
   atomic weight of, compared with that of hydrogen, i. 100
   one of an isomeric triad with rubidium and potassium, i. 105
   M. Bunsen’s discovery of the metal by spectrum analysis, i. 134, 135
   mode of distinguishing it from potassium, i. 136
   its properties, i. 136
   changes in its spectrum by high temperature, i. 144

 Caffeine, the neutral crystallisable principle of coffee, i. 428

 Caladium distillatorium, water secreted by at night, i. 429

 Calanus, structure and social habits of the, ii. 206
   bloody water formed by the, ii. 206

 Calanus pontilla, structure of, ii. 206

 Calanus sanguineus, structure of, ii. 206

 Calcarina, intermediate skeleton of, ii. 42, 43

 Calcium, i. 3, 4
   one of an isomeric triad with strontium and barium, i. 105
   spectrum analysis of the rarefied vapour of, i. 142
     effect of high temperature, i. 143
   chloride of, spectrum of, i. 146

 Caliciei, characters and habitat of the order, i. 309

 Calicium inquinans, spores of, i. 309

 Calicium tympanellum, organs of reproduction of, i. 301

 Calico-printing, use of oxalic acid in, i. 116

 Callithamnion, structure of the genus, i. 231

 Callithamnion corymbosum, structure and reproductive organs of, i. 231,
    232

 Callithamnion sparsum, structure of, i. 231

 Calorescence, property of, in some solids and liquids, i. 61

 Calyx of flowering plants, i. 378

 Cambium of plants, structure of, i. 407, 417

 Camptosorus, or walking fern, structure and mode of reproduction of, i.
    352

 Campylopus lamellinervis, leaves of, i. 330

 Canada, enormous quantities of petroleum in, i. 126

 Caoutchouc, whence obtained, i. 426

 Carbazotic acid, constituents of, i. 121

 Carbohydrates, combinations forming the, i. 116

 Carbolic acid, combination forming, i. 107, 120, 121, 128
   its property of arresting putrefaction, i. 121
   becomes solid when dried and purified, i. 121

 Carbon, i. 13
   allotropism of, i. 15
   at variance with the geometrical system of crystallization, i. 15
   analogy between carbon and silicon, i. 18
   effect of the combination of the atoms of carbon and oxygen in
      combustion, i. 30
   property of the bisulphide of, in the transmission of heat, i. 36
   effect of a discharge of the nitric acid electric battery, with
      carbon balls on the terminals, i. 84
   will not combine directly with hydrogen, i. 97
   atomic weight of, compared with that of hydrogen, i. 100
   spectra of the compounds of carbon, i. 146
   bisulphide of, absorption of radiant heat by, i. 40
     opacity of, to the invisible rays, i. 65
   in coal gas, i. 118
   how freed from coal gas, i. 118

 Carbonic acid gas, i. 14
   reduced to a solid, i. 15
   weight of the atoms of, compared with those of hydrogen, i. 99
   absorption of radiant heat by, i. 41
   how freed from coal gas, i. 118
   one of the illuminants in coal gas, i. 118

 Carbonic oxide, absorption of radiant heat by, i. 41
   action of different thicknesses of, on radiant heat, i. 48
   the poisonous quality of coal gas, i. 118

 Carcinus mœnas, structure and reproduction of, ii. 192, 195
   moulting and power of reproducing limbs, ii. 197

 Cardium, or cockle, structure and mode of reproduction of, ii. 230, 231

 Carpenteria, structure and habitat of the, ii. 57
   a link between the Foraminifera and the sponges, ii. 57

 Carrigeen, or Irish moss, structure and mode of reproduction of, i.
    235, 236

 Caryophyllia Smithii, its formidable artillery, ii. 133, 134
   structure and development of, ii. 134, 135

 Caryophyllia Smithii, locomotion of, ii. 135

 Caseine, or cheese, used as a mordant, i. 125
   obtained also from pease and beans, i. 125
   formation and structure of, in vegetable organisms, i. 423

 Cassava, or Manihot, food obtained from the, i. 426

 Cassis, or helmet shells, structure of, ii. 234

 Catalysis, effect of, on molecular arrangement, i. 91
   instances of, i. 91, 92

 Catasetum, structure and mode of reproduction of, i. 601

 Caterpillars, fungus destructive of, i. 282

 Caulerpas, where found, i. 219

 Celidium, organs of reproduction of, i. 307

 Cellipora, structure of, ii. 218

 Cells, vegetable. _See_ Vegetation

 Cellulose, produced by plants, i. 419

 Cellularia ciliata, structure of, ii. 219, 220

 Cephalopoda, naked, characters of, ii. 245

 Ceramiaceæ, beauty and habitat of, i. 231
   characters of the genus, i. 232

 Ceramium ciliatum, structure and organs of reproduction of, i. 232

 Ceratopteridineæ, or Parkeriaceæ, characters, structure, and habitat of
    the group, i. 345, 363

 Cerealia, the grasses from whence they were derived unknown, i. 383

 Cestum Veneris, structure and mode of reproduction of, ii. 103

 Ceterach, characters of the genus, i. 354
   structure and mode of reproduction of, i. 354
   brought by ocean currents to Europe, i. 355

 Ceterach officinarum, or scaly spleen-wort, structure, organs of
    fructification, and habitat of, i. 354, 355

 Cetraria, or Iceland moss, characters of the genus, i. 304

 Cetraria tristis, structure of, i. 304

 Ceylon, the black mildew of, i. 297

 Chamomile flowers, amount of radiant heat absorbed by the perfume of,
    i. 44

 Chantarelle, veins of the, i. 264

 Characeæ, structure and habitat of, i. 312
   fluid currents of, i. 312, 313
   reproductive organs of, i. 313

 Chara fragilis, structure and mode of reproduction of, i. 314, 315

 Charcoal, i. 13
   as a conductor of electricity, i. 32

 Cheese, blue and brick-red moulds on, i. 285

 Cheilanthes odora, scent of, i. 347

 Chelidonine, whence obtained, i. 427

 Chemistry, laws placing it on a strictly numerical basis, i. 93

 Chimneys of bad construction, i. 14

 Chiodecton monostichum, structure and habitat of, i. 309

 Chiodecton myrticola, habitat of, i. 309

 Chirodota, structure and habitat of, ii. 186

 Chirodota lævis, wheels of the, ii. 186

 Chirodota myriotrochus, wheels of the, ii. 186

 Chloride of lime, i. 19

 Chlorides, spectra of the, i. 146, 147

 Chlorine, i. 18
   whence obtained, i. 18, 19
   properties of, i. 19, 22
   affinity for hydrogen, i. 19, 20
   substances produced by the combination of chlorine with oxygen, i. 20
     and with nitrogen, and with sulphur, i. 20
   Professor Schönbein’s views as to, i. 20, 21
   spectrum of, i. 21
   absorption of radiant heat by, i. 41, 42
   its affinity for iodine, i. 96
   atomic weight of, compared with that of hydrogen, i. 100
   spectrum analysis of, i. 140

 Chlorine gas, substances which take fire spontaneously in, i. 19
   combustion of, i. 19

 Chlorospermeæ, characters of, i. 180
   structure and development of, i. 181

 Chondrus crispus, or Carrigeen, structure and mode of propagation of,
    i. 235, 236
   effect of fresh water on the form of, i. 243

 Chorda filum, structure and habitat of, i. 245

 Chordaria divaricata, spore cysts of, i. 246

 Chordariæ, structure of, i. 245

 Chrysophonic acid, whence obtained, i. 303

 Chylocladia kaliformis, structure and mode of propagation of, i. 235

 Cibotium Barometz, caudex of, i. 350

 Cidaris, spines of the, ii. 180

 Cinchoneæ, alkaloids obtained from, i. 427

 Cinchonine, structure of, and whence obtained, i. 427

 Cirripedia, characters of the, ii. 213

 Cladocarpi, characters of the group, i. 328

 Cladocera, characters of the order, ii. 208

 Cladonia, structure of, i. 306

 Cladophora, structure of the genus, i. 222

 Cladophora pellucida, structure of, i. 222

 Claudea, structure of, i. 242

 Clavariei, structure and habitat of, i. 266

 Clio pyramidata, structure of, ii. 242, 243

 Cliona, structure and burrowing apparatus of, ii. 60

 Clione borealis, structure and habitat of, ii. 243
   fossil remains of, ii. 244

 Closterium, double circulation of the internal fluid in, i. 193

 Closterium lunula, structure and development of, i. 193

 Cloud, force of the chemical combination requisite to form a, i. 98

 Cloves, oil of, amount of radiant heat absorbed by the perfume of, i.
    44

 Club mosses. _See_ Lycopodiaceæ

 Coal, i. 13
   effect of the consumption of a pound of, in a steam-engine, i. 29, 30
   energy in abeyance in the coal existing in the whole globe, i. 30

 Coal gas, i. 117, 118
   poisonous quality of, i. 118
   explosive quality of, i. 118
   impurities from which it is freed, i. 118
   uses of the black fœtid gas water resulting from the distillation of
      coals, i. 119

 Coal tar, i. 120
   substances produced from, by distillation, i. 120
   colours, i. 121, 122

 Cobalt, i. 4, 5
   crystals of, formed artificially by electricity, i. 74
   effect of heat in the magnetism of, i. 77

 Coccocarpei, characters of, i. 307

 Coccocarpia smaragdina, section of, i. 300

 Coccospheres, found at a vast depth in the ocean, ii. 51

 Cockle, structure and mode of life of the, ii. 230, 231

 Codium, structure and habitat of, i. 219-224

 Codium Bursa, structure of, i. 224

 Codium tomentosum, structure of, i. 214

 Coffee, active principle of, i. 428

 Cohesion, properties of, i. 25
   in solids, i. 25
   in liquids, i. 25

 Cohesion: reciprocal attractions between solids and liquids, i. 25
   instance of the power of the cohesive force, i. 91

 Colchicum, or meadow saffron, mode of reproduction of, i. 388
   poisonous alkaloid obtained from, i. 427

 Collemacei, structure and habitat of, i. 307

 Colloid bodies, i. 109, 110
   less diffusible than crystalloids, i. 109
   permeable to solutions of crystalloids, but impermeable to solutions
      of colloids, i. 110
   characteristics of colloids, i. 111

 Collomia grandiflora, fructification of, i. 380

 Colocasia, water secreted by, i. 430

 Colour, causes of, in flowers, green leaves, dyed cloth, gold, and
    copper, i. 35, 36

 Colouring matter of flowers and plants, i. 428, 429

 Comatula, structure of, ii. 175, 176

 Combination, chemical, i. 94
   the law of definite proportions, i. 94, 95
   force of, i. 97, 98

 Combustion, effect of, in the consumption of coal in the steam-engine,
    i. 29, 30
   combustion a case of impetus, i. 30

 Comets, spectra of, i. 159
   _note_, 163

 Conferva glomerata, development of, i. 207

 Confervaceæ, habitat of, i. 206, 207
   structure and development of, i. 207
   reproduction of, i. 208
   modes of action of the vital forces of, i. 210

 Confervæ, cells and development of, i. 171

 Confervæ, marine, structure, habitat, and reproduction of, i. 221, 222.

 Conidium, or spore-dust cell, of fungi, i. 279

 Coniomycetes, characters of the family of, i. 275

 Conjugatæ, structure of the genus, i. 216

 Conjugation, propagation of diatoms by, i. 200, 204

 Constantinea rosa marina, i. 235

 Constantinea sitchensis, i. 235

 Copepoda, characters of the, ii. 204

 Copper, properties of, i. 4
   colour of, in reflected and absorbed light, i. 35, 36
   its power of transmitting electricity, i. 90
   atomic weight of, compared with that of hydrogen, i. 100
   spectra of copper and its compounds, i. 145, 146

 Copper, peroxide of, combination forming, i. 104

 Coral, structure of the coral polypes, ii. 133, 134
   composition of the stony substance of, ii. 137
   reef-building corals, ii. 138
   red, ii. 125
   white, ii. 127

 Corallina officinalis, mode of reproduction of, i. 230
   structure and development of, i. 240

 Corallines, structure and mode of propagation of, i. 240

 Corallium Johnstoni, structure and habitat of, ii. 127

 Corallium rubrum, structure and mode of reproduction of, ii. 125, 126,
    127
   coral fishing in the Mediterranean, ii. 126

 Corallium secundum, structure of, ii. 127

 Cordyceps miliaris, characters of, i. 283, 293

 Cordyceps purpurea, the second form of ergot, i. 293
   structure of, i. 293

 Cordyceps Robertsii, form of, i. 293

 Coremium glaucum, production of, i. 287

 Corolla of flowering plants, i. 378

 Corundum, i. 4

 Corynidæ, characters of the family, ii. 90

 Cosmarium, structure and development of, i. 194

 Cotton, dyes for, i. 125
   necessity for mordants for fixing cotton dyes, i. 125
   enormous manufacture of cotton in Britain, i. 125

 Cowries, shells of, ii. 234, 235

 Cow-tree, beverage obtained from the, i. 426

 Crabs, structure and mode of reproduction of, ii. 189-197
   hermit, structure of, ii. 197
   king, fossil, ii. 211
   spider, structure of, ii. 211

 Creosote, i. 121
   its property of preventing the decay of organic matter, i. 121

 Cressylic acid, how produced, i. 120, 121
   chemical composition of, i. 128

 Cristallaria compressa, form of, ii. 28
   structure of, ii. 39

 Cristata, siliceous skeleton of, ii. 60

 Crocus, structure and mode of reproduction of, i. 387, 388

 Cruoria pellita, tetraspores of, i. 237

 Crustacea, characters of the, ii. 188
   fossil crustacea, ii. 211

 Crustacea, Decapod. _See_ Decapods

 Cryptogamia, spores of, i. 177, 178

 Cryptonemiaceæ, multitude of forms of, i. 234
   structure of, i. 234

 Crystal, rock, its transmission of chemical solar rays, i. 65
   absorption of invisible rays by, i. 65
   change of position of the optical axes of the crystals of, by heat,
      i. 73

 Crystallization, relation of polarization of light and heat to
    crystallization, i. 70, 71
   axes of symmetry of crystals, i. 71, 72
   change of position of the optical axes of crystals by heat, i. 72
   effect of pressure on the optical axes of crystals, i. 73
   influence on the aggregation of, i. 73, 74
   probable origin of the crystalline form, i. 74
   causes of the variety of forms assumed by matter, i. 74
   deviation of dimorphous crystals from the general law of
      crystallization, i. 75
   proof of the connection between the magnetic forces and crystalline
      structure, i. 76
   conditions of the position which crystals take with regard to the
      magnetic force, i. 76

 Crystalloids, or crystalline substances, diffusibility of, i. 109

 Crystals, water an essential element in, i. 107
   alterations in crystals by heat, i. 108

 Cunina octonaria, larvæ of, parasites of the Turritopsis nutricula, ii.
    100

 Cusconine, structure of, and whence obtained, i. 427

 Cuthbert’s, St., beads, ii. 175

 Cutaneous diseases caused by fungi, i. 274

 Cutleria multifida, structure, mode of reproduction, and habitat of, i.
    248

 Cuttle fish, structure of the, ii. 245-247

 Cyanogen, combines with other substances and simple atoms, as if it
    were a simple element, i. 106
   combination forming it, i. 106

 Cyathea medullaris, used as food in New Zealand, i. 360

 Cyatheæ, structure of, i. 360

 Cyatheineæ, characters of the group, i. 344
   sporangia of, i. 343
   structure and fructification of, i. 360

 Cycloclypeus, structure and habitat of, ii. 48

 Cyclops quadricornis, structure and mode of reproduction of, ii. 205,
    206

 Cydippe pileus, structure of, ii. 101, 102

 Cymothea, their food and mode of taking it, ii. 203

 Cyprææ, or cowries, shells of, ii. 234, 235

 Cypris, structure and mode of reproduction of, ii. 207, 208

 Cystopteris, characters and habitat of the genus, i. 348, 349

 Cystopus candidus, or Uredo candida, structure, habitat, and mode of
    reproduction of, i. 278

 Cystopteris fragilis, or brittle bladder fern, structure and habitat
    of, i. 349

 Cystoseira, structure and habitat of, i. 255

 Cystoseireæ, habitat of, i. 255

 Cyttaria, the food of the Fuegians, i. 292
   habitat of the, i. 292


                                   D

 Dactylocalyx pumiceus, spicula and skeleton of, ii. 60

 Daphnia pulex, or arborescent water-flea, structure and mode of
    reproduction of, ii. 208

 Dasya, structure of, i. 241, 242

 Dasyglœa, structure of, i. 215

 Datura sanguinea, intoxicating effect produced by a drink obtained
    from, i. 427

 Davallia canariensis, or hare’s-foot fern, i. 351

 Davallieæ, structure and habitat of, i. 350, 351

 Dead bodies, agents in the decomposition of our, ii. 67

 Decapoda, structure of, ii. 245

 Decapods, tribes of the, ii. 188
   characters of the, ii. 189

 Delesseria alata, structure of, i. 243

 Delesseria angustissima, structure of, i. 243

 Delesseria sanguinea, structure and habitat of, i. 239

 Dematiei, structure of, i. 283

 Dendritina elegans, form of, ii. 28
   structure of, ii. 32

 Dendritine variety of the Peneroplis, characters and habitat of, ii. 32
   fossils of, ii. 33

 Dennstædtia, indusium of, i. 350

 Deparia prolifera, sorus and indusium of, i. 350

 Desmidiaceæ, structure and development of, i. 191 _et seq._
   their habitat, i. 195

 Dextrine, production of in plants, i. 421

 Dialysis, as a method of separating and analysing substances, i. 108
   what constitutes dialysis, i. 110
   Prof. Graham’s instrument for, i. 110
   an extraordinary result of, i. 110

 Diamagnetism, i. 75

 Diamond, i. 13, 15
   heat required to consume the, i. 15
   crystallisation of the, i. 15
   its resistance to electricity, i. 90

 Dianthine, production of, i. 127

 Diastase, production of, in plants, i. 420

 Diatoma vulgare, structure and development of, i. 197

 Diatomaceæ, or Brittleworts, found all over the globe, i. 196, 204
   variety of forms of, i. 196
   structure of, i, 197
   development of, i. 200
   social plants, i. 205
   food for many aquatic animals, i. 205
   fossil deposits of shells of, i. 206
   enormous geological changes caused by, i. 206

 Dicksonia antarctica, structure of, i. 349

 Dicksonia lanata, structure of, i. 349

 Dicksonia squarrosa, habitat of, i. 349

 Dicksonieæ, i. 349
   structure of, i. 350

 Dicotyledonous, or exogenous, plants, structure and mode of
    reproduction of, i. 404-428

 Dicranei, structure of, i. 329

 Dictyopodium trilobum, structure and habitat of, ii. 20

 Dictyota, structure of the genus, i. 246

 Dictyota dichotoma, structure and mode of reproduction of, i. 247

 Dictyoteæ, structure and mode of reproduction of, i. 246

 Dictyurus purpurascens, structure of, i. 242

 Difflugia, structure and minuteness of the shells of, ii. 22
   their architecture, ii. 23

 Difflugia pyriformis, structure, and habitat, and mode of propagation
    of, ii. 22, 23

 Diffusibility, most substances differ in, i. 109
   partial decomposition of definite chemical compounds by, i. 109
   reciprocal diffusion of gases through porous plates, i. 111-113
   the diffusing instrument used by Prof. Graham, i. 112

 Digestion, chemical powers causing, ii. 4

 Dimorphism, contrasted with isomorphism, ii. 99

 Diphyidæ, structure and habitat of the, ii. 103

 Diplazieæ, characters of the group, i. 352

 Diplazium, structure and fructification of, i. 352

 Distillation, ordinary, i. 117
   destructive, i. 117
     of coal, i. 117

 Distomata, characters of the, ii. 146

 Dracæna Draco, or dragon tree of Teneriffe, i. 387

 Drummond’s light, how produced, i. 30
   the continuous spectra of, i. 132

 Dry rot in wood, cause of, i. 266
   on various substances, i. 285

 Dulses, red, i. 235

 Dumontia filiformis, structure of, i. 235

 D’Urvillæa, structure, fructification, and habitat of, i. 256

 Dyes obtained from aniline, i. 122-124
   vegetable dyes, i. 124
   mordants for cotton when dyed, i. 125
   effects of electricity and the east wind on the process of dyeing, i.
      126
   obtained from preparations of petroleum, i. 127
   obtained from lichens, i. 303
   blue dye obtained from some club mosses, i. 374


                                   E

 Ear-shell, structure of the, ii. 234

 Earth, quantity of heat which would be generated if it were arrested in
    its orbit, i. 27
   and if it struck the sun, i. 28

 Earth light, causes of, i. 68

 Earth-worm, structure and food of the, ii. 151, 152

 Echinodermata asteroïdea, structure, mode of reproduction, and habitat
    of, ii. 169-174

 Echinodermata crinoïdea, or stone-lilies, structure, habitat, and mode
    of reproduction of, ii. 175, 176
   fossil remains of, ii. 175

 Echinodermata Echinoïdea, structure and mode of reproduction of, ii.
    177-182

 Echinodermata: fossil Echinidæ, ii. 182, 183

 Echinodermata Holothuroïdea, structure and mode of reproduction of, ii.
    183

 Echinodermata Sipunculidæ, characters of, ii. 186, 187

 Echinodermata Synaptidæ, characters of the order of, ii. 184, 185

 Echinus, structure of, ii. 176, 177

 Echinus miliaris, spines of the, ii. 181

 Ecklonia, structure and habitat of, i. 252

 Ecklonia buccinalis, structure of, i. 250

 Ectocarpeæ, form, structure, and habitat of, i. 244, 245

 Ectocarpus pulsillus, fruit of, i. 245

 Effusion of gases by pressure, i. 109, 110
   Prof. Graham’s experiments, i. 113

 Electricity, i. 30
   permanent and regular current of, over the earth, in the atmosphere
      and in the surface of the earth, i. 31
   thunder and lightning, i. 31
     force exerted in the creation of the deep, i. 31
   the voltaic battery and the electro-magnetic induction apparatus, i.
      31, 32
   reciprocity of action of electricity and heat, i. 31
   electricity produced by chemical action, and conversely, i. 32
   intensity of the light and heat of the electric spark, i. 32
     produced by fifty Bunsen elements, i. 32
   conducting power of charcoal, i. 32
   air and glass non-conductors, i. 32
   proof of the correlation of heat and electricity, i. 33
   motion of the atoms of a conducting wire during the passage of an
      electric current, i. 33
   effect on a conducting wire of an invariable transit of electricity
      sent from the same pole of an inductive apparatus, i. 33
   difference between electric and magnetic currents, i. 33
   influence of electricity on the aggregation of matter, i. 74
   ratio between the specific heat and weight of the atoms of matter, i.
      74
   influence of magnetism on the stratified appearance of the electric
      light, i. 78
   extreme heat and light of electric discharges, i. 84
     the arc discharge, i. 84
   cause of the stratified discharge, i. 85
   effect of varied intensity on electric discharges, i. 86, 87
   effect of varied resistance on electric discharges, i. 88, 89
   varied facility with which substances transmit electricity, i. 90
   illustration of the action of electricity and magnetism on light, i.
      90
   instances of the correlation of electricity and heat, i. 91
   voltaic electricity, and its combination and resolution of substances
      according to the law of definite proportions, i. 94
   Faraday’s law of the quantity of electricity required to separate and
      unite the same atoms, i. 94
   superiority of voltaic over static electricity, i. 95
   electricity the most powerful instrument of analysis, i. 95
   capacity of atoms for electricity, i. 100
     a given quantity of electricity required to separate combined
        substances into their component parts, i. 101
   spectrum analysis of the electric spark, i. 138
   development of electricity in plants and flowers, i. 430
   electric currents in the nerves and muscles of animals, ii. 7

 Electro-chemical action in heterogeneous atoms of matter, i. 95

 Electro-magnetic induction apparatus, i. 31
   Ruhmkorff’s, i. 32

 Elvellacei, characters of the, i. 290

 Empusa Muscæ, or fly fungus, i. 274

 Encalypta vulgaris, organs of fructification of, i. 326

 Encrinites, structure of, ii. 175

 Endocarpei, structure and habitat of, i. 310

 Endocarpon lacteum, structure and organs of reproduction of, i. 299

 Endogenous plants, i. 383-403

 Enteromorpha, characters of the genus, i. 223

 Enteromorpha intestinalis, structure and habitat of, i. 223, 224

 Entomostraca, characters of the, ii. 203

 Entophytes, characters of the group of, i. 275, 276
   sporangia, or spore-cells, of, i. 279

 Entozoa, characters of the order of, ii. 144
   transformations of the young of the, ii. 146

 Eolis, the crowned, structure of, ii. 240, 241

 Eozoön Canadense, may be regarded as the first appearance of animal
    life upon the earth, ii. 54
   found in fundamental quartz rocks, ii. 54
   structure of, ii. 55
   range of its existence, ii. 56

 Epipactis palustris, structure and mode of reproduction of, i. 397, 398

 Epiphytes, characters of the group of, i. 275

 Epithemia, mode of development of, i. 202

 Epizoa, or suctorial crustacea, structure, habitat, and mode of
    reproduction of, ii. 212

 Equisetaceæ, or horse-tails, characters of, i. 367
   contrasted with ferns, i. 369
   large quantity of silex in, i. 369
   size and habitat of fossil and existing species, i. 369

 Equisetum giganteum, structure and fructification of, i. 368

 Ergot, Cordyceps the second form of, i. 293

 Errantia, structure and habitat of the, ii. 156, 157, 161

 Erysiphe, mildew formed by, i. 295

 Eschara, structure of, ii. 218

 Ether, sulphuric absorption of radiant heat by, i. 40

 Eucamptodon perichætialis, leaves of, i. 330

 Eucyrtidium, structure of, ii. 20

 Euglena acus, structure of, ii. 72

 Euglena sanguinea, structure of, ii. 72

 Euglena, structure of the genus, ii. 72

 Euglyphæ, structure and habitat of, ii. 22

 Eunice, structure of, ii. 157
   mode of reproduction of, ii. 160

 Euparmeliaceæ, characters of the group, i. 304

 Euphorbiaceæ, or spurgeworts, poisons and food supplied by the, i. 425,
    426

 Evernia flavicans, colour of, i. 303

 Evernia jubata, structure of, i. 302

 Evernia vulpina, i. 303
   brown dye obtained from, i. 303

 Exchange, law of, i. 35
   independent proofs of the, i. 35, _note_

 Exidia Auricula Judæ, or Jew’s Ear fungus, structure and fructification
    of, i. 266

 Excœcaria Agallocha, poison of the, i. 426

 Exogenous plants, i. 404-408

 Eyes of man, fungi in, i. 275


                                   F

 Fairy rings of the fields, i. 262

 Faujasina, structure of, ii. 45

 Felspar, fluorescence of, i. 66

 Fermentation, fungi producing, i. 286-288
   minuteness and lowness of organization of the ferments, ii. 68
   habitat of the, ii. 68

 Fibrin, formation and structure of, in vegetable organisms, i. 423

 Fig, common, juices of fruit of, changed into sugar, i. 426
   poison of the white juice of the, i. 426

 Filariæ, structure and mode of reproduction of, ii. 147

 Filices, or ferns, structure and habitat of, i. 335
   range of non-arborescent ferns, i. 335
   number of species in North America, Britain, and in other places, i.
      336
   development of spores, i. 336, 337
   roots and stems of, i. 339
   leaf-stalks of, i. 340
   fronds of, i. 340, 341
   structure of tree-ferns, i. 341
   fructification of, i. 341
     sori, i. 341, 342
     sporangia, i. 342, 343
   foundation of the systematic arrangement of the ferns, i. 343
     annulate and exannulate ferns, i. 344

 Film fern, structure and habitat of, i. 362

 Fire-damp of coal mines, i. 118

 Fireworks, mode of the obtaining of different colours in, i. 132

 Fish, phosphorescence of, i. 67

 Flowering fern, i. 364

 Flowers, absorption of radiant heat by the perfumes of, i. 44
   weight of the perfumes, i. 45
   chemical combinations forming the perfumes of, i. 97
   general structure of flowering plants, i. 378
   chemical nature of the colours of, i. 428, 429

 Fluorescence, property of, in some solids and liquids, i. 60
   Sir D. Brewster’s discovery of, i. 66
   Professor Stokes’s examination of the fluorescent spectra of metals,
      i. 64
   employed in tracing substances in impure chemical solutions, i. 67
   rapid absorption accompanied by copious fluorescence, and the
      converse, i. 67
   essential difference between fluorescence and phosphorescence, i. 67,
      68

 Fluorine, i. 18

 Fluor spar, i. 18
   crystals of, i. 18
   acid obtained from, i. 18
   fluorescence of, i. 60, 66
   absorption of invisible rays by, i. 65
   phosphorescence of, i. 66

 Flustra, or sea-mat, structure of the, ii. 218

 Fly fungus, i. 274

 Fontinalei, structure and habitat of, i. 331

 Food, miraculous descent of, i. 305
   lichens as, i. 305, 308
   importance of, to the animal frame, ii. 3

 Foot-pound, the, of Mr. Joule, i. 26
   unit of mechanical force, i. 26

 Foraminifera, structure and geological importance of, ii. 27
   various forms of, ii. 28
   order of porcellanous foraminifera, ii. 30
   order of arenaceous foraminifera, ii. 36
   order of vitreous foraminifera, ii. 37
   abundance of fossil foraminifera in the sedimentary strata, ii. 52
   mode of obtaining casts of, ii. 53
   comparison of foraminifera recent and fossil, ii. 53
   the Eozoön Canadense, ii. 54
   the Carpenteria a link between the foraminifera and sponges, ii. 57

 Force, i. 23
   store of, eternal and unchangeable, i. 24, 25
   cohesion, i. 25
     in solids, i. 25
     in liquids, i. 25
     reciprocal attraction between solids and liquids, i. 25
   unit of mechanical force, i. 26
   heat generated by impetus, i. 27
   unit, or mechanical equivalent of heat, i. 29
   combustion a case of impetus, i. 30
   energy existing in the coal fields on the globe, i. 30
   magnetism and electricity, i. 30
   influence of force on the aggregation of matter, i. 73
     power of electricity in this respect, i. 74
     relations between the force of magnetism and atoms of matter, i. 75
   effect of the physical forces on molecular arrangement, i. 91
     electricity, i. 91
     motion, i. 91
     catalysis, i. 91
   force of chemical combination, i. 97, 98
   relation between chemical affinity and mechanical force, i. 98

 Formic acid, synthetical formation of, i. 424

 Fragillaria, development of, i. 201

 Fraunhofer’s lines, i. 129

 Frond, or thallus, of lichens, i. 301
   of ferns, i. 340, 341

 Fruit, chemical combinations forming the perfumes of, i. 97
   preserved, greenish and grey moulds on, i. 285
   fungus on decayed, i. 290

 Fucaceæ, structure and fructification of, i. 244, 250, 251

 Fuci, sexual fructification of, i. 253

 Fucoideæ, habitat of some, i. 256

 Fucus platycarpus, male and female cells of, i. 253, 254

 Fucus vesiculosus, or bladder-wrack, structure and fructification of,
    i. 252, 253
   form of, in the Mediterranean, i. 258

 Fuegians, staple food of the, i. 292

 Funaria hygrometrica, structure of, i. 324, 327

 Fungi, enormous numbers of, i. 260
   structure of, i. 260
   two principal groups of, i. 260
   families of, i. 261, _et seq._
   poisonous matter in all, i. 264
   luminous, i. 264
   stems of, i. 268
   spawn of the gelatinous or creamy fungi, i. 269
   universality of the lower fungi, i. 273
     destruction caused by them, i. 273
     parasitic fungi, i. 274
   conidium, or spore-dust cell, i. 279
   propagation of fungi by fragments of spawn, i. 281
   extreme minuteness and profusion of fungi, i. 297
   sudden and often disastrous appearance of the lower fungi, i. 297
   conditions necessary to the development of fungi, i. 297

 Furcellaria fastigiata, mode of reproduction of, i. 229
   structure and mode of reproduction of, i. 235, 238

 Fusarium tremelloides, its identity with the nettle Peziza, i. 292


                                   G

 Galeolaria lutea, structure, development, and mode of reproduction of,
    ii. 107, 108

 Garancine, from what obtained, i. 124

 Gas coal. _See_ Coal gas.

 Gases, absorption and radiation of radiant heat by gases and vapours,
    i. 38
   relation between the density of the gas and the quantity of heat
      extinguished or absorbed, i. 40
   experiments on coloured gases, i. 40
   table showing the absorption of various gases at a common pressure,
      or tension of one atmosphere, i. 41
   table of absorption for one inch of tension, i. 4
   causes of the difference between the absorptive power of compound and
      simple gases and vapours, i. 42
   radiation equal to absorption, i. 46
   action of different thicknesses of the same gas or vapour on radiant
      heat, i. 47
   dynamic absorption and radiation, i. 49
   great opacity of a gas to radiations from the same gas, i. 52
   the specific heat of compound gases generally greater than that of
      their component elements, i. 101
   law of equivalency in weight of, i. 102
   diffusion of, i. 111
     Prof. Graham’s experiments, i. 112
   effusion of, 109, 110, 113
     Prof. Graham’s experiments, i. 113
   atmolysis, or method of analysing gases, i. 111, 112, 114
   internal movement of molecules of matter in a gaseous state, i. 113
   coal gas, i. 117
     spectrum analysis of, i. 139
     oxygen, i. 139
     hydrogen, i. 139
     nitrogen, i. 140
     chlorine, i. 140
     bromine, i. 140
     iodine, i. 140
     superposed spectra of rarefied compound gases, i. 141
     researches of M.M. Bunsen and Kirchhoff, i. 141
   spectra of the rarefied vapours of:—
     sodium, i. 141
     iron, i. 142
     calcium, i. 142
     strontium, i. 142
     lithium, i. 142
     barium, i. 142
     magnesium, i. 142
     effect of high temperature on various spectra, i. 142, 143
     effects of pressure on various spectra, i. 145

 Gasteromycetes, characters of the family, i. 267

 Gastropoda, structure of the shells of, ii. 234
   structure of the animal, ii. 235
   tongue of the, ii. 239

 Gelidiaceæ, structure and mode of propagation of, i. 238

 Gelidium corneum, structure and mode of propagation of, i. 238
   form of, i. 243

 Gems, crystals of, formed artificially by electricity, i. 74

 Geranium, amount of radiant heat absorbed by the perfume of, i. 44

 Glass, a non-conductor of electricity, i. 32
   property of, in regard to the transmission of light, i. 36
   impervious to chemical solar rays, i. 63

 Gleichenineæ, sporangia of, i. 34
   structure and habitat of, i. 344, 360

 Globigerina bulloïdes, form of, ii. 28

 Globigerina, structure of the genus, ii. 40, 41

 Globigerinæ, abundance of at vast depths in the ocean, ii. 49-51

 Globigerinidæ, characters of the family of, ii. 40

 Glycerine, chemical combination forming, i. 97

 Glyphidei, characters of the order, i. 309

 Glucinum, i. 4

 Gold, colour of, in reflected and in absorbed light, i. 35, 36
   crystals of, formed artificially by electricity, i. 74

 Gonidia, propagation of diatoms by, i. 202, 204
   of lichens, i. 300

 Goniometer, i. 38

 Gorgonia graminea, structure and habitat of, ii. 124, 125

 Gorgonia verrucosa, structure, habitat, and mode of reproduction of,
    ii. 124

 Gorgoniidæ, structure and mode of reproduction of, ii. 123, 124
   the three natural groups of, ii. 124

 Gracilaria compressa, i. 239

 Gracilaria lichenoides, or Ceylon moss, i. 239

 Gracilaria armata, mode of reproduction of, i. 230

 Graminaceæ, structure and mode of reproduction of, i. 385, 386

 Grammatophora serpentina, structure and development of, i. 197

 Graphidei, characters of the order, i. 308
   habitat of the, i. 309

 Graphite, form of the crystals of, i. 15
   natural, little or no porosity of, i. 112
   porosity of artificial graphite, i. 112

 Grasses, silica in the stalks and leaves of the, i. 17

 Grasses, structure and mode of reproduction of, i. 385-387

 Gravitation, force of, less powerful than that of molecular attraction,
    i. 109

 Gravity, specific, of atoms, i. 100

 Green dye, obtained from the buckthorn, i. 124

 Griffithsia, structure, habitat, and organs of reproduction of, i. 233

 Grimmiei, structure of the tribe, i. 329

 Grinnelia americana, structure and mode of reproduction of, i. 230, 239

 Gromia oviformis, structure of, ii. 26, 27

 Gromiæ, structure of the genus, ii. 25-27

 Guano, mauve dye obtained from, i. 125

 Guernsey, richness of the iodine obtained from the sea-weeds of, i. 258

 Guinea worm, structure and mode of reproduction of, ii. 147

 Gums, formation of, i. 422

 Gymnogramma rutæfolia, remarkable distribution of, i. 336

 Gyrophora, structure of the genus, i. 308


                                   H

 Hairs of plants, structure of, i. 411

 Halichondria panicea, mode of propagation of, ii. 60, 61

 Haliomma, structure of, ii. 21

 Haliotis splendens, or ear-shell, structure of, ii. 234

 Haliseris, structure of, i. 247

 Halogens, spectra of the, i. 146

 Halurus, structure and mode of propagation of, i. 233

 Hare’s-foot fern, i. 351

 Hart’s-tongue fern, caudex of, i. 340
   structure, habitat, and fructification of, i. 351, 352

 Heart, and organs representing it in the lower animals, ii. 4

 Heat generated by impetus, i. 27
   quantity of heat which would be generated if it were arrested in its
      orbit, i. 27
   probable cause of the heat of the sun, i. 28
   effect of the absorption of heat on a body in expansion and
      contraction, i. 28
   specific heat, i. 28
   mechanical equivalent of heat, i. 29
     causes of the heat which is the motive force of the steam-engine,
        i. 29
   reciprocal action of heat and electricity, i. 31
   intensity of the heat of the electric spark, i. 32
   proof of the correlation of heat and electricity, i. 33
   constancy in the amount and refrangibility of the light and heat
      absorbed and radiated, i. 34
   property of some substances in the transmission of heat, i. 36
   substances which transmit radiant heat freely but radiate badly, and
      vice versâ, i. 37
   Tyndall’s experiments on the radiation and absorption of radiant heat
      by gases and vapours, i. 38
   relation between the density of the gas and the quantity of heat
      extinguished or absorbed, i. 40
   absorption of radiant heat by the vapours of volatile liquids, i. 40
   Prof. Tyndall’s experiments showing the radiation to be equal to the
      absorption of radiant heat, i. 46
   action of different thicknesses of the same gas or vapour on radiant
      heat, i. 47
   dynamic absorption and radiation, i. 49
   dynamical evolution of heat, i. 52
   experiment illustrating the change of heat into light, i. 62
   polarization of heat and light, i. 68, 69
   by reflection and refraction, i. 69
   effect of heat on the magnetism of iron, nickel, and cobalt, i. 77
   extreme heat and light of electric discharges, i. 84
   instances of the correlation of electricity and heat, i. 91
   capacity of atoms for heat, i. 100
     Mr. Croll’s experiments, i. 101
   effects of heat on vegetation, i. 169
   large amount of heat evolved by vegetables, i. 265
   cause of animal heat, ii. 63

 Helix, or snail, tentacles of, ii. 236

 Helix aspersa, structure of the tongue of, ii. 237

 Helix pomatia, teeth of, ii. 237

 Hellebore, white, poisonous alkaloid of, i. 427

 Helmet shell, structure of, ii. 234

 Helminthosporium Hoffmanni, spores of, i. 285, 286

 Helminthosporium nodosum, spores of, i. 286.

 Helvella, structure and habitat of, i. 291

 Hepaticæ, or liverworts, characters of the, i. 316

 Hepialus virescens of New Zealand, Cordyceps Robertsii of the, i. 293

 Herschel, Sir W., his discovery of invisible rays of light of high
    heating power, i. 36

 Hewardia, structure of, i. 359

 Himanthalia lorea, structure, fructification and habitat of, i. 256

 Holly fern, structure of, i. 347

 Holothuridæ, or sea-cucumbers, structure and mode of reproduction of,
    ii. 183-186

 Hop, fungus constituting the mildew of the, i. 295

 Hormosiphon arcticus, wide distribution of, i. 212

 Hormotrichum, structure of the genus, i. 222

 Hormotrichum collabens, structure and mode of reproduction of, i. 222

 Hyacinth, structure and mode of reproduction of the, i. 388

 Hyalæa, structure of the, ii. 242

 Hydra fusca, structure and mode of propagation of, ii. 84, 85

 Hydra viridis, structure and mode of propagation of, ii. 85

 Hydra vulgaris, structure and mode of propagation of, ii. 85

 Hydræ, structure of, ii. 81, 84
   mode of propagation of, ii. 84
   compound fresh-water Hydræ, ii. 85
   development of medusa-buds, ii. 95
   alternation of generation of hydræ and medusæ, ii. 96

 Hydraulic machine, bog moss acts as a, i. 333

 Hydridæ, characters of the group, ii. 81

 Hydrobromic acid, amount of absorption of radiant heat by, i. 42

 Hydrochloric acid, absorption of radiant heat by, i. 41, 42

 Hydrodictyon utriculatum, structure and habitat of, i. 211
   reproduction and development of, i. 211

 Hydrofluoric acid, i. 18

 Hydrogen gas, i. 11
   affinity of chlorine for hydrogen, i. 19
   absorptive power of, i. 39, 41
   proportion of hydrogen to oxygen in the composition of water, i. 94
   will not combine with carbon, i. 97
   weights of the atoms of, as compared with those of oxygen, i. 99
   probable cause of its greater cooling power than that of oxygen, i.
      115
   one of the illuminants in coal gas, i. 118
   spectrum analysis of, i. 139, 144
     and of hydrogen, carburetted and attenuated, during and after
        decomposition, i. 141
   carburetted, one of the illuminants in coal gas, i. 118
   marsh-gas and fire-damp, i. 118
   peroxide of, i. 9
     combination forming, i. 104
   combines with other substances and simple atoms as if it were a
      simple element, i. 106
   sulphuretted, amount of absorption of radiant heat by, i. 41
     opacity of, to the invisible rays, i. 65
     one of the illuminants in coal gas, i. 118
     how freed from coal gas, i. 118

 Hydrozoa, campanograde, characters of, ii. 103
   ciliograde, characters of the, ii. 101
   mode of reproduction of, ii. 103
   oceanic, structure of, ii. 81, 86
   compound oceanic, structure of, ii. 87
   medusiform zooids, ii. 88, 89
   mode of reproduction of, ii. 89, 90

 Hymenomycetes, family of, i. 261

 Hymenophyllum, stem of, i. 339

 Hymenophyllum tunbridgense, or film fern, structure and habitat of, i.
    362

 Hymenophyllum unilaterale, habitat and structure of, i. 362

 Hyphomycetes, characters of the family, i. 282

 Hypogæi, structure and fructification of the subterranean sub-order of,
    i. 267, 268

 Hyponitrous acid, combination forming, i. 95

 Hypopterygium Smithianum, leaves of, i. 330


                                   I

 Ice, absorption of invisible rays by, i. 65
   believed to be a colloid body, i. 111

 Iceland moss, characters of, i. 304

 Iceland spar, polarization of light and heat in, by refraction, i. 69
   change of position of the optical axes of the crystals of, by heat,
      i. 73

 Ileodictyon, eaten as food, i. 268

 Impetus, i. 26, 27
   heat generated by, i. 27
   combustion a case of, i. 30

 Indian rubber, dissolved by naphtha for waterproofing, i. 120

 Indium, discovery of the metal so called, i. 137
   properties of, i. 137

 Infusoria, structure, form, and habitat of, ii. 63
   abundance of infusoria in the atmosphere, ii. 63, 64
   different states of development in one and the same animal, ii. 65
   groups of, of opposite characters in the same liquid, ii. 66
   the agents in the decomposition of animal matter, ii. 67
   minuteness of the ferments, ii. 68
   cilia of Infusoria, ii. 68
     as organs of locomotion, ii. 69
   cell constituting the body of Infusoria, ii. 70
   food and organs of digestion of, ii. 70
   transparent contractile vesicles, ii. 71
   modes of propagation, ii. 74 _et seq._
   in a state analogous to hybernation, ii. 78, 79
   functions assigned to Infusoria in the economy of nature, ii. 79

 Insects, phosphorescence of, i. 167

 Intelligence, or the mental principle, of animals, ii. 11, 12

 Iodine, i. 18, 21
   whence obtained, i. 19
   properties of, i. 21, 22
   spectrum of, i. 21
   atomic weight of, compared with that of hydrogen, i. 100
   preparation of, from sea-weeds, i. 128
   spectrum analysis of, i. 140, 141
   iodine richer at Guernsey than elsewhere, i. 258

 Iris germanica, vertical section showing the cellular tissue of, i. 173

 Iron heated by percussion or impetus, i. 27
   effect of electricity on, i. 32
   effect of magnetism of, i. 75, 76, 77
   and of heat on the magnetism of, i. 77
   feeble affinity of iron for mercury, i. 98
   atomic weight of, compared with that of hydrogen, i. 100
   spectrum analysis of the rarefied vapour of, i. 142

 Isaria, its structure and habitat, i. 282
   its destruction of caterpillars, i. 282, 293

 Isaria crassa, characters of, i. 283

 Isaria farinosa, characters of, i. 283

 Isariacei, structure and habitat of, i. 282

 Isidæ, structure, habitat, and mode of reproduction of, ii. 125

 Isis, structure of the genus, ii. 125

 Isomeric substances, i. 104
   law of the sequence of three isomeric bodies, and their respective
      atomic numbers, i. 105

 Isomorphism, property of, in determining atomic weights, i. 99

 Isomorphous substances, conduct of, under magnetic influence, i. 76

 Isopoda, characters of the order, ii. 202


                                   J

 Jam, greenish, and grey moulds on, i. 285

 Jungermanniaceæ, or scale mosses, structure, fertilization, and
    development of, i. 320-322

 Jupiter, the planet, spectrum of, i. 157, 161
   constitution of, i. 158


                                   K

 Katsup made from the morel, i. 292

 Kelp, i. 258
   former and present uses of, i. 128

 Kerona silurus, structure and food of, ii. 69


                                   L

 Labellum of orchids, i. 401

 Lactarii, laticiferous vessels of, i. 263

 Lady Fern, herbaceous caudex of, i. 340
   structure and fructification of, i. 352, 353

 Lagenidæ, characters and habitat of the family, ii. 39

 Lagoons formed by corals, ii. 140-143

 Laminaria bulbosa, structure and fruit of, i. 248

 Laminaria debilis, structure of, i. 248

 Laminaria digitata, or oar-weed, structure of, i. 249
   richness of the iodine obtained from, at Guernsey, i. 258

 Laminaria radiata, structure, habitat, and fructification of, i. 250

 Laminaria saccharina, or devil’s apron, structure and mode of
    reproduction of, i. 249

 Laminariæ, submarine forests of, i. 248

 Lanosa nivalis, the probable cause of the mildew in rye, i. 297

 Lastrea, structure of the genus, i. 346

 Lastrea æmula, structure of, i. 346

 Lastrea filix-mas, structure of, i. 340

 Lastrea rigida, scent of, i. 347

 Lastrea Thelypteris or marsh fern, structure of, i. 346

 Latex, structure of the, in plants, i. 417

 Laughing-gas, or protoxide of nitrogen, combination forming, i. 95

 Laurel, oil of, amount of radiant heat absorbed by the perfume of, i.
    44

 Laurencia dasyphylla, antheridia of, i. 241

 Laurencia pinnatifida, or pepper dulse, structure, habitat, and mode of
    propagation of, i. 241

 Laurencia tenuissima, antheridia of, i. 241

 Laurenciaceæ, characters of, i. 240, 241

 Laurentian system, ancient formation of the crystallized limestone near
    the base of the, ii. 54

 Lavender, absorption of radiant heat by the perfume of, i. 44

 Lead, its resistance to electricity, i. 90
   chloride of, spectrum of, i. 146

 Leathesia, structure of, i. 245

 Leaves of plants, structure and functions of, i. 410

 Lecanora affinis, section of, i. 300
   structure and habitat of, i. 305

 Lecanora esculenta, habitat of, i. 305

 Lecidea, characters of the genus, i. 306

 Lecidea geographica, great age of, i. 306

 Lecidinei, characters of the order, i. 306

 Leeches, structure of, ii. 149-151

 Lemon, amount of radiant heat absorbed by the perfume of, i. 44

 Lepadidæ, structure of, ii. 213, 215

 Lepas anatifera, structure and mode of reproduction of, ii. 215

 Lepidodendron, structure of the fossil, i. 375

 Lepidostrobus ornatus, structure of, i. 375

 Lepraliæ, structure of, ii. 219

 Leptopteris, characters of, i. 364

 Lessonia, arborescent, forests of, i. 251

 Lessonia nigrescens, structure of, i. 251
   rings of growth of, i. 252

 Leucobryum, structure of, i. 329

 Leucobryum glaucum, leaves of, i. 330

 Leveillea, structure of, i. 242

 Libanea crab, a parasite of the medusæ, ii. 100

 Lichens, characters and habitat of, i. 298
   two groups of—Gymnocarpei and Angiocarpei, i. 299
   horizontal lichens, i. 299
   gonidia, i. 300
   asci, i. 301
   thallus or frond, i. 301
   dyes obtained from, i. 124, 303
   brilliancy of the colours of, i. 304

 Lichina, characters of the genus, i. 310

 Lichinei, characters of the group, i. 310

 Life, the mystery of, i. 97
   instances of individual combined with a common life in the lower
      orders of animals, ii. 38

 Light, probable cause of the, of the sun, i. 28
   Drummond’s light, how produced, i. 30
   intensity of the, of the electric spark, i. 32
   chemical action of, i. 34
     amount of force exerted by the sun’s light within the limits of the
        terrestrial atmosphere, i. 34
   constancy of the amount and refrangibility of light and heat absorbed
      and radiated, i. 34
   causes of colour of flowers, leaves, dyed cloth, and gold and copper,
      i. 35, 36
   property of some substances in the transmission of radiant light and
      heat, i. 36
   invisible rays of high heating power existing beyond the red end of
      the solar spectrum, i. 36
   Melloni’s investigation of the laws of radiation and absorption of
      radiant heat in solid and liquid matter, i. 38
   chemical power of the light of the moon and stars, i. 55
   and of solar light, i. 56
     effect of the opalescence of the atmosphere on the light of the
        sun, i. 55
   probable causes of the blueness of the sky, and the brightness of the
      tints at sun-rise and sun-set, i. 58
   solar spectrum, i. 58
     myriads of ethereal waves constituting the seven colours of the, i.
        58
   fluorescent or degraded light, i. 60
   experiment illustrating the change of heat into light, i. 62
   absorption of invisible rays, by solids, liquids, and gases, i. 65
   causes of earth light, i. 66
   polarization of light and heat, i. 68, 69
     by reflection and refraction, i. 69
   relation of polarization to crystallization, i. 70, 71
   M. Gassiot’s experiments on stratified electric light, i. 78
   influence of magnetism on the stratified light, i. 78
   cause of the stratified discharge, i. 85
   effect of varied intensity on electric discharges, i. 86
   action of magnetism and electricity on light, i. 90
   delicate power of analysis in light, i. 95
   Dr. Young’s establishment of the undulatory theory of light, i. 129
   Drummond’s light, i. 132
   coloured light obtained from the combustion of the salts of different
      metals, i. 132
   effects of light on vegetation, i. 168, 169
   effects of light as an exciting cause in the vegetable world, i. 431

 Lightning, spectrum of, i. 147

 Ligneous tissue, i. 174, 175

 Liliaceæ, structure and development of the, i. 388

 Limboriei, structure and habitat of the, i. 310

 Lime, carbonate of, different modifications assumed by, i. 74
   chloride of, properties of, i. 19
   oxalate of, formation of, i. 117

 Limestones, probable history of the oldest, ii. 52
   the nummulitic, ii. 53
   the limestone formed by the Eozoön Canadense, ii. 54

 Limnoria lignorum, structure of, ii. 203

 Limpet, structure, food, and habitat of, ii. 238, 239

 Limulus, fossil remains of, ii. 211

 Lindsæeæ, characters of, i. 351

 Lingbya, structure of, i. 213

 Lingula flags, fossil animals composing the, ii. 53

 Listera ovata, structure and mode of reproduction of, i. 399

 Lithium, i. 3
   spectrum analysis of, i. 133
   existence of lithium in all three kingdoms of nature, i. 134
   spectrum analysis of the rarefied vapour of, i. 142
   effect of high temperature, i. 142, 143
   chloride of, spectrum of, i. 146

 Litmus, or orchil, from what obtained, i. 124, 303

 Lituola, structure of the genus, ii. 37

 Lituolidæ, characters of the family of, ii. 36, 37

 Liver worts. _See_ Hepaticæ

 Lobophylla angulosa, structure of, ii. 135, 136

 Locomotion of some diatoms, i. 204

 Lo-hao, a Chinese green dye, i. 124

 Loligo vulgare, or squid, structure of, ii. 245, 246

 Lomaria, characters of the genus, i. 357

 Lomariopsis, characters of, i. 360

 Loxades bursaria, structure of, ii. 72

 Loxades bursaria, mode of propagation of, ii. 75

 Lucifer, characters of the genus, ii. 200
   matches, i. 17

 Luminosity of the medusæ in warm seas, ii. 99
   of Annelida, ii. 160
   of Pyrosomidæ, ii. 226

 Lycoperdon giganteum, structure, habitat, and fructification of, i. 267

 Lycoperdon, structure and fructification of, i. 267

 Lycopodiaceæ, or club mosses, characters of, i. 373
   habitat and fructification of, i. 373, 374
   uses to which they have been applied, i. 374

 Lycopodium, or wolf’s-claw, characters of the genus, i. 374

 Lycopodium clavatum, structure and habitat of, i. 374
   inflammability of the dried spores of, i. 374

 Lycopodium inundatum, structure and habitat of, i. 374

 Lygodium articulatum, structure of, i. 363

 Lymnæa, or pond snail, distoma of the, ii. 146


                                   M

 Macrocystis pyrifera, structure, habitat, and fructification of, i. 250

 Macrura, characters of the, ii. 189
   fossil remains of, ii. 211

 Madder, dyes obtained from, i. 124, 125

 Magenta, or aniline red, Dr. Hofmann’s discovery of, i. 122
   how produced, i. 122
   purple dye produced by mixing it with aniline, i. 123

 Magnesium, i. 3, 4
   spectrum of, i. 65
   spectrum analysis of the rarefied vapour of, i. 142
   shown to be one of the metals existing in the sun, i. 151-153
   spectrum of the light of a magnesium flame, i. 153, 154
   its use in photography, i. 154

 Magnetism, i. 30
   periodic and secular variations of, i. 30
     probably stand in some periodic connection with the solar spots, i.
        30, 31
   probable cause of terrestrial magnetism, i. 31
   difference between magnetic and electric currents, i. 33
   relations between the force of magnetism and the atoms of matter, i.
      75
   all substances either magnetic or diamagnetic, i. 75
   proof of the connection between the magnetic forces and crystalline
      structure, i. 76
   conduct of amorphous and isomorphous substances under magnetic
      influence, i. 76
   action of magnets upon matter most powerful in the line of maximum
      density, i. 76
   conditions of the position which crystals take with regard to the
      magnetic force, i. 76
   magnetic changes in the relations and distances between the ultimate
      atoms of matter, i. 77
   effect of magnetism on the stratified appearance of the electric
      light, i. 78
   causes of the polarity of a magnet, according to Ampère, i. 80
   action of magnetism on the stratified discharges of electric light,
      i. 82
   illustration of the action of magnetism and electricity on light, i.
      90

 Maiden’s hair fern, structure and habitat of, i. 359

 Malaxis paludosa, or bog malaxis, structure and mode of reproduction
    of, i. 400

 Malic acid, chemical combination forming, i. 97

 Manchineel, poison of the, i. 426

 Manganese, peroxide of, relative weight of the atoms of oxygen and
    metal in, i. 99
   atomic weight of, compared with that of hydrogen, i. 100
   peroxide of, combination forming, i. 104

 Manihot, or Cassava, food obtained from the, i. 426

 Marasmius Oreades, spawn of, i. 262

 Marattia salicina used as food, i. 365

 Marattiaceæ, characters of the group, i. 364

 Marchantia polymorpha, structure, development, fructification, and
    habitat of, i. 317-320

 Marchantiaceæ, characters of the order, i. 317

 Mars, the planet, spectrum of, i. 158, 161

 Marsh-gas, i. 118
   amount of absorption of radiant heat by, i. 41

 Marsilea, characters of the genus, i. 372

 Marsileaceæ, or Rhizospermæ, characters of the tribe, i. 371

 Matonineæ, characters of the group, i. 344

 Matter, molecules of, i. 2
   agency of electricity in the chemical composition and decomposition
      of, i. 32
   decomposing and elective power of, i. 35

 Mauve, Dr. Hofmann’s discovery of the aniline colour, i. 122
   at first made from orchil, i. 124
   derived from guano, i. 125

 Mediterranean sea, zones of algæ in the, i. 259
   foraminifera, the ooze in the bed of the, ii. 51

 Medusæ, form and structure of, ii. 91

 Medusæ, pulmograde, form and structure of, ii. 91, 92
   naked-eyed medusæ, ii. 91-96
   mode of reproduction of, ii. 94
   armed or stinging medusæ, ii. 99
   luminosity of, in warm seas, ii. 99
   development of medusa-buds, ii. 96
   alternation of generation of hydræ and medusæ, ii. 96
   covered-eyed medusæ, ii. 97
   parasites of, ii. 99
   abundance of species of, ii. 100
   food of medusæ, ii. 101

 Medusa-buds, development of, ii. 95

 Medusiform zooids of hydrozoa, ii. 88, 89

 Melanconiei, characters of the group of, i. 281, 282

 Melanogaster, the red truffle of Bath, i. 268

 Melanospermeæ, characters of, i. 181
   marine forests of, i. 244
   structure of the, i. 244, _et seq._

 Mercury, conditions under which he may be habitable, i. 55

 Mercury, its feeble affinity for iron, i. 98

 Meridion circulare, structure and development of, i. 200, 201

 Merulius lacrymans, the cause of dry rot in wood, i. 266

 Mesocarpus, reproduction of, i. 217

 Mesogloias, structure of, i. 245

 Metals and their properties, i. 3
   alkaline metals, i. 3
   of alkaline earths, i. 3
   from non-alkaline earths, i. 4
   avidity of some of them for oxygen, i. 4
   metals whose oxides are not reducible by heat, i. 4
     diatomic metals, i. 4
     triatomic metals, i. 5
   conduction of heat and radiation, i. 5
   vaporization, i. 5
   spectra of volatilized metals, i. 64
   coloured light obtained from the combustion of the salts of different
      metals, i. 132
   spectrum of, not always the same, i. 145
   number of the metals of which the spectra has been determined i. 147
   metals shown to exist in the sun, i. 151-153
     and not to exist in it, i. 154
   metals contained in every plant, i. 414

 Meteorites probably not of solar origin, i. 153

 Mica, opacity of, to the invisible rays, i. 65

 Mignonette, weight of the perfume of, i. 45

 Mildew of the vine, hop, &c., i. 295
   the black mildews of the Azores and Ceylon, i. 297

 Miliola, structure and habitat of, ii. 30
   abundance of, in the seas of the Eocene period, ii. 31

 Miliolidæ, characters of the order, ii. 30

 Milk sap of plants, i. 425
   vessels of plants, i. 417

 Milleporæ complanata or palmipora, structure of, ii. 141

 Mimosa pudica, irritability of the tissues of, i. 432

 Mind and matter connection between, ii. 5

 Molecules of matter, i. 2
   cohesion of, i. 25
   unit of mechanical force, i. 26
   causes of the ethereal undulating motions of, i. 34, 35
   _See_ Atoms

 Mollusca, structure, habitat, and mode of reproduction of, ii. 229, _et
    seq._
   shells of the, ii. 232
   naked, ii. 240
   winged, ii. 240

 Monas corpusculus, extreme minuteness of, ii. 63, 67
   and of its ova, ii. 67

 Monocotyledonous, or endogenous plants, structure, growth, and
    reproduction of, i. 383
   seeds of this class, i. 383
   stems, or axes, i. 384
   remarkable plants belonging to the monocotyledons, i. 387

 Monostoma, or one-mouthed medusæ, structure and mode of reproduction
    of, ii. 97

 Monormia, habitat and structure of, i. 212

 Moon, interception of the heat radiated by the full, by the earth’s
    atmosphere, i. 55
   its effect on the higher regions of the atmosphere, i. 55
   chemical power of the moon’s light, i. 55

 Moonstone, or adularia, fluorescent property of, i. 66

 Moonwort, structure, and habitat of, i. 366

 Morchella esculenta, the morel, i. 291
   katsup made from, i. 292

 Mordants for fixing dyes in cotton cloth, i. 125

 Morphine, the probable narcotic principle of opium, i. 427

 Mother-of-pearl, composition of, ii. 234

 Motion, effect of, on molecular arrangement, i. 91

 Moulds, fungus, on various substances, i. 285

 Mucedines, structure of the, i. 283
   the origin of all fermentation, i. 289

 Mucorini, or moulds, structures of the, i. 296
   habitat of the, i. 296, 297

 Muriatic acid, formation of, i. 20
   action of, upon ammonia, i. 120

 Musci, or Mosses, structure and mode of reproduction of, i. 3
   antheridia of, i. 324
   sporangia of, i. 325
   gemmæ or buds, i. 327
   leaves of, i. 330
   aquatic mosses, i. 331
   peat, i. 333
   uses of mosses, i. 334

 Muscle, structure and functions of, ii. 2
   functions of the muscles, ii. 5
   electric currents formed in the muscles, ii. 7
   muscular respiration, ii. 8

 Mushrooms, i. 261
   mycelium, or spawn, i. 262

 Musk, power of absorption of radiant heat by the perfume of, i. 45

 Mussel, structure of the gills of the common, ii. 230

 Mycelium or mushroom spawn, i. 261, 262

 Mysis, or opossum shrimps, structure of, ii. 199

 Myxogastres, structure, habitat, and reproduction of, i. 269, 270
   their Amœba-like motions, i. 270


                                   N

 Naïs, structure and habitat of the, ii. 152, 153

 Naphtha, manufacture of, i. 120
   uses to which it is applied, i. 120
   sources of, in various parts of the world, i. 126
   naphtha procured by the distillation of petroleum, i. 127

 Naphthalin, production of, i. 127

 Narcotics, exciting, known to almost all people, however savage, i. 427

 Navicula, structure of, i. 198

 Naviculæ, spontaneous locomotion of, i. 202

 Nebulæ, spectra of various, i. 158-160, 163
   constitution of the, i. 163
   probable existence of primordial nebulous matter according to the
      theories of Sir W. Herschel and La Place, i. 164

 Nectria, characters of the genus, i. 295

 Nectria Peziza, structure of, i. 295

 Nematoid entozoa, structure and mode of reproduction of, ii. 146

 Nemertes gigas, or great band worm, nervous system of, ii. 158

 Neotteæ, characters of the British tribe of, i. 397, 398

 Neottia Nidus-avis, structure and mode of reproduction of, i. 400

 Nephrolepis tuberosa, characters and habitat of, i. 348

 Nereis, structure of, ii. 157

 Nereis diversicolor, mode of reproduction of, ii. 160

 Nereocystis Lutkeana, structure and habitat of, i. 249

 Nerve-force of animals, ii. 4, 6
   chemical powers generated by, ii. 6

 Nerves of animals, structure and functions of, ii. 5
   electric currents in the, ii. 7
   structure and functions of the brain and spinal cord, ii. 8
   the nervous systems of the higher and lower animals, ii. 9-11

 Nervous system of animals, ii. 5

 Nickel, i. 4, 5
   crystals of, formed artificially by electricity, i. 74
   effect of heat on the magnetism of, i. 77
   atomic weight of, compared with that of hydrogen, i. 100

 Nidulariacei, characters of the order of, i. 272, 273
   structure and mode of reproduction of, i. 273
   habitat of, i. 273

 Nitella flexilis, structure and mode of reproduction of, i. 312

 Nitophyllum, structure and fronds of, i. 238, 239

 Nitric acid, combination forming, i. 95

 Nitrogen gas, i. 12
   combination of nitrogen with chlorine, i. 20
   absorptive power of, i. 39, 41
   fulminates compounds of, i. 92
   number of combinations of, with oxygen, i. 95
   atomic weight of, compared with that of hydrogen, i. 100
   one of the illuminants in coal gas, i. 118
   spectrum analysis of, i. 140, 145
     in high temperature, i. 144
   binoxide of, combination forming, i. 95
   chloride of, catalysis of, i. 91
   iodide of, catalysis of, i. 91
   protoxide of, or laughing gas, combination forming, i. 95

 Nitrous oxide, absorption of, radiant heat by, i. 41, 43
   combination forming, i. 95

 Noctiluca miliaris, structure and mode of propagation of, ii. 73, 74

 Noctiluci, their food, i. 205

 Nodosaria, characters of the genus, ii. 39

 Nodosaria rugosa, shell of, ii. 28
   structure of, ii. 39

 Nodosaria spinicosta, form of, ii. 28
   structure of, ii. 39

 Nostoc commune, wide distribution of, i. 212

 Nostochineæ, structure and habitat of, i. 211
   reproduction of, i. 212
   wide distribution of, i. 212

 Nullipores, structure and habitat of, i. 142

 Nummulites, structure of the, ii. 44-46
   circumstances favouring their existence in the Tertiary period, ii.
      53


                                   O

 Ocean, force exerted in the creation of the, i. 31

 Octoblepharum albidum, leaves of, i. 330

 Octopods, structure of, ii. 245

 Octopus vulgaris, or poulpe, structure of, ii. 245-247

 Odonthalia, structure of, i. 242

 Œdogonium capillare, reproduction of, i. 216

 Oïdium, structure and habitat of, i. 290

 Oïdium Tuckeri, or vine mildew, fungus producing, i. 295

 Oils, dead, manufacture of, i. 120
   essential, chemical combinations forming, i. 97
   vegetable, i. 422
   formation of fixed and essential, artificially, i. 424

 Oleandra, characters of the genus, i. 347

 Oleandra neriiformis, structure of, i. 347

 Olefiant gas, absorptive power of, i. 39, 40, 41
   amount of absorptive power, i. 41
   great absorption of, i. 47
   formation of, i. 97
   amount of carbon in, i. 105
   one of the illuminants in coal gas, i. 118
   chemical composition of, i. 128

 Olefiant oil, i. 104

 Olive oil, i. 422

 Onion, structure and mode of reproduction of, i. 388

 Oniscus, common wood-louse or slater, structure of the, ii. 202

 Onoclea, characters of the genus, i. 348

 Onoclea sensibilis, characters and habitat of, i. 348

 Oolina claxata, shell of, ii. 48

 Opacity in liquids and solids synonymous with accord, i. 37
   causes of opacity, i. 37

 Operculina, structure of, ii. 38, 46
   an instance of an individual combined with a common life, ii. 38.

 Ophioglossaceæ, characters of the group, i. 365

 Ophioglossum, characters of the genus, i. 365

 Ophioglossum vulgatum, structure and habitat of, i. 365

 Ophionyx, structure of, ii. 173

 Ophiuridæ, or snake stars, structure of, ii. 172, 173
   power of reproducing rays, ii. 173

 Ophrys apifera, or bee ophrys, structure and mode of reproduction of,
    i. 397

 Ophrys, fly, structure and mode of reproduction of, i. 396

 Opium, alkaloids obtained from, i. 427
   almost universal use of, in the East, i. 427

 Orange, amount of radiant heat absorbed by the perfume of, i. 44
   fructification of the, i. 381

 Oranges, fungus of decayed, i. 290

 Orbitolite, characters of the, ii. 33
   a fossil gigantic one found in Canada, ii. 54

 Orbitolites complanatus, structure of, ii. 33, 34
   development and varieties of, ii. 35, 36
   habitat of, ii. 36

 Orchids, their structure, habitat, and mode of reproduction, i. 388-403
   theoretical structure of, i. 403

 Orchil, whence obtained, i. 303
   from what obtained, i. 124

 Orchis mascula, structure and mode of reproduction of, i. 389, _et
    seq._

 Orchis pyramidalis, structure and mode of reproduction of, i. 393, _et
    seq._

 Oscillatoria littoralis, structure and motions of, i. 213

 Oscillatoria spiralis, motions of, i. 213

 Oscillatoriæ, structure, habitat, and motions of, i. 213
   reproduction of, i. 214

 Osmunda regalis, structure and habitat of, i. 363

 Osmundineæ, characters of, i. 363
   sporangia of, i. 343
   characters of the group, i. 345

 Ostrapods, characters of the, ii. 207

 Otolites of Gastropoda, ii. 236

 Otolites of Thaumantias, ii. 93

 Ovulata, siliceous skeleton of, ii. 60

 Oxalic acid, former and present modes of making, i. 116
   produced by lichens, i. 303
   synthetical formation of, i. 424

 Oxygen, i. 6-11
   effect of the combination of the atoms of oxygen and carbon in
      combustion, i. 30
   absorptive power of ozonized oxygen, i. 43
   absorptive power of, i. 39, 41
   proportion of oxygen to hydrogen in the composition of water, i. 94
   limit to the number of combustions of, with nitrogen, i. 95
   weight of the atoms of, in the peroxide of manganese, i. 99
   weight of the atoms of, compared with those of hydrogen, i. 99, 100
   spectrum analysis of, i. 139
   inhalation and exhalation of, by plants, i. 416, 417

 Oysters, iodine found in, i. 19

 Ozone, i. 7
   effect of the combination of ozone and oxygen in the absorption of
      radiant heat, i. 43
   Prof. Tyndall’s conjecture as to the production of ozone, i. 44
   its affinity for iodine, i. 96
   its possible effect on the solar spectrum, i. 132
   production of, i. 17


                                   P

 Padina Pavonia, or Peacock’s tail laver, structure of, i. 247

 Pagurus, or hermit crab, structure and mode of reproduction of, ii. 197

 Palm, stem or axis of a, i. 384
   growth and reproduction of palms, i. 384, 385

 Palmoglœa macrococca, structure and development of, i. 182

 Palmyra palm, growth of the stem of, i. 385

 Pandanus or screw pine, roots and habitat of, i. 387

 Papillaris, siliceous skeleton of, ii. 60

 Paraffin, i. 105
   crystals of, how produced, i. 127

 Paraffin oil and candles, i. 119, 127

 Paramœcium caudatum, cilia and mouth of, ii. 68, 69
   immense propagation of, ii. 74

 Parkeriaceæ, or Ceratopteridineæ, structure and habitat of, i. 363

 Parmelia saxatilis, structure and habitat of, i. 305

 Parmeliacei, structure of the, i. 302
   characters of the group, i. 304

 Parmelia parcolerina, yellow dye obtained from the, i. 124

 Parmelia parietina, dye obtained from, i. 303

 Passalus cornutus, parasitic fungus in the stomach of, i. 274

 Patchouli, amount of radiant heat absorbed by the perfume of, i. 44

 Patellidæ, or limpets, structure, food, and habitat of, ii. 238, 239

 Paulia perforata, organs of reproduction of, i. 301

 Pea mildew, fungus producing the, i. 295

 Peach, cause of blistered leaves of the, i. 291

 Pear, cause of blistered leaves of the, i. 291

 Pearl oyster, nacreous lining of shell of, ii. 234

 Pease, caseine obtained from, i. 125

 Peat, and peat mosses, i. 333

 Pecten, or scallop, eyes of the, ii. 235

 Pediastrum, structure and development of, i. 194

 Pedicellaria globosa, structure of, ii. 180

 Pelagia, structure and mode of reproduction of, ii. 97

 Peltigeri, characters of the group, i. 305

 Peneroplis, structure of, ii. 31, 32

 Penicillia mould, structure of the, i. 285

 Penicillium armeniacum, spores of, i. 285, 286

 Penicillium candidum, production of, i. 287

 Penicillium glaucum, structure and habitat of, i. 286, 287
   its polymorphous character, i. 287
   in the yeast of beer, i. 288
   its production of acetic fermentation, i. 288

 Pennatula phosphorea, structure and habitat of, ii. 128
   modes of reproduction of, ii. 129

 Pennatulidæ, or sea-pens, structure, habitat, and mode of reproduction
    of, ii. 128, 129

 Pentacrinites, structure of, ii. 175

 Pentacrinus caput-Medusæ, structure and habitat of, ii. 175

 Pentacrinus Europæus, structure of, and change to a comatula, ii. 176

 Peppermint, absorption of radiant heat by the perfume of, i. 44

 Peranemeæ, or Woodsieæ, characters of the group, i. 350

 Perfumes of flowers and plants, absorption of radiant heat by the, i.
    44
   weight of the perfumes, i. 45
   chemical combinations forming, i. 97

 Peridinium, structure and habitat of, ii. 72
   scarlet colour it gives to the sea, ii. 72, 75
   mode of propagation of, ii. 75

 Perisporacei, structure and habitat of, i. 295

 Periwinkle, tongue of, ii. 239

 Peronospora infestans, its destruction of the potato, i. 284
   structure and mode of working, i. 284, 285
   its effects on the branches and wood of trees, i. 285

 Perophora Listeri, structure, habitat, and development of, ii. 222, 223
   larva of, ii. 224

 Petroleum, enormous quantities of, in North America, i. 126
   the Babylonian petroleum fountains of Is, i. 126
   geological formation in which it occurs, i. 126
   dangers of petroleum wells, i. 127
   substances yielded by it on destructive distillation, i. 127

 Peyssonnelia, habitat of, i. 237

 Peziza aurantia, cells of staff-shaped particles of, i. 291

 Peziza elegans, beauty and habitat of, i. 291

 Peziza vesiculosa, force of its ejection of its sporidia, i. 292

 Pezizæ, structure and habitat of the genus, i. 290, 291
   fructification of, i. 291
   eel-shaped particles or antherozoids of, i. 292
   force with which many of them eject their sporidia, i. 292

 Phacopsis, organs of reproduction of, i. 307

 Phalloidei, structure of, i. 268

 Phallus Mokusin, a food of the Chinese, i. 268

 Phascei, characters of, i. 329

 Phenic acid, i. 123

 Phenyle, chemical composition of, i. 128

 Philomedusa Vogtii, a parasite of the medusæ, ii. 100

 Phonolite stone, on the Rhine, of what it consists, i. 206

 Phosphorescence, property of, i. 66
   of insects, fish, and plants, i. 67
   of inorganic substances, i. 67
   essential difference between phosphorescence and fluorescence, i. 67,
      68

 Phosphorescent light of luminous fungi, i. 264
   causes of this, i. 264

 Phosphorus, i. 17
   whence procured, i. 17
   red allotropic, i. 17
   takes fire spontaneously in chlorine gas, i. 19
   atomic weight of, compared with that of hydrogen, i. 100

 Photography, importance of the magnesium light in, i. 154

 Phyllodoce, structure of, ii. 157

 Phyllopoda, characters of the order, ii. 209

 Physalia, or ‘Spanish man-of-war,’ structure, and modes of locomotion
    and reproduction of, ii. 111-114

 Physaliidæ, characters of the order, ii. 111

 Physomycetes, characters of the order, i. 296

 Physophora hydrostatica, structure and habitat of, ii. 109, 110

 Physophoridæ, characters of the family of the, ii. 109-111

 Pillwort, structure and habitat of, i. 371, 372

 Pilularia globulifera, or pillwort, structure and habitat of, i. 371

 Pilularia minuta, structure of, i. 371

 Pinna, structure of the shell of, ii. 233

 Pitch, manufacture of, i. 120

 Planets, spectra of the, i. 157, 158, 161, 162

 Plants, absorption of radiant heat by the perfumes of, i. 44
   weight of the perfumes, i. 45
   phosphorescence of, i. 67
   their synthetic process of rearing their fabrics, i. 96
   this process imitated by artificial means, i. 96
   chemical nature of the colouring matter of, i. 428, 429

 Platinum, properties of, i. 5
   how vaporized, i. 30
   effect of electricity on, i. 32
   crystals of, formed artificially by electricity, i. 74
   its resistance to electricity, i. 90

 Pleurocarpi, characters and habitat of the group, i. 328

 Pleurosigma angulatum, structure of, i. 198, 199

 Plumbago, or natural graphite, little or no porosity of, i. 112

 Podocyrtis Schomburgi, structure and habitat of, ii. 20, 21
   fossil and existing species of, ii. 20

 Poisons, vegetable, i. 425, 426

 Polarization of light and heat, i. 68, 69
   relation of polarization of light and heat to crystallization, i. 70,
      71

 Pollen of flowering plants, i. 379

 Polyatomic theory, the, i. 107

 Polycystina, structure and habitat of the, ii. 19, 20

 Polygastria, a name for the Infusoria, ii. 71

 Polyides rotundus, mode of reproduction of, i. 229
   structure and mode of propagation of, i. 237, 238

 Polynoë, structure of, ii. 160

 Polypary of the Alcyon zoophytes, ii. 123

 Polypes. _See_ Hydrozoa

 Polypodiaceæ, characters of the order, i. 344
   sporangia of, i. 343

 Polypodineæ, characters of the group, i. 344
   habitat of the, i. 345

 Polypodium, structure and habitat of the genus, i. 345
   vulgare, fronds of, i. 340, 345
   sori of, i. 342, 345
   structure of, i. 345

 Polyporei, structure, habitat, and growth of, i. 264, 265

 Polysiphonia, structure of the genus, i. 233

 Polysiphonia elongata, structure and mode of propagation of, i. 234

 Polystichum, characters of the genus, i. 347
   sorus and indusium of, i. 347

 Polystichum aculeatum, characters of, i. 348

 Polystichum angulare, characters of, i. 348

 Polystichum Lonchitis, structure of, i. 347

 Polystichum proliferum, organs of reproduction of, i. 348

 Polystomella, characters of the genus, ii. 47

 Polystomella crispa, form of, ii. 28
   structure and habitat of, ii. 47, 48
   cristata, form of, ii. 28
   striato-punctata, habitat of the, ii. 48

 Polytrichei, characters of the tribe, i. 329

 Polytrichum commune, organs of fructification of, i. 325

 Polytrichum dendroides, structure of, i. 329

 Polyzoa, or Bryozoa, characters of, ii. 218

 Polyzonia cuneifolia, structure of, i. 242, 243

 Porites, or reef-building corals, ii. 140

 Porphyra laciniata, structure and habitat of, i. 227

 Porphyra vulgaris, structure, habitat, and mode of reproduction of, i.
    227

 Porpita, characters of the genus, ii. 117
   glandifera, structure, habitat, and mode of reproduction of, ii. 117,
      118

 Potash, caustic, effect of the heating of, by an electric discharge, i.
    84
   its effect on electricity in a vacuum tube, i. 86
   cyanate of, combination forming, i. 106
   nitrate of, its opacity to the invisible rays, i. 65
   oxalate of, formation of, i. 117
   violet coloured light obtained by the combustion of, i. 133
   in the land plants, i. 414

 Potassium, i. 3
   affinity for oxygen, i. 96
   atomic weight of, compared with that of hydrogen, i. 100
   one of an isomeric triad with cæsium and rubidium, i. 105
   iodide of, whence obtained, i. 19

 Potato, fungi causing the murrain in the, i. 284, 285
   mode in which the injury is done, i. 284
   poisonous nature of the leaves and berries of the, i. 426, 427

 Poulpe, structure of the, ii. 245

 Praya diphys, structure and mode of reproduction of, ii. 103-106

 Propagation of diatoms by bisection, by conjugation, and by gonidia, i.
    200

 Protein produced by plants, i. 419

 Protococcus pluvialis, structure and development of, i. 184
   cycles of reproduction, i. 186

 Protococcus viridis, abundance of, found in dust from Egypt, ii. 65

 Protophytes, structure and development of, i. 182

 Protoplasm of plants, structure and function of, i. 412, 413

 Protozoa, structure of the, ii. 13

 Pterideæ, characters of the group, i. 357

 Pteris, characters of the genus, i. 357

 Pteris aquilina, stems of, i. 339
   or bracken, structure, mode of reproduction, and habitat of, i. 357,
      358

 Pteris esculenta of New Zealand, i. 358

 Pteris serrulata, development of spores of, i. 337
   antheridium and spermatozoids of, i. 338

 Pteris serrulata: archegonium of, i. 338

 Pucciniæ, structure and habitat of the sub-order, i. 276, 277

 Puccinia Amorphæ, spores of, i. 276

 Puccinia Fabæ, structure, habitat, and method of reproduction of, i.
    280
   alternation of generations of, i. 280

 Puccinia Graminis, spore-cases of, i. 280

 Puccinia lateripes, spores of, i. 276

 Puff-balls of the meadows, i. 267

 Purple dyes, i. 123, 124

 Pycnogonoïdea, or spider crabs, structure and habitat of, ii. 211

 Pyrosomidæ, structure and mode of reproduction of the, ii. 225

 Pyxinei, characters of the, i. 307


                                   Q

 Quartz, rock-crystal the purest form of, i. 17
   certain thickness of, not transparent to invisible rays of light, i.
      65
   axis of symmetry of, i. 72
   aqueous solution of, i. 110
     characteristics of the, i. 111

 Queen conch, shell of, ii. 234

 Quinine, structure of, and whence obtained, i. 427

 Quinqueloculina Bronniana, form of, ii. 228


                                   R

 Radiation of light and heat, i. 34, 35
   effects of, i. 35, 36
   generally independent of colour, i. 36
   Professor Tyndall’s experiments, i. 38
   experiments showing radiation to be equal to absorption, i. 46
   dynamic radiation, i. 49
   absorption a phenomenon irrespective of aggregation, i. 53

 Radicles, compound, i. 106

 Ramalina calicaris, gluten of, i. 303

 Ramalina polymorphum, dyes obtained from, i. 303

 Ramalina scopulorum, dyes of, i. 303

 Raphides, structure and formation of, i. 424, 425

 Red Sea, foraminifera in the, ii. 51

 Reed, Cordyceps on the ergot of the, i. 293

 Reed, Italian, vegetable tissues represented in, i. 175, 176

 Reef-building corals, ii. 138-142

 Reefs, barrier, formation of, ii. 143

 Resins, formation of, i. 422

 Respiration, chemical powers causing, ii. 4

 Respiration of the muscles, ii. 8

 Reticularia, characters of the order, ii. 24

 Reticularia maxima of cucumber beds, i. 269

 Rhabdonia Coulteri, mode of propagation of, i. 236

 Rhamnus cathartica, M. Charwin’s discovery of a green dye obtained from
    the, i. 124

 Rhizocrinus Lofotensis, structure of, ii. 175

 Rhizopoda, structure of the, ii. 13
   simple Rhizopods, forms of, ii. 22

 Rhizoselenia, enormous masses of, in the Indian Ocean, i. 205

 Rhizospermæ. _See_ Marsileaceæ

 Rhizostoma, or many-mouthed medusæ, structure and mode of reproduction
    of, ii. 97, 98
   food of, ii. 98

 Rhodomelaceæ, structure &c. of, i. 241

 Rhodospermeæ, characters of the, i. 180
   structure and mode of reproduction of, i. 227, _et seq._
   causes which affect the form and limits of, i. 243

 Rhodymenia palmata, structure and mode of reproduction of, i. 236

 Rhodymeniaceæ, structure of, i. 236

 Ricciaceæ, or Crystalworts, characters of the order, i. 316

 Richmond, in Virginia, siliceous deposit upon which it stands, i. 206

 Riella, characters of the genus, i. 316, 317

 Rivularia, structure of the genus, i. 213, 215

 Rivularia nitida, structure and motions of, i. 213, 215

 Rocella fuciformis, dye and orchil obtained from, i. 303

 Rocella tinctoria and fusiformis, blue and purple dyes obtained from,
    i. 124

 Rock-crystal, formation of, i. 17

 Rock-salt, chlorine obtained from, i. 18
   permeable to radiant heat but radiates badly, i. 37

 Roots, downward tendency of, i. 382
   structure and functions of, i. 409

 Rosalina ornata, structure of, ii. 41

 Rosaniline, or roseine, the base of aniline, i. 123
   production of, i. 123

 Rosaniline, chemical composition of, i. 128

 Rosemary, amount of radiant heat absorbed by the perfume of, i. 44

 Roses, otto of, amount of radiant heat absorbed by the perfume of, i.
    44

 Rotalia, structure of, ii. 38

 Rotalia Beccarii, habitat and structure of, ii. 42

 Rotaline group of foraminifera, ii. 44

 Rotifer, common, structure of the, ii. 167

 Rotifera, characters of the, ii. 162
   mode of reproduction of the, ii. 162

 Rubia tinctorum, madder obtained from the roots of, i. 124

 Rubidium, i. 3, 4
   atomic weight of, compared with that of hydrogen, i. 100
   one of an isomeric triad with cæsium and potassium, i. 105
   M. Bunsen’s discovery of the metal by spectrum analysis, i. 134, 135
   mode of distinguishing it from potassium, i. 135
   properties of, i. 136
   where found, i. 136

 Ruby, i. 4

 Ruhmkorff’s electro-magnetic induction apparatus, i. 32

 Russulæ, laticiferous vessels of, i. 263

 Rust of wheat, i. 281

 Rye, probable cause of the mildew of, i. 297

 Rytiphlæa pinastroides, antheridia of, i. 243

 Rytiphlæa tinctoria, antheridia of, i. 242


                                   S

 Saffron, meadow, poisonous alkaloid obtained from, i. 427

 Sagartia miniata, structure and habitat of, ii. 132
   its deadly weapons, ii. 132, 133

 Salpa maxima, structure of, ii. 226, 227

 Salpa mucronata, vast shoals of, ii. 228

 Salpa zonaria, young of, ii. 227

 Salpi, their food, i. 205

 Salpidæ, characters of the, ii. 226-228

 Salt, change of volume of, by chemical combination, i. 20
   partial decomposition of, by diffusion, i. 111
   yellow coloured light produced by the combustion of, i. 132
   spectrum analysis of, i. 134

 Salt: more universally diffused than any other matter, i. 134
   spectrum analysis of the rarefied vapour of, i. 141

 Sand of the sea-shore, the debris of quartz rocks, i. 17

 Sandal wood, amount of radiant heat absorbed by the perfume of, i. 44

 Sandhopper, structure of the, ii. 201, 202

 Sap milk, composition and formation of, i. 425

 Sapindaceæ, or soapworts, fruits of, innocuous, i. 426

 Sapphire, i. 4

 Sapphirina fulgens, structure and habitat of, ii. 204, 205

 Sarcode, structure and functions of, ii. 2

 Sarcophycus of the Antarctic Ocean, i. 256

 Sarcophycus potatorum, fruit of, i. 256

 Sargassum, habitat of the, i. 255

 Sargassum bacciferum, structure, fructification, and habitat of, i. 257

 Sargassum vulgare, structure, fruit, and habitat of, i. 257

 Saturn, the planet, spectrum of, i. 158, 161

 Sausages, fatal effects caused by, i. 285

 Saw-dust, manufacture of into oxalic acid, i. 116, 117

 Scalariæ, tongues of, ii. 239

 Scallop, or pecten, eyes of the, ii. 235

 Schistocarpi, characters of the group, i. 328

 Schistostegei, structure and habitat of, i. 331

 Schizæa, structure of, i. 363

 Schizæineæ, characters of the, i. 363
   sporangia of, i. 343
   characters of the group, i. 345

 Sclerogen, i. 175
   production of, in plants, i. 421

 Scolopendrieæ, characters of, i. 351, 352

 Scolopendrium, structure and development of the caudex of, i. 340

 Scolopendrium vulgare, or Hart’s-tongue fern, structure,
    fructification, and habitat of, i. 351, 352

 Screw pine, roots and habitat of, i. 387

 Scutella, spines of the, ii. 180

 Scutula, organs of reproduction of, i. 307

 Sea-cucumbers, structure and mode of reproduction of, ii. 183

 Sea-eggs, or sea-urchins, structure of, ii. 176 _et seq._

 Sea-fans, structure of the, ii. 125

 Sea-mat, structure of, ii. 218

 Sea-nettles, vast shoals of, ii. 100, 101

 Sea-pens, structure of, ii. 128, 129-131

 Sea-salt, chlorine obtained from, i. 18

 Sea-serpent of Celebes, ii. 186

 Sea-slugs, structure of, ii. 239, 240

 Sea-weeds, former and present uses of, i. 128

 Seeds of two lobes, i. 177
   of one lobe, i. 177
   of plants, i. 381, 382, 383, 404

 Selaginella, structure and habitat of, i. 374

 Selenium, atomic weight of, i. 105
   its properties analogous with those of sulphur and tellurium, i. 105

 Sepedonium mycophilum, spores of, i. 286

 Sepia, or cuttle fish, structure of, ii. 245, 246, 247

 Serpentine marble of Tyree and of Connemara, composition of, ii. 56

 Serpula, structure of, ii. 155

 Sertularia cupressina, structure of, ii. 87

 Sertulariidæ, characters of the family of, ii. 90, 91
   modes of propagation of, ii. 91

 Shells of mollusks, ii. 234

 Shrimps, opossum, ii. 199, 200

 Sigillaria, structure of the fossil, i. 375

 Silex, quantity of, in the Equisetaceæ, i. 369
   in the grasses, i. 386

 Silica, abundance of, i. 17
   in the stalks and leaves of the grasses, i. 386
   effect of electricity on, i. 32

 Silicon, i. 17
   combined with oxygen gas forms rock-crystal, i. 17
   three different states in which it exists, i. 18
   analogy between silicon and carbon, i. 18
   Professor Graham’s limpid solution of, i. 18

 Silk, dyes for, i. 125
   mode of preparing it if it is to be moiré, i. 125
   advantages of the climate of Lyons in the manufacture of, i. 126

 Silk-worm, a fungus parasite of the, i. 274

 Silver, affinity of, for oxygen, i. 5
   conduction of heat and radiation, i. 5
   iodine found in combination with, i. 19
   crystals of, formed artificially by electricity, i. 74
   transmissive power of, of electricity, i. 90

 Sipunculidæ, structure and mode of reproduction of, ii. 186, 187

 Sky, probable cause of the blue colour of the, i. 58

 Slate, polishing, of Bilin, of what it consists, i. 206

 Smoke, i. 14

 Smut of wheat, i. 281

 Snails, structure of, ii. 235-237

 Soda, chlorate of, singular property of, in crystallization, i. 75

 Soda, oxalate of, formation of, i. 117

 Soda, silicate of, i. 110
   Prof. Graham’s dialysis of, i. 110

 Soda, former and present mode of procuring, i. 128
   yellow coloured light produced by the combustion of, i. 132
   abundance in the Algæ, i. 414

 Sodium, i. 3
   atomic weight of, compared with that of hydrogen, i. 100
   spectrum analysis of the rarefied vapour of, i. 141
   chloride of, spectrum of, i. 146
   reversion of the coloured lines of sodium burning in air, i. 150

 Solar spectrum, i. 58
   myriads of ethereal waves constituting the seven colours of the, i.
      58
   rayless spaces crossing the, at right angles, i. 59
   length of the undulations of the ether producing the impression of
      the colours of the solar spectrum, i. 59
   mode of bringing the invisible rays of the chemical spectrum before
      the human eye, i. 59
   fluorescence and calorescence, i. 60-62
   experiments producing the long spectrum, i. 63
   spectra of volatilized metals, i. 64
   absorption of invisible rays, i. 65
   discoveries of Sir Isaac Newton of the solar spectrum and the laws of
      coloured rings, i. 129
   Fraunhofer’s lines, i. 129, 148
     MM. Bunsen and Kirchhoff’s experiments, i. 130, 148
     Mr. Glaisher’s experiments, i. 130
   absorption bands, i. 131
     Sir D. Brewster’s discovery, i. 131
   coincident dark and coloured lines, i. 148
   reversion of the coloured lines, i. 149
   Prof. J. P. Cook’s discoveries, i. 149
   M. Foucault’s discoveries, i. 149
   M. Kirchhoff’s discovery of the law of exchanges, i. 150
   metals shown by MM. Kirchhoff and Angström to exist in the sun, i.
      151-153
     and those proved to have no existence in the sun, i. 154

 Solferino, production of, i. 127

 Solids, the specific heat of compound, generally greater than that of
    their component elements, i. 101

 Solorina, structure of the genus, i. 305

 Solorina crocea, structure of, i. 306

 Solorina saccata, structure of, i. 305

 Space, matter wandering in, i. 28

 Spearmint, absorption of radiant heat by the perfume of, i. 44

 Specific gravity, i. 26
   unit of, i. 26

 Spectra, continuous, from glowing solids and liquids, i. 132
   spectrum analysis, i. 133
     experiments of Sir David Brewster and Mr. Fox Talbot, i. 133
     spectrum of rubidium and cæsium, i. 136
       of thallium, i. 136, 137
       of indium, i. 137
       of the flame of the iron in the manufacture of the Bessemer
          steel, i. 137, 138
       of gases, i. 139, _et seq._
         effect of high temperature on various spectra, i. 142-144
         effects of pressure on a variety of gases and vapours, i. 145
       of metals not always the same, i. 145
       of the halogens, i. 146
       of the vaporized mixture of five chlorides, i. 147
       of lightning, 147
       of the sun. _See_ Solar spectrum
       of the fixed stars, i. 155, 162, 163
       of temporary and periodic stars, i. 156, 157
       of the planets, i. 157, 158, 161
       of various nebulæ, i. 158-160

 Spermatozoids of confervaceæ, i. 210

 Sphæria aquila, structure and habitat of, i. 295

 Sphæria bombarda, structure of, i. 295

 Sphæria, candle-snuff, habitat of, i. 295

 Sphæria Desmazierii, development of, i. 294

 Sphæria miliaris, characters of, i. 283

 Sphæriacei, characters and habitat of the order, i. 293
   development of, i. 294

 Sphærobolus, force with which its sporangium is ejected, i. 273

 Sphærococcoideæ, characters of the, i. 238

 Sphærococcus coronopifolius, mode of reproduction of, i. 236, 238

 Sphærococcus, habitat of the genus, i. 239

 Sphærophorei, characters of the, i. 310

 Sphæroplea annulina, structure and reproduction of, i. 208, 209

 Sphagnei, characters of, i. 331

 Sphagnum latifolium, leaves of, i. 330

 Sphagnum, or common bog-moss, structure, fructification, and habitat
    of, i. 332
   uses of, in Lapland, i. 334

 Spikenard, amount of radiant heat absorbed by the perfume of, i. 44

 Spinal cord, structure and functions of the, ii. 8

 Spines of plants, structure and formation of, i. 411

 Spirogyra, mode of reproduction of, i. 218

 Spirulina tenuissima, motions of, i. 213

 Splachnei, characters of the tribe, i. 330

 Splachnum ampullaceum, structure and habitat of, i. 331

 Splachnum vasculosum, structure and colour of, i. 330

 Spondylus gædaropus, eyes of the, ii. 235

 Sponges, iodine found in, i. 19

 Sponges, the Carpenteria, a link between the foraminifera and the, ii.
    57
   structure and development of, ii. 57
   varieties in size, structure, and habits of the marine sponges, ii.
      60
   propagation of, ii. 60, 61
   structure of fresh-water sponges, ii. 61
   fossil sponges, ii. 62

 Spongilla fluviatilis, structure and development, and propagation of,
    ii. 61

 Spongiocarpeæ, structure and mode of propagation of, i. 237, 238

 Spores of Cryptogamia, i. 177, 178

 Sporidia, or spore-bearing cells, of Ascomycetes, i. 290

 Sporidiiferi, structure and fructification of, i. 260
   orders of, i. 260, 261

 Sporiferi, structure and fructification of, i. 260
   orders of, i. 260

 Sporopodium Leprieurii, ascus of, i. 300

 Spurgeworts, poisons and food supplied by the, i. 425, 426

 Squamariæ, structure and modes of propagation of, i. 237

 Squid, structure of the, ii. 245, 246

 Squilla Desmarestii, structure of, ii. 198

 Squilla mantis, structure of, ii. 198

 Stamens of flowering plants, i. 379

 Starch, production of, by plants, i. 420

 Star-fishes, structure of, ii. 169

 Stars, absorption of the heat radiated by the, by the earth’s
    atmosphere, i. 55
   chemical power of the light of the, i. 55

 Stars, falling, i. 28

 Stars, fixed, constitution and spectra of the, i. 154, 155, 162, 163
   spectra of periodic stars, i. 156

 Staurastrum, structure and development of various species of, i. 192

 Steam-engine, equivalence between the mechanical work and heat of the,
    as between cause and effect, i. 29
   causes of the motive force in the, i. 29

 Steel, effect of magnetism on, i. 77
   importance of the spectrum analysis in the manufacture of, by
      Bessemer’s process, i. 137, 138

 Stegobolus Berkeleianus, structure and organs of development of, i. 299

 Stelleridæ, structure and mode of reproduction of, ii. 169-172, 174

 Stems of plants, structure of, i. 384, 405

 Stenochlæna, stem of, i. 339

 Stephanosphæra pluvialis, structure and development of, i. 187-189

 Stickleback, entozoön of the, ii. 145

 Sticta, structure of the genus, i. 304

 Sticta pulmonacea, structure and habitat of, i. 304

 Stilbacei, structure and habitat of, i. 283

 Stomach, human, fungus in the, i. 275

 Stomapoda, characters of the, ii. 198

 Stomata of plants, structure of the, i. 405, 406

 Stone-lilies, structure of, ii. 174, 175

 Strawberry, fructification of the, i. 381

 Strombus gigas, or queen conch, shell of, ii. 234

 Strontium, i. 3
   one of an isomeric triad with calcium and barium, i. 105
   red-coloured light obtained by the combustion of, i. 133
   spectrum analysis of, i. 133
   spectrum analysis of the rarefied vapour of, i. 142
     effect of high temperature, i. 142, 143

 Strychnos nux vomica, fruit of, food for birds, i. 426

 Substitution, direct or indirect, the basis of the modern doctrine of
    equivalents, i. 104

 Succinic acid, chemical combination forming, i. 97

 Sugar, production of, in plants, i. 421

 Sugar-cane, size and structure of, i. 386

 Sulphur, i. 16
   extensive range of affinities of, i. 16

 Sulphur: dimorphism and allotropism of, i. 16
   combination of chlorine with sulphur, i. 20
   atomic weight of, compared with that of hydrogen, i. 100, 105
   its analogous properties with selenium and tellurium, i. 105
   spectrum analysis of, in different temperatures, i. 144

 Sulphuretted hydrogen gas, i. 16

 Sulphurous acid, amount of absorption of radiant heat by, i. 41

 Sun, probable cause of the light and heat of the, i. 28
   probability of the periodic connection of the solar spots with
      magnetic phenomena, i. 30, 31
   amount of force exerted by the sun’s light within the limits of the
      terrestrial atmosphere, i. 34
   chemical action of the light of the sun, i. 56
   effect of the opalescence of the atmosphere on the chemical power of
      the sun’s light, i. 57

 Sun, spectrum of the, i. 58
   thirteen terrestrial substances in the sun’s atmosphere, i. 59
   metals shown by M. Kirchhoff to exist in the sun, i. 151-153
   and those proved not to exist in it, i. 154
   the structure of the sun in some respects still a mystery, i. 164
   the luminous gaseous atmosphere of the, i. 164
   mottled appearance of the photosphere of the sun, i. 164, 165
   the faculæ, i. 165
   the red flames or protuberances round the edge during a total
      eclipse, i. 165
   the solar spots, i. 166
     their periodicity, i. 166
     appear to be influenced by the planet Venus, i. 166

 Sun-rise and sun-set, causes of the bright tints at, i. 58

 Surirella, mode of development of, i. 202

 Synapta, structure of the genus, ii. 185

 Synapta digitata, structure and habitat of the, ii. 185

 Syncladei, structure of the branches of, i. 328

 Syncoryna Sarsii, structure and development of zooids of, ii. 90

 Synthesis, in the animal and vegetable creation, i. 96


                                   T

 Tænia, or tape-worm, structure and mode of reproduction of, ii. 145
   transformation of the young of, ii. 146.

 Tænioïdæ, characters of, ii. 144

 Talitrus, or sand-hopper, structure of, ii. 201, 202

 Tar, coal. _See_ Coal tar

 Tardigrada, characters of the, ii. 161

 Tartaric acid, chemical combination forming, i. 97

 Tea, active principle of, i. 428

 Tellurium, atomic weight of, i. 105
   its properties analogous to those of sulphur and selenium, i. 105

 Terebella conchilega, structure of, ii. 153-155

 Textularia, structure and habitat of the genus, ii. 41
   fossils of, ii. 42

 Textularia Mayeriana, form of, ii. 28

 Thalassicolla morum, structure of, ii. 21

 Thalassicollæ, structure of, ii. 21

 Thalassiophyllum Clathrus, habitat and structure of, i. 250

 Thallium, i. 3, 4
   effect of the spectrum from an electric spark between points of, i.
      64
   atomic weight of, compared with that of hydrogen, i. 100
   Mr. W. Crookes’s discovery of the metal so called, i. 136
   spectrum analysis of, i. 137
   properties of, i. 137
   where found, i. 137
   changes in the spectrum of, by high temperature, i. 144

 Thallus or frond, of lichens, i. 301

 Thaumantia pilosella, form and structure of, ii. 92
   otolites of, ii. 93
   mode of reproduction of, ii. 94

 Thorinum, i. 4

 Thunder-dirt of the New Zealanders, i. 268

 Thyme, absorption of radiant heat by the perfume of, i. 44

 Timmia, leaves of, i. 330

 Tobacco, spectrum analysis of, i. 136

 Tobacco, narcotic effect of, and as a relief from hunger, i. 428

 Toluidine, property of, in producing the aniline colours, i. 122

 Toluol, constituents of, i. 121

 Topaz, oriental, i. 4

 Torula cerevisiæ, structure, development, and cell-multiplication of,
    i. 287

 Transparency in liquids and solids synonymous with discord, i. 36
   causes of transparency, i. 37

 Transpiration of gases, i. 110, 114

 Tree ferns, structure of, i. 341

 Trees, constitution of the stems of, i. 174

 Tremellini, structure, habitat, and fructification of, i. 266

 Trepang, used as food, ii. 184

 Triceratium favus, structure and habitat of, i. 199

 Triceratium genus, structure and habitat of, i. 199

 Trichodesmium erythræum, structure, habitat, and reproduction of, i.
    215, 216

 Trichogastres, characters of the group, i. 267

 Trichomanes radicans, or bristle fern, structure and habitat of, i. 361

 Trichomanineæ, characters of the group, i. 345
   characters and habitat of, i. 360, 361

 Trilobites, structure of, ii. 203

 Tripe de Roche, structure of, i. 308

 Triphragmium dubens, spores of, i. 276

 Tripoli stone on the Rhine, of what it consists, i. 206

 Trochus granulatus, structure of, ii. 237, 238

 Trochus zizyphinus, palate of, ii. 237

 Truffle, structure and habitat of the, i. 292, 293
   modes of tracing them, i. 293
   the red, of Bath, i. 268

 Trypethelium Sprengelii, pustules and sporidia of, i. 299

 Tuberacei, characters and habitat of the order, i. 292

 Tubicola, structure of, ii. 153
   found in all seas, ii. 161

 Tubipora, characters of the family of, ii. 129

 Tubipora musica, structure and mode of reproduction of, ii. 130

 Tubipora purpurea, habitat of, ii. 130

 Tubularia, form and structure of the family, ii. 91
   mode of propagation of, ii. 91

 Tulip, structure and mode of reproduction of, i. 388

 Tunicata, or Ascidians, characters of, ii. 222
   groups of, ii. 222, _et seq._

 Turbellariæ, structure of the, ii. 148

 Turkey red, from what obtained, i. 124

 Turpentine, oil of, takes fire in chlorine gas, i. 19

 Turritopsis nutricula, larvæ of the Cunina octonaria, parasites of, ii.
    100


                                   U

 Ulva, structure and reproduction of, i. 171, 225

 Ulva bullosa, reproduction of, i. 227

 Ulva lactuca, structure and mode of reproduction of, i. 226

 Ulva latissima, structure and mode of reproduction of, i. 225-227

 Ulva Linza, structure and mode of reproduction of, i. 226

 Umbilicaria, structure and habitat of, i. 308

 Upas-tree of Java, causes of the virulence of, i. 426
   fruit of, innocuous, i. 426

 Uranium, i. 4, 5
   nitrate of, phosphorescence of, i. 67

 Uranite, yellow, fluorescent property of, i. 66

 Uredines, structure and habitat of the, i. 277

 Uredo, structure and mode of reproduction of, i. 277

 Uredo candida, or Cystopus candidus, structure, habitat, and mode of
    reproduction of, i. 278

 Uredo linearis, the rust on the leaves and chaff scales of wheat, i.
    281

 Uromyces appendiculatus, or fungus of the common bean, i. 280

 Usnea, structure of, i. 302

 Usnea melaxantha, splendour of, i. 304

 Usnea Taylori, splendour of, i. 304

 Usneæ, brilliancy and wide diffusion of the, i. 304

 Usnic acid, produced by lichens, i. 303

 Uva di mare, fruit of, i. 257


                                   V

 Vacuum tubes, i. 78, _note_
   electric discharges in, i. 78, 79

 Valvulina, structure of, ii. 37

 Vanilla, reproduction of, i. 402

 Vapours, absorption of radiant heat by gases and vapours, i. 38
   radiation of, equal to absorption, i. 46

 Variolaria dealbata, litmus or orchil obtained from, i. 124

 Vaucheria, character of the genus, i. 218, 224
   structure, habitat, and modes of reproduction of, i. 218

 Vaucheria marina, structure and habitat of, i. 224

 Vaucheria sessilis, mode of reproduction of, i. 219

 Vaucheria velutina, habitat of, i. 224

 Vegetable world, microscopic structure of the, i. 167

 Vegetation, i. 167
   effects of light and heat on, i. 168, 169
   the primordial cell, the universal framework or skeleton of the
      vegetable world, i. 170, 172
   formation of cellular tissue, i. 171, 173
   fibro-vascular bundles constituting the wood of trees, i. 173
     the vascular ducts, i. 73
     the pitted tissue, i. 174
     the woody fibre, i. 174
       sclerogen, i. 175
   the laticiferous vessels, or vasa propria, i. 176, 177
   seeds and spores, i. 177
   cycles of existence in the vegetable world, i. 178, 186
   variation of marine vegetation, horizontally and vertically with the
      depth, i. 258
   instance of mechanical power exerted by vegetable matter, i. 269
   seeds brought to Europe by the great Atlantic currents, i. 355
   gradual changes of structure from the lowest to the highest
      cryptogamic forms, i. 375
     reproduction and fructification, i. 376, 377
   general structure of flowering plants, i. 378
     calyx and corolla, i. 378
     stamens and pollen, i. 379
     pistil, ovary, and style, i. 380
     fructification, i. 381
   monocotyledonous, or endogenous plants, their structure, growth, and
      reproduction, i. 383
     the earliest dawn of plant life, as shown in the colourless
        protoplasm of the grasses, i. 387
     bulbous plants, structure and mode of reproduction of, i. 388
     orchids, their structure and fructification, i. 389
   dicotyledonous, or exogenous plants, i. 404-428
     seeds, i. 404
     stems, i. 405
     stomata, i. 405, 406
     bark, i. 406, 407
     cambium, i. 407
     wood, i. 408
     roots, i. 409
     leaves, i. 410
     buds, spines, and hairs, i. 411
     protoplasm, i. 412
     chemical elements of vegetable matter, i. 413
     sap and sap motion, i. 415-417
     inhalation and exhalation of oxygen, i. 416, 417
     latex, i. 417
       milk vessels, i. 418
     chemical functions, i. 419
       cellulose, i. 419, 420
       starch, i. 420
         diastase and dextrine, i. 420, 421
       sclerogen, or colouring matter of wood, i. 421
       sugar, i. 421
       oils, resins, and wax, i. 422, 423
       albumen, fibrin, and casein, i. 423
       raphides, i. 424
       milk sap, i. 425
         poisons, i. 425, 426
         food and other purposes, i. 426
         alkaloids, chemical structure of, i. 427
       colouring matter of flowers, chemical nature of, i. 428, 429
     water secreted by plants night and morning, i. 429
     electricity developed by plants and flowers, i. 430
     irritability of the tissues of plants, i. 430
       light the most universal and important exciting cause in the
          vegetable world, i. 431

 Velella spirans, structure and modes of locomotion and reproduction of,
    ii. 114-116
   food of, ii. 116
   habitat of, ii. 117

 Velellidæ, characters of the order, ii. 111, 114

 Venus, the planet, spectrum of, i. 158, 162

 Veratrum album, or white hellebore, poisonous alkaloid of, i. 427

 Verrucariei, structure of, i. 310

 Verrucaria muralis, structure and habitat of, i. 310

 Verrucaria variolosa, structure and development of, i. 299, 300

 Vibrissea, motions of the sporidia in, i. 292

 Vibrios found in dust from Egypt, ii. 65
   agents in the decomposition of organic matter, ii. 66, 67
   difference between vibrios and mycoderms, ii. 66
   extreme tenacity of life of the vibrios, ii. 67

 Vine, fungus constituting the mildew of the, i. 295, 297

 Vinegar plant, fungus producing the, i. 288

 Virgularia juncea, ii. 129

 Virgularia mirabilis, mode of reproduction of, ii. 129

 Virgulariæ, structure and mode of reproduction of the, ii. 129

 Vis viva, or impetus, i. 26, 27
   heat generated by, i. 27
   combustion, a case of, i. 30

 Vittarieæ, characters of the group, i. 356

 Volatility of a compound, law of, i. 22

 Voltaic battery, i. 31

 Voltaic electricity. _See_ Electricity

 Volvocineæ, structure and reproduction of, i. 187, 189

 Volvox globator, structure and development of, i. 189

 Vorticella nebulifera, diversity of their reproductive powers, ii. 75,
    76

 Vorticellæ, structure of the, ii. 76
   food of, and mode of taking it, ii. 76, 77
   mode of reproduction of, ii. 77

 Vulpinic acid, whence obtained, i. 303


                                   W

 Walking fern, structure and mode of reproduction of, i. 352

 Walnut, cause of blistered leaves of the, i. 291

 Wasp, a West Indian, killed by fungi, i. 293, 294

 Water almost impervious to heat, i. 36
   proportions of oxygen and hydrogen in, i. 94
   combination and decomposition of, i. 94
   force of the chemical combination requisite to form a gallon of, from
      the combustion of the two gases, i. 97, 98
   force required to freeze water, i. 98
   amount of voltaic electricity required to separate a given quantity
      of water into hydrogen and oxygen, i. 101
   the most common radicle in the inorganic and organic world, i. 107
   the water of crystallization, and the effect of heat upon it, i. 108
   production of aqueous solutions of organic and inorganic matter, i.
      111
   water secreted night and morning by plants, i. 429

 Water-flea, arborescent, structure of the, ii. 208

 Waterproofing, India rubber prepared for, i. 120

 Wax, vegetable, formation of, i. 422, 423
   functions of, i. 423

 Wheat, probable causes of the rust and smut of, i. 281
   Cordyceps on the ergot of, i. 293

 Whelks, tongues of, ii. 239, 240

 Wood-louse, or slater, structure of, ii. 202

 Wood of trees, fibro-vascular bundles constituting the wood of, i. 173,
    174
   structure of, i. 408

 Woodsia, structure of, i. 349

 Woodsia ilvensis, structure and fructification of, i. 350

 Woodsieæ, or Peranemeæ, characters of the group, i. 350

 Worms, or annulosa, characters of, ii. 144

 Wormskioldia sanguinea, structure and habitat of, i. 239, 240

 Wormwood, absorption of radiant heat by the perfume of, i. 44

 Wrangelia penicillata, spores of, i. 237

 Wrangeliaceæ, structure and mode of reproduction of, i. 237


                                   X

 Xenodochus parodoxus, spores of, i. 276


                                   Y

 Yeast plant, i. 286, 288
   yeast of beer, i. 287, 288
   German yeast, i. 288

 Yellow dye, obtained from aniline, i. 124

 Yttrium, i. 4


                                   Z

 Zinc, i. 4
   spectrum of volatilized zinc, i. 64
   atomic weight of, compared with that of hydrogen, i. 100
   peroxide of, combination forming, i. 104
   seleniate of, different forms assumed by, according to the
      temperature of the water united with, i. 107, 108

 Zirconium, i. 4

 Zonaria, structure of, i. 247

 Zooids of Hydrozoa, ii. 88-90

 Zoophytes, characters of, ii. 81
   hydrozoa, ii. 81, 86
   actinozoa, ii. 130
   anthozoa, ii. 119
     alcyon, ii. 119

 Zoospores of Confervaceæ, i. 208

 Zostera marina, or sea wrack, crimson fringe of the, i. 231
   characters of the flowers of, i. 387

 Zygnema quininum, mode of reproduction of, i. 217

 Zygodesmus fuscus, spores of, i. 286


                                THE END.


                  ------------------------------------

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                            Transcriber’s Note


  This book uses inconsistent spelling and hyphenation, which were
  retained in the ebook version except in the Index and List of
  Illustrations, where spelling was altered to match the body of the
  text where differences were noted. Index entries were re-ordered to
  preserve alphabetical order when the spelling changes resulted in out
  of order entries. Some corrections have been made to the text,
  including normalizing punctuation. Further corrections are noted
  below:

  D added to Fig. 97 before Cristellaria compressa
  Fig. 97: Foraminifora -> Foraminifera
  p. 124 the Isidinæ is jointed -> the Isidæ is jointed
  p. 142 the depths of the lagoon -> the depth of the lagoon
  p. 236 entirely ontained -> entirely contained

  Book catalogue: Sale’s Brigade in Affghanistan -> Sale’s Brigade in
     Afghanistan
  Book catalogue: carborough, Whitby -> Scarborough, Whitby
  Book catalogue: MELVILLE’S (HERMANN) -> MELVILLE’S (HERMAN)
  Book catalogue: the Kiver Yukon -> the River Yukon