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			(Voght)[B]	to face 103
118.	Galeolaria lutea				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

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.